Abstract
Growing concern for public health has increased the need to change the paradigm towards a healthcare system that advocates holistic practices while reducing adverse effects. Herbal therapy is becoming an integral part of the therapeutic arsenal, and several successful plant-derived compounds/molecules are being introduced into the market. The medicinal plants belonging to the genus Thymus are among the most important species within the Lamiaceae family. One of them is Thymus algeriensis which is mainly distributed in the Mediterranean region. For a long time, this species has been used in traditional medicine to treat several disorders and diseases including inflammation, diabetes, rheumatism, digestive, and respiratory affections. This review describes the traditional uses, phytochemical composition, and biological and pharmacological activities of T. algeriensis extracts. Data were obtained using electronic databases such as SciFindern, ScienceDirect, Scopus, and Web of Science. Several plant-based extracts and a broad spectrum of identified secondary metabolites were highlighted and discussed with respective activities and modes of action. T. algeriensis represents a promising natural resource for the pharmaceutical industry mainly for antioxidant, anti-inflammatory, antimicrobial, and anticancer activities. Considering these findings, more research is needed to transmute the conventional uses of T. algeriensis into scientifically sound information. Moreover, extensive preclinical, clinical, toxicological, and pharmacokinetic trials on this species and its derivatives compounds are required to underpin the mechanisms of action and ensure its biosafety and efficiency. This comprehensive review provides a scientific basis for future investigations on the use of T. algeriensis and derived compounds in health maintenance and promotion and disease prevention.
1. Introduction
According to the World Health Organization (WHO), about 80% of the earth’s population relies on folk medicine. Most of ethnopharmacological practices involve the use of plant-based extracts and their bioactive constituents as natural healing remedies [1]. Plants have been used for therapeutic purposes worldwide for thousands of years and still provide the largest drugs to humankind. Therefore, scientists have dedicated a lot of effort to drug discovery willing to identify natural molecules/compounds from plants [2]. Until now, the international market of medicinal and aromatic plants has reached over 60 billion dollars per year, and it is still increasing gradually [3]. In addition, the pharmaceutical industry values medicinal plants for their bioactive constituents such as flavonoids, polyphenols, alkaloids, tannins, and glycosides, which are used as agents in drugs’ synthesis [4]. Nowadays, plant-derived molecules are continuously enriching our drug arsenal (e.g., galantamine, vinblastine, vincristine, and artemisinin) [5].
Thymus is a large plant genus comprising up to 400 species of aromatic and medicinal herbaceous, perennials, and shrubs. They are widely distributed in the Mediterranean region and Asia. Thymus species are used traditionally as herbal teas, culinary spices, and condiments [6]. Additionally, their essential oils are listed as one of the world’s top ten essential oils and known for their broad spectrum of biological activities including antioxidant, antibacterial, and age-delaying properties [7].
One of the most renowned North African Thymus species is T. algeriensis. The previous investigations carried out on this plant species have been mainly oriented towards its biological activities and clinical attributes. According to the Scopus database, more than 43.4% of the works relate to biological, pharmacological, biochemical, microbiological, and immunological aspects. The largest number of these studies was carried out by North African researchers and institutions, particularly Algerian, Moroccan, and Tunisian. In the Scopus database, the TITLE-ABS-KEY (thymus AND algeriensis) research resulted in 951 documents gathering articles (95.5%), reviews (2.2%), book chapters, and data papers (2.2%) with an increasing trend of document numbers over the years. For instance, it went from 4 to 15 documents per year between 2013 and 2021 indicating the importance that this species arouses among researchers. Therefore, T. algeriensis is a promising endemic resource for drug discovery and healthcare systems. Since this Thymus species has been the subject of a multitude of studies using both in vitro and in vivo approaches, and the data related to its phytochemistry and biological properties are distributed in several documents, we thought, here, to comprehensively summarize and review the phytochemical composition of T. algeriensis tissues with reference to the biological and pharmacological activities of its various extracts to have a holistic and synoptic view of its benefits and curative potentialities and track down research gaps and future prospects.
2. Botanical Description and Distribution
T. algeriensis is an endemic species of North Africa (Morocco, Algeria, Tunisia, and Libya) (Figure 1) [8]. The Thymus genus is represented by numerous aromatic plant species, including T. algeriensis which is a short lived, diploid () and gynodioecious shrub [9, 10] belonging to the Hyphodromi section and the Subbracteati subsection [11]. It grows wildly in diverse bioclimatic areas extending from the subhumid to the lower arid and on poor fertile calcareous soils [12]. In Morocco, it is found in the Middle, the High, and the Western Anti-Atlas, the Rif, and the Oriental (Figure 1).
T. algeriensis is a perennial plant with 4 to 7 mm long internodes emerging as a tuft from the short woody stump [13] (Figure 2(a)). The leaves are opposite with linear-lanceolate (6–12 mm) and have both green surfaces. The flowers are small (5 –7 mm) and have a white purplish or pinkish purple corolla color, with the upper lip cleft and the lower one divided into three lopes (Figure 2(b)). Flowering and fruiting time takes place from late April to June [14, 15].
(a)
(b)
Due to the anthropic pressures (overcollection, overgrazing, clearing, etc.), Thymus populations and cultivars from natural areas are severely affected and tend to occur in scattered metapopulations, often characterized by a low size [9]. Many factors influence the level of differentiation and the genetic drift of T. algeriensis populations, mainly habitat fragmentation, special isolation, ecological conditions, and gene flow limitation reducing their adaptation to ecological changes [16]. Several chemotypes were described in T. algeriensis according to their phytochemical composition (essential oils and main compounds) [15]. The genetic diversity among populations are also reported to be influenced by the level of site destruction, the number of initial founders in populations, and their dispersal and reproductive potentials [16].
3. Sources, Search Strategy, and Eligibility Criteria
The chemical composition and the biological and pharmacological activities of T. algeriensis were obtained using the electronic databases SciFinderⁿ®, Web of Science, Google Scholar, and Scopus. The search term used was Thymus algeriensis Boiss. & Reut. (1210 records). When crosslinked with specified terms, mainly “chemical compounds” (1010 records) and “activities” (1050 records), the number of documents decreased to 989. The cumulative results were then crosslinked with biological activities, in vitro, in vivo, or pharmacology which resulted in 155 records. In addition, titles and abstracts were screened and subjected to inclusion criteria that were as follows: phytochemical constituents of Thymus algeriensis extracts and their biological and pharmacological activities both in vitro and in vivo. Exclusion criteria were also considered and included other applications of the plant such as agriculture, non-English documents, duplicated papers, and the inability to locate full text. This selection resulted in relevant literature of 87 records that was used to retrieve data represented in this review. Other than the aforementioned records, many other references were sourced from citations of eligible studies.
4. Phytochemical Composition of T. algeriensis
The phytochemical analysis of different parts of T. algeriensis has shown the presence of diverse phytochemicals like polyphenols, flavonoids, terpenoids, sterols, and volatile compounds. This might be attributed to different factors such as geographic location, temperature, and harvesting time. Furthermore, phytochemical content is reported to vary with the extraction method employed and compounds identified in various parts of T. algeriensis. The major active constituents are flavonoids (Figure 3). The plant is also known by the volatile compounds characterizing the essential oil. Tables 1 and 2 demonstrate the reported compounds from T. algeriensis of different parts including aerial parts, leaves, flowers, and stem bark. The presence of various phytochemicals in T. algeriensis suggests its pluripharmacological properties, and a comprehensive assessment of the various activities of different phytochemicals is included in the sections below.
4.1. Aerial Parts
Most of the studies have been focused on the aerial parts of T. algeriensis. Different studies reported that the hydroalcoholic extracts were shown to contain kaempferol-O-glucuronide, apigenin-6,8-C-dihexoside and apigenin-7-O-glucuronide, and naringenin, identified from the Algerian plants [7, 17, 18], while apigenin was annotated in the polar extracts of the Tunisian plants [14, 19] (Figure 3). In the Tunisian plants, catechin, epicatechin, rutin, flavone, and (+)-catechin hydrate have been identified in the hydroalcoholic and methanolic extracts (Figure 3) [14, 18, 19].
Phenolic acids such as O-coumaric, p-coumaric, salvianolic, and t-ferulic were documented from the Algerian flora [7, 18], while gallic, rosmarinic, syringic, and vanillic acids were detected in the extracts from the Tunisian and Egyptian plants (Figure 4). In contrast, caffeic and ferulic acids characterized the methanolic extract of the plant harvested from Tunisia [20]. In the same line, 2,3-dimethoxybenzoic acid, 3-hydroxybenzoic acid, and 4-hydroxybenzoic acid were identified in the ethanol, hydroalcoholic, and butanolic extracts of the Algerian plants (Figure 5) [18].
4.2. Leaves
T. algeriensis has a high antioxidant activity mainly due to its high content in flavonoids (Figure 3). Most of the flavonoids present in the leaves are in the flavanol and glycoside forms. Kaempferol and rutin were detected in the polar leaves’ extract of the plants from Algeria and Tunisia. Kaempferol-O-hexoside and kaempferol-O-hexuronide have been identified in the methanolic extract of the leaves collected from Tunisia [21], while luteolin glucuronide was identified in the Algerian plants (Figure 3) [22]. From all phenolic acids (Figure 4), the leaves of T. algeriensis are rich in vanillic and rosmarinic acids, well-known phenolic acids with previously confirmed biological and pharmacological activities. These two compounds were characterized in the Algerian and Tunisian plants [7, 18–21, 23, 24]. The plant also contained phenolic acid derivatives such as rosmarinic acid glucoside, characterized in the methanolic extract of the Algerian plants [22]. Ursolic acid, a triterpenoid, was identified in the methanolic extract of the leaves. In addition, sterols such as β-sitosterol have been identified in the methanolic extract of the leaves. In addition, oleanolic acid, a triterpenoid, was identified in the leaves [25] (Figure 5).
4.3. Essential Oils of T. algeriensis
Essential oils are naturally defined as volatile secondary metabolites of plants and characterized by a strong aromatic nature and a complex chemical composition. The aroma of T. algeriensis is strong and contains large quantities of volatile compounds [26, 27]. More than 40 volatile compounds have been identified in T. algeriensis (Table 2 and Figure 6).
Several investigations reported the phytochemical composition of the essential oils of different specimen of T. algeriensis from different regions including Algeria, Libya, Tunisia, and Morocco (Table 2). The monoterpenoids and sesquiterpenoids dominated the oil. For instance, the main components of the essential oils of the aerial parts from the Tunisian flora were 1,8-cineole and 4-terpineol [9, 15], while borneol, camphene, camphor, and carvacrol dominated the Algerian and Moroccan plants. p-Cymene and thymol were detected in all flora. In the Tunisian flora, caryophyllene oxide and cis-sabinene hydrate were identified from different parts of the plant with different percentages [15, 28, 29]. Limited studies focused on the isolation and identification of bioactive compounds from the flowers of T. algeriensis. Like the leaves, the flowers were shown to contain 1,8-cineole identified in the Algerian plants. Elemol was identified from the stem bark from the Tunisian plants while acorenone and α-pinene were characterized in the stem bark of the Algerian plants [23, 30].
The variations in the phytochemical composition of the essential oils could be attributed to the harvesting time and stage and the drying methods as well as extraction methods. Some factors like environmental conditions, genetic variations, physiological condition of plants, regions, and evolution also influence the phytochemical variability of T. algeriensis [31]. Overall, T. algeriensis alongside other Lamiaceae species contains high contents of polyphenols, flavonoids, terpenoids, sterols, and volatile compounds.
5. Traditional Uses and Ethnomedicinal Properties
Thymus species have been used by the populations of the Mediterranean and Asian countries for thousands of years [32, 33]. Traditional health uses of Thymus species show high applicability as flu controller, anti-inflammatory, sedative, antirheumatic, analgesic, antiseptic, astringent, and diuretic agents. Generally, Thymus flowers and leaves are mainly used as herbal teas and aromatic and flavoring preparations to treat common cold, cough, sore throat, and indigestion symptoms [33]. Thymus leaves are also used as astringent, expectorant, antiseptic, antirheumatic, diuretic, analgesic, and cicatrizing agents [18]. T. algeriensis is also used either fresh or dried for its antispasmodic, antiabortive, and antifungal properties [34, 35]. In Morocco, its vernacular name is either Zîtra, Tazouknit, or Mantha [36]. Infusion and decoction of its stems and/or leaves are traditionally used to treat diabetes and digestive and respiratory infections [37, 38]. To sum up, T. algeriensis preparations are traditionally known for their multiple benefits and uses in gastronomy, digestive and cold problems, analgesia, microbial infections, and perfume preparations.
6. In Vitro Pharmacological Properties
Many studies have shown that T. algeriensis extracts have several in vitro biological properties mainly antioxidant, anti-inflammatory, antimicrobial, and anticancer activities.
6.1. In Vitro Antioxidant Activity
The oxidation process causes cellular damage by interacting with biological materials within the cell leading to several disorders and chronic diseases such as cardiovascular diseases and cancer. In addition, oxidation forms secondary reaction products in food and alters its nutritional quality and safety [4].
Antioxidant activity was defined as a delay or inhibition of the oxidation of cell molecules mainly proteins, lipids, DNA, and sugars by limiting the oxidative chain reactions. Due to their phytochemicals, plants are generally known as the best source of active antioxidants [39]. This is mainly attributed to their capacity to prevent the oxidation of a substrate by neutralizing reactive oxygen species (ROS) such as superoxide radical and hydroxyl radical. Common mechanisms by which plant-based extracts/molecules block ROS formation include mainly free radical scavenging (e.g., lipoxygenase inhibition, transition-metal-chelating activity, and singlet-oxygen-quenching capacity) and lipid peroxidation inhibition [40, 41]. Hence, there is a growing interest in the antioxidant activities of plant-based compounds and their role in promoting health and preventing diseases.
The in vitro antioxidant activities of T. algeriensis extracts have been extensively explored (Table 3). They have been determined by various methods mainly 2,2-diphenyl-1-picrylhydrazil (DPPH) radical scavenging, ferric-reducing antioxidant power (FRAP), β–carotene bleaching, oxygen radical absorbance capacity (ORAC), thiobarbituric acid reactive substances (TBARS), reducing power (RP), phosphomolybdenum, lipid peroxidation inhibition, total antioxidant capacity (TAC), hydroxyl radical scavenging (HRS), metal ion chelation, and superoxide anion scavenging assays (Table 3).
Many studies have been carried out on the T. algeriensis aerial parts using mainly EO. However, other extracts such as methanolic, aqueous, and ethanolic were also tested (Table 3). Recently, Ouakouak et al. [42] showed that the EO from the leaves of Algerian T. algeriensis are endowed with a moderate antioxidant activity using DPPH (), ABTS (), and TAC () assays. This activity was corroborated in other studies that showed that the EO extracted from the aerial parts of T. algeriensis grown in Libya possessed a strong antioxidant activity () better than thymol [43] and inhibited the deoxyribose degradation better than mannitol [35]. The only study on the antioxidant potential of EO from T. algeriensis stem bark was conducted in Algeria and showed that the plant exhibited moderate DPPH scavenging activity () [30]. Furthermore, Mokhtari et al. [44] tested the antioxidant activities of other different extracts and showed that, using the DPPH assay, chloroform, petroleum ether, and n–BuOH extracts demonstrated IC50 values of , , and , respectively. Noteworthy, the antioxidant potential of all these extracts was dependent on the method used. For instance, using the CUPRAC assay, the chloroform extract was the most effective while the n–BuOH extract was the most active using the FTC assay. In another work, the aqueous extract of leaves of Algerian T. algeriensis was active only using ABTS () while the ethanolic extract was active using both the DPPH and ABTS assays with IC50 (μg/mL) of and , respectively [45]. In contrast, Ziani et al. showed that both aqueous and hydroethanolic extracts exhibit antioxidant activities using DPPH, RP, β–carotene bleaching, and TBARS assays, with a high efficiency of the hydroethanolic fraction except when using β–carotene bleaching in which the aqueous extract demonstrated the highest activity [7]. It was also shown that the ethyl acetate extract is also endowed with an antioxidant capacity () higher than the n–BuOH extract () and ascorbic acid [46]. Interestingly, Rezq et al. [47] demonstrated that methanolic extracts of T. algeriensis leaves induced significant increase in the nuclear levels of Nrf–2 (nuclear factor erythroid 2) transcription factor (Table 3). The Nrf–2 upregulates the antioxidant response element-mediated expression of antioxidant enzymes and cytoprotective proteins and protects against oxidative pulmonary injury, abnormal inflammatory and immune responses, and apoptosis [48]. The various antioxidant activities of T. algeriensis extracts were largely studied and correlated with their phytochemical composition. In this regard, it has been shown that the Algerian T. algeriensis EO with poor content of thymol and carvacrol exhibited significantly lower antioxidant effect [35]. Similarly, Guesmi et al. [20] concluded that the higher the polarity of the Tunisian T. algeriensis extract, the stronger is the antioxidant effect. In addition, the antioxidant effect was chemotype dependent [20]. This was also reported by Jaouadi et al. [21] who showed that T. algeriensis plants growing in upper arid climatic zones of Tunisia were endowed with the best reducing, antiradical, and β–carotene bleaching inhibition potential. Likewise, the antioxidant effect of EO from leaves was reported to vary according to localities [28]. Furthermore, the antioxidant capacity was also reported to be influenced by the development stage of the plant. For instance, the aqueous extract extracted during the flowering stages of Tunisian T. algeriensis had the highest scavenging potential, while the extracts of both vegetative and flowering periods were able to reduce Fe3+ to Fe2+ [14].
Several studies have compared Thymus species to other plants in terms of their antioxidant activities. For instance, Ahmed et al. observed that EO of T. algeriensis from Tunisia presented a moderate DPPH scavenging effects compared to EO from other plants such as Eucalyptus globulus, Pinus halepensis, Pituranthos tortuosus, Rosmarinus officinalis, and Tetraclinis articulata [49]. Similarly, the EO of Moroccan T. algeriensis had a lower antioxidant activity comparatively to other species mainly T. capitatus, T. ciliatus, and T. bleicherianus [50]. However, because of the bioassay used, it is often biased and difficult to compare the antioxidant activities reported by many different studies [51].
The antioxidant effect of T. algeriensis extracts and EO is attributed to their free radical scavenging and metal chelating properties having a chemoprotective activity against oxidative stress [35]. This should be due to the richness of T. algeriensis in phytochemical active compounds dominated by certain phenolic constituents as well as the synergetic effect between its compounds even at small concentrations such as δ-cadinene and germacrene D [52]. Next to T. algeriensis, other Thymus species such asT. vulgaris, T. serpyllum, T. bleicherianus, and T. hyemalis have been widely explored for their antioxidant potential [50, 53–56]. As a result, the antioxidant potential has diverse downstream biological effects, including anti-inflammatory, anticarcinogenic, and antiatherosclerosis activities [57]. Same as other Lamiaceae species as well as most medicinal plants, T. algeriensis can serve as a good source for antioxidant compounds/molecules with potential applications in pharmaceutical, nutraceutical, and cosmeceutical industries.
6.2. Anti-Inflammatory Activity
Inflammation is a healing process induced by inflammation-inducing factors such as pathogens, toxic substances, and irradiation. These stimuli trigger the immune system and induce inflammatory responses in the host’s organs which may lead to cell damage and/or diseases [58]. Many diseases are associated with inflammatory processes including type 2 diabetes, asthma, rheumatoid arthritis, neurodegenerative diseases, chronic inflammatory bowel diseases, and cancer [59]. Prominent anti-inflammatory molecules have been isolated from plants and clinically tested in humans. They mainly include curcumin, resveratrol, colchicine, quercetin, capsaicin, and epigallocatechin-3-gallate [5]. The in vitro anti-inflammatory effect of T. algeriensis has been reported by many studies. It was evaluated using egg albumin denaturation, cyclooxygenase (COX–1 and COX–2), and lipoxygenase inhibition assays (Table 4).
Recently, Mokhtari et al. [44] showed that petroleum ether, chloroform, and n–BuOH extracts from the aerial parts of Algerian T. algeriensis inhibited egg albumin denaturation in a concentration-dependent effect. The highest inhibitory effect was observed using the chloroform extract with 45.27% inhibition, followed by petroleum ether (30.26%) and then n–BuOH (26.03%) extracts [44]. This activity could be useful in inhibiting protein denaturation during inflammatory conditions characterized by the production of autoantigens during disorder conditions such as rheumatic arthritis, cancer, and diabetes [60]. As previously suggested, the in vitro antidenaturation of protein activity of these extracts could be due to the interactions of the phytocompounds and/or molecules such as polyphenols, phenylpropanoids, and the alpha-lipoic acid with the aliphatic regions surrounding the lysine residue of the albumin protein [61].
Additionally, it was revealed that the methanolic extract of T. algeriensis leaves showed higher selectivity against cyclooxygenase (COX–2) () than both diclofenac and indomethacin used as standard references. Moreover, T. algeriensis extract induced similar potency like zileuton and diclofenac to inhibit 5–lipoxygenase (5–LOX) in vitro () [22]. Similarly, the EO of the aerial parts from T. algeriensis grown in Algeria were the most potent in inhibiting the lipoxygenase comparatively to those extracted from Mentha spicata and Ocimum basilicum [62]. This could be associated with the volatile compounds in EO such as 1,8-cineole, borneol, and camphor which were reported to be determinant for the anti-inflammatory activity [63]. For instance, carnosol and rosemary oil inhibited the adhesion of TNF-α-induced monocytes to endothelial cells during inflammation and suppressed the expression of intercellular adhesion molecule at the transcriptional scale [64]. Using molecular docking of some T. algeriensis compounds characterized in the methanolic extract of leaves, Ref. [22] demonstrated that salvianolic acid A and apigenin 6,8-di-C-hexosides interact with amino acid residues of COX–1 and 5–LOX, while rosmarinic acid glucoside interacts with amino acid residues of COX–2. In addition, salvianolic acid A and quercetin pentoside target the amino acid residues of the 5–LOX activating protein (FLAP). This in silico study showed that identified compounds especially rosmarinic acid glucoside, salvianolic acid A, quercetin pentoside, and apigenin 6,8-di-C-hexosides could be developed as safer anti-inflammatory agents. Noteworthy, the anti-inflammatory potential of T. algeriensis species growing in other North African countries, namely, Morocco, Tunisia, and Libya, has not been evaluated yet (Table 4). In conclusion, T. algeriensis is endowed with an exceptional anti-inflammatory power thanks to its compounds and active molecules that target key stages and mediators of the inflammatory process of different pathologies.
6.3. Antimicrobial Activities
The overuse of antibiotics has become one of the greatest challenges in human health. In addition, the rapid spread of antibiotic resistant pathogens is alarming. Therefore, research and development of a new generation of antimicrobials has become imperative. Antimicrobial agents are a group of materials that selectively destroy pathogens by interfering with their growth or survival. With the emergence of resistance phenomenon to current antibiotics, new alternative compounds such as plant-derived compounds are being explored. Due to the advantages of their inherent biochemical and biophysical properties including biocompatibility, biodegradability, and low cytotoxicity, plant biomolecules have huge potential for the antimicrobial application and have been extensively studied in recent years.
Several methods are used to evaluate the antimicrobial activities of plant extracts mainly agar disk diffusion, agar well diffusion, and macrodilution or microdilution methods [65]. These methods determine the inhibition zone diameter and the minimal inhibitory (MIC) and bactericidal concentrations (MBC). The antimicrobial activities of the T. algeriensis extracts and EO against a broad set of pathogenic bacteria, fungi, and yeasts were exhaustively studied (Tables 5 and 6).
According to literature, EO were the most explored for the antimicrobial potential. For instance, Ait-Ouazzou et al. tested the effect of the EO from the aerial parts of T. algeriensis growing in Morocco against seven bacteria and one fungus and showed their very high effectiveness () towards S. aureus, E. faecalis, L. monocytogenes, and C. glabrata and a moderate activity () against P. aeruginosa, S. enteritidis, and E. coli O157:H7 [66]. These bacteria especially P. aeruginosa were sometimes shown to exhibit a remarkable resistance against OE of T. algeriensis from other countries such as Tunisia and Algeria [20, 28, 67]. Interestingly, Hazzit et al. [35] noticed that Helicobacter pylori strains J99 and 26695 were more sensitive () to EO from Algerian T. algeriensis aerial parts than all the other five tested strains () including Salmonella sp. and S. aureus CFSA2. Similarly, it was found that Bacillus cereus (ATCC10876), Micrococcus luteus (NRLL B-4375), and Proteus mirabilis (ATCC35659) were inhibited at 2.34, 7.03, and 4.68 mg/mL, respectively, of the EO extracted from the aerial parts of T. algeriensis growing in the north of Tunisia [68]. In addition, Zaïri et al. revealed that the best antibacterial activity was obtained using the EO from T. algeriensis leaves () which also inhibited the fungus Aspergillus flavus by up to 42.86% [69]. Moreover, Rezzoug et al. [23] and Guesmi et al. [14] observed that the EO from the aerial parts of Algerian and Tunisian T. algeriensis demonstrated better antimicrobial activity that the ethanolic and aqueous extracts, respectively. For instance, S. epidermidis ATCC12228 and S. aureus ATCC25923 were inhibited at 128 μg/mL using the methanolic extract and at only 32 μg/mL using EO. More importantly, the antimicrobial efficiency of T. algeriensis EO from Libya was shown to be stronger than reference antibiotics such as ampicillin and streptomycin [6]. Additionally, Messaoudi et al. reported that the aerial part’s ethanolic extract of T. algeriensis collected in the Algerian southwest has no effect on the growth of E. cloacae ATCC 49452 while it inhibited widely the growth of both E. faecalis ATCC 49452 and S. aureus ATCC 25923 [70]. However, aqueous extracts from leaves and aerial parts of Algerian T. algeriensis showed no inhibitory effect (no halo zone) against five human pathogens. Using methanol as the solvent, the leaves of T. algeriensis from the upper arid area of Tunisia showed a bacteriostatic effect against seven human pathogenic bacterial strains () while the bactericidal activity was restricted to E. coli [71, 72].
Extract characterization revealed that samples containing carvacrol elicited a remarkable antibacterial activity [21]. Elsewhere, Nikolić et al. found a positive correlation between the antimicrobial activity of selected EO of T. serpyllum, T. algeriensis, and T. vulgaris and their chemical composition, which indicated that the activity may be ascribed to the presence of thymol because it occurs in high proportions [73].
The synergistic combination effect of T. algeriensis EO from Morocco and heat or pulsed electric fields against E. coli and L. monocytogenes was shown to inactivate the bacterial growth by of 5 log10 cycles [66]. Moreover, T. algeriensis EO were capable of inactivating 4–5 log10 cycles of the initial cell populations (E. coli O157:H7 VTEC (phage type 34) and L. monocytogenes (EGD-e)) in combination with high hydrostatic pressure [74]. Comparatively to other plants such as E. globulus and R. officinalis, EO from T. algeriensis were the most potent in inhibiting seven human pathogenic bacteria [75].
Plant pathogens such as Phytophthora megakarya (black pod of cocoa) were dose-dependently inhibited using both aqueous and ethanolic extracts of T. algeriensis leaves bought at a local market in Cameroon and completely inhibited at 60 and 125 mg/mL, respectively [76]. Similarly, Botrytis cinerea was inhibited by the EO from wild Tunisian T. algeriensis at >300 μL/L [77]. Another investigation showed that 0.5% EO of T. algeriensis collected in northern Algeria decreased the rate of infestation of Varroa destructor, causing a mortality rate of 32.6% [78] (Table 6). Taken together, the EO of T. algeriensis are more potent than the different other extracts against human and plant pathogenic strains as well as other microbial species. Antibacterial activity of other Moroccan Thymus species was also reported in the literature such as T. bleicherianus [79], T. serpyllum [56], T. munbyanus [80], T. hyemalis, T. vulgaris [54], and T. zygis [81].
The mechanisms by which plant extracts trigger microbe survival and growth involve the integrity and the permeability of the membranes as well as the efflux pump systems of resistant Gram-negative bacteria such as E. coli, E. aerogenes, K. pneumoniae, P. aeruginosa, and S. enterica Typhimurium [82–84]. These effects are often explained by the hydrophobic nature of plant extracts allowing them to accumulate in bacterial cell membranes and disrupt their structure. In addition, the ability of some compounds to chelate transition metals such as copper and iron reduces their bioavailability to microorganisms [85]. Some researchers have observed that the antibacterial activity is correlated with the high contents of monoterpenes, phenols, aldehydes, and ketones in plant extracts that affect the cell membrane of microorganisms [86]. Others have been reported to induce a deficiency in microbial enzyme systems due to leakage of intracellular constituents leading to apoptosis [87]. Plant extracts can also trigger the phospholipids of the bacterial cell membrane, coagulate the cytoplasm, and attack lipids and proteins [88]. For instance, a study reported that Salix tetrasperma stem bark extract impaired P. aeruginosa virulence by inhibiting swimming and swarming mobilities as well as by reducing its hemolytic and proteolytic activities [89]. The antibacterial activities of plant extracts have been associated with several bioactive compounds such as polyphenols and molecules, namely, furocoumarins and furanocoumarins [85]. In general, the nature of a phytochemical may influence its mechanism of action and therefore its antibacterial activity. Finally, most antimicrobial activities of T. algeriensis extracts have been tested in vitro. However, it is interesting to evaluate their potential in vivo using animal models.
The antimicrobial effects of T. algeriensis extracts against human pathogenic bacteria and fungi make it an important plant to be explored using fractionation and testing procedures in order to isolate potentially active molecules that can be tested in vivo and through clinical trials.
6.4. Anticancer and Cytotoxic Activity
Nowadays, cancer is the most lethal disease [90]. Although several chemotherapeutic molecules are being used to manage cancer, development of drug resistance, problems of selective toxicity, and severe side effects are still challenging. Therefore, discovering new anticancer drugs is becoming more and more imperative [91]. In this regard, plant-derived compounds are a promising source of chemotherapeutic drugs. In the market today, many of the currently used chemotherapeutic medicines such as taxanes (docetaxel and paclitaxel), vinca alkaloids (vincristine, vinblastine, and semisynthetic drugs vindesine and vinorelbine), camptothecin derivatives (camptothecin and irinotecan), and epipodophyllotoxins (teniposide and etoposide) were initially isolated from medicinal plants [92].
The cytotoxic activity of T. algeriensis against cancer cells has been extensively studied using the methyl tetrazolium test (MTT), sulforhodamine B assay, and histopathological analysis (Table 4). Cytotoxicity was mostly assessed using EO. According to Ouakouak et al. [42], leaves’ EO from the Algerian T. algeriensis showed high toxicity against the HCT116 tumor cell line () while limited effect was seen towards the HePG2 hepatocellular carcinoma cell line (). Similarly, it was reported that the EO of T. algeriensis leaves and flowers exhibited cytotoxic activities at concentrations higher than 400 μg/mL with and , respectively [69]. Toxicities against other tumor cells such as NCI-H460 (non-small cell lung cancer), MCF-7 (breast), HCT-15 (colon), and AGS (gastric) were also reported using Libya’s T. algeriensis EO with CC50 ranging from 62.12 to 64.79 μg/mL. However, no toxicity was noticed on the porcine liver primary cell lines at [73]. Noteworthy, Jaafari et al. observed that the EO from the chemotype-rich Moroccan T. algeriensis which is carvacrol were more toxic than the other chemotypes’ EO against the p815 mastocytoma tumor model cell line (up to 100% lysis) and had an important proliferative effect on the peripheral blood mononuclear cells (PBMC) [93]. Besides EO, other T. algeriensis extracts were also shown to be potent against cancer cell lines. For instance, the leaf methanolic extract of Algerian T. algeriensis was slightly toxic on A431 and SVT2 cancer cells at 100 μg/mL. However, biocompatibility on both the immortalized tested cell lines HaCaT and BALB/c-3T3 was observed [47]. The anticancer potential of Tunisian T. algeriensis extracts of leaves and flowers on the HCT116 cell line revealed that both the aqueous and methanolic extracts are toxic with CC50 (μg/mL) of and , respectively. However, using T. capitatus and R. officinalis extracts, the aqueous extracts showed very low toxicities compared with the methanolic extracts [19].
The mechanisms underpinning the cytotoxic activity against cancer cells were linked to the nature of secondary metabolites in the plant extracts. For instance, berberine alkaloids are known to activate the apoptosis-inducing enzymes (caspase 3/9) of the human leukemia HL-60 cells by downregulating telomerase activity and nucleophosmin/B23 [94]. Additionally, some plant molecules such as curcumin have the potential to downregulate miR-21 expression in MCF-7 cells through the upregulation of the PTEN/Akt signaling pathway [95]. The anticancer potential of phytochemicals and their regulatory aspects were exhaustively described by Khan et al. [94]. In 2021, a study showed that even at low concentrations, T. algeriensis (from Tunisia) oily fraction inhibited HCT116 cell growth. Cleavage of poly(ADP-ribose) polymerase (PARP) and activation of the initiator and effector caspases show that T. algeriensis causes apoptotic cell death (caspases 3, 8, and 9). It also increased the expression of death receptors (DRs) while decreasing the expression of TNF-α-related apoptosis-inducing ligand (TRAIL) decoy receptors, according to the findings (DcRs). This study also showed that T. algeriensis increased MAPK pathway signaling molecules (p38 kinase, ERK, and JNK), downregulated c-FLIP, and overexpressed SP1 and CHOP. The in vivo model of cancer showed that intragastric injection of T. algeriensis extract (12.5 and 50 mg/mL) inhibited colorectal carcinogenesis in an animal model by preventing multiple phases in the carcinoma [96]. To sum up, the anticancer potential of T. algeriensis extracts is mainly attributed to their capacity to enhance the apoptosis-stimulating enzymes and receptors in tumor cells, to downregulate the oncogenic miRNA and genes, and to inhibit the antiapoptotic proteins. The anticancer and cytotoxic activity of T. algeriensis extracts is evident as has been widely proven. It turns out that this plant hides an enormous potential in the induction of apoptosis of cancer cells; that said, it will be extremely interesting to go further through in vivo and clinical experiments and to identify, like the compounds mentioned above, potential anticancer molecules with promising medical applications.
6.5. Miscellaneous Activities
T. algeriensis appears to be endowed with other biological activities such as antihemolytic, antiacetylcholinesterase, antilipase, insecticidal, leishmanicidal, and angiotensin converting enzyme (ACE) inhibitory activities (Table 4). Using erythrocyte osmotic fragility test, petroleum ether, chloroform, and n–BuOH extracts of the Algerian T. algeriensis (aerial part) induced resistance of erythrocytes to hemolysis. The highest antihemolytic effect was noted using the n–BuOH extract (), followed by the chloroform extract (). The petroleum ether extract displayed the lowest activity with 19.51% inhibition at 800 μg/mL [44]. The antihemolytic effect of T. algeriensis extracts could be linked to their chemical composition, particularly flavonoids and polyphenols. In fact, previous research has shown that the presence of polyphenols and flavonoids in the crude extracts enhanced the stability of the erythrocyte membrane and inhibited hemolysis [97]. However, the exact mechanism of action of the membrane stabilization by the extract is yet to be elucidated [98]. Studies have also related the antihemolytic effect to the occurrence of molecules having antioxidant activities due to their ability to scavenge free radicals [99].
Previous studies showed that the methanolic extract from the T. algeriensis leaves (Tunisia) showed various abilities to inhibit acetylcholinesterase (AChE) with significant variations among T. algeriensis populations. It was observed that populations growing in the arid zone were the most potent () [21]. The modulation of the activity of AChE has been linked to the phenolic compounds [100, 101]. These latter effects have been reported to protect against neurodegenerative diseases [102]. For instance, rosmarinic acid was reported as an Aβ aggregation inhibitor [103, 104]. Besides, Benguechoua et al. showed, for the first time, that an acetone/water extraction mixture with ethyl acetate and butanol fractions from Algerian T. algeriensis inhibited Candida rugosa lipase [105].
Leaves’ EO from Tunisian T. algeriensis used as a fumigant exhibited insecticidal activity against Spodoptera littoralis Boisd. with LC50 ranging from 41.75 to 131 μL/L air [28]. The insecticidal effect of T. algeriensis EO is likely due to their major constituents such as α-pinene, 1,8-cineole, caryophyllene oxide, camphor, linalool, camphene, and p-eugenol, which were found to be toxic in many previous studies [106, 107]. This level of larvae mortality is higher than those observed using other medicinal plants such as Thymus mastichina, Origanum majorana, and Salvia sclarea [108]. According to Al-Nagar et al., terpene compounds constitute a major group of insect feeding deterrents and display a very strong inhibitory activity against larvae of S. littoralis [109]. Tunisian OE from the T. algeriensis aerial parts were also endowed with leishmanicidal activity against two Leishmania species: L. infantum and L. major with IC50 of 0.25 and 0.43 μg/mL, respectively [49]. Some investigations indicated that the plant extracts including EO can target the metabolic pathways involved in Leishmania species survival such as MAP kinases, glyoxalase, metacaspases, and sterol pathways [110, 111]. These plant-based compounds include flavones, azoles, and staurosporine [111, 112]. Similarly, Melo et al. demonstrated that oleanolic acid, which possessed a good activity against L. amazonensis, L. braziliensis, and L. infantum (IC50<70 μM in promastigotes and amastigotes), did not provoke FN-γ–assisted macrophage activation [113]. These findings indicated that this type of triterpene most likely acts on targets located in Leishmania parasites rather than in the host’s macrophages.
Finally, Zouari et al. showed that T. algeriensis EO could be used as angiotensin I-converting enzyme (ACE) inhibitor for hypertension treatment () [29]. To elucidate the mechanisms underlying ACE inhibition, it was suggested that the flavonoids such as vitexin and isovitexin isolated from various plants that inhibited ACE activity at 0.33 mg/mL by 20% and 45%, respectively, act by competing with the substrate for the ACE active site [114]. In addition, flavonoids are reported to form chelate complexes with the zinc atom within the active site of zinc-dependent metallopeptidases to which ACE belongs [115, 116].
It can be concluded that T. algeriensis extracts act as an antihemolytic by protecting the integrity of the erythrocyte’s membrane, as antiacetylcholinesterase agents by inhibiting the Aβ aggregation, as an insecticidal and leishmanicidal by targeting the metabolic pathways for insects’ survival, and as angiotensin converting enzyme (ACE) inhibitors by competing with the ACE active site. In the light of these various potentialities, some of which are being used traditionally, it would be original to investigate T. algeriensis extracts more deeply for these activities to spotlight the underlying mechanisms both in vitro and in vivo.
7. In Vivo Pharmacological Activities
In the biological systems, there is a growing need for in vivo experiments as most studies focus on in vitro and cell culture-based assays. However, these studies require verification using animal experiments to avoid misleading data and toxicity issues [117]. Indeed, the in vivo studies carried out on T. algeriensis extracts were comparatively limited and were only performed on species growing in Algeria and Tunisia using aerial part and leaf hydromethanolic extracts and EO. Reported studies are summarized in Table 7.
7.1. Safety Studies
As the interactions of plant constituents with biological components such as tissues, cells, proteins, and DNA could disrupt host’s immune response and metabolism, assessing their in vivo behavior would provide a basis for beneficial as well as toxic responses and, more importantly, could lead to predictive models to assess their pharmacokinetics and intended effects [117].
Here, the in vivo toxicological evaluation of T. algeriensis extracts was investigated using animal models mainly Albino male mice and Sprague Dawley rats. Toxicity parameters were mostly related to hepatic and renal functions, mortality, body and organ weights, and histopathological evaluation of tissues (Table 7). Righi et al. showed that the hydromethanolic extract of T. algeriensis aerial parts (Algeria) administrated orally to Albino male mice at 200–800 mg/kg did not influence the hepatic (AST and ALT) and renal functional markers (urea and creatine) [24]. Similarly, Guesmi et al. observed no sign of toxicity or mortality after 15 days of oral administration of EO from Tunisian T. algeriensis at 180 mg/kg per day dissolved in normal saline to Sprague Dawley rats with no pathological change and normal histoarchitecture [118]. In another study, they showed that T. algeriensis EO exhibited no signs of acute toxicity with the absence of abnormal organic damage to the rats’ organs and prevented H2O2-induced liver, kidney, and organ weight loss [20].
The leaf aqueous extract of Algerian T. algeriensis administered by oral route to Albino Wistar male and female mice at the doses of 2000 mg/kg and 5000 mg/kg induced no significant differences in body weight, no mortality, and/or no signs of toxicity. In addition, treated mice showed no change in the biochemical parameters and no histological abnormalities in their organs compared to control animals [119].
The major challenge encountering plant-derived agents is the prediction of their trajectory and location after in vivo administration. Hence, the relationship between the properties of a molecule and its distribution and behavior with the biological system should be studied as it is developed by pharmaceutical approaches for drugs. This is known as structure–function/toxicity relationship models [120]. Such data could be investigated before evaluating the systematic effect [117].
According to these studies, it seems that T. algeriensis is far from being toxic at least in tested models and at administered doses. However, more in-depth studies using pharmacokinetics and downstream targets of hepatic metabolites resulting from its compounds will have to be explored to ensure its biosafety for humans.
7.2. Antioxidant Activity
As mentioned above, T. algeriensis extracts are reported to be endowed with a high antioxidant activity in vitro. This was corroborated using in vivo experiments especially by Guesmi et al. [20, 118, 121, 122]. Various methods were used to investigate the antioxidant effect in vivo, mainly the assessment of the antioxidant machinery (enzymatic and nonenzymatic antioxidants), lipid peroxidation, and ferric reducing ability of plasma (Table 7). For instance, in 2016, these researchers demonstrated that Tunisian EO administrated orally to Sprague Dawley rats (150 mg/kg per day) subjected to low- and high-dose H2O2 treatment recovered the catalase (CAT), superoxide dismutase (SOD), and glutathione peroxidase (GPx) activities and glutathione (GSH) levels in liver and renal tissues of treated animals. In another study, they showed that T. algeriensis EO administered to adult male Wistar rats preserved the sperms’ viability, improved the morphological abnormalities, and prevented induced DNA fragmentation in their testis. An increase in total protein in the H2O2-treated group was also noticed [121]. These researchers’ investigations corroborated the same finding [20, 118]. Moreover, it was reported that the aerial part hydromethanolic extract of Algerian T. algeriensis increased the plasma antioxidant levels by 3-folds following treatment of Albino male mice with the dose of 800 mg/kg. At the same concentration, the iron reducing ability (908 μM FeSO4eq/mL) was twofolds higher than that of the untreated group (405 μM FeSO4/mL). In addition, a significant improvement in CAT activity and GSH levels was seen in mice treated with 400 and 800 mg/kg compared to the nontreated group accompanied with a significant decrease in the MDA levels in the plasma of treated groups by 200 and 400 mg/kg [24]. To sum up, T. algeriensis extracts displayed substantial antioxidant activities in several experimental models (in vitro: DPPH, FRAP, β–carotene bleaching, ORAC, TBARS, RP, phosphomolybdenum, lipid peroxidation inhibition, TAC, HRS, metal ions chelation, and superoxide anion scavenging assays, and in vivo: rats and mice) via diverse molecular targets/mechanisms by increasing the activities of antioxidant enzyme activities such as SOD, GST, and CAT and nonenzymatic antioxidants like GSH, reducing the MDA levels, and preventing the DNA damage. All these protective effects of T. algeriensis extracts against oxidative stress sustain the in vitro findings and highlight its benefits in the prevention and protection against oxidative stress and its radicals in related disorders and diseases.
7.3. Anti-Inflammatory Activity
The in vivo effectiveness of T. algeriensis as a source of anti-inflammatory agents has been investigated by Sobeh et al. using the leaf extracts of Algerian species [22] (Table 7). The study showed that the oral administration of the 80% MeOH extract to male rats injected with carrageenan induced a significant mild reduction in edema thickness using the doses of 200 and 400 mg/kg compared to the control rats. In addition, the increased vascular permeability induced by acetic acid injection in mice was reduced by 63 and 58% when animals were pretreated with T. algeriensis extracts at 400 or 600 mg/kg, respectively. These effects on the first phase of inflammation were attributed to the capacity of T. algeriensis extracts to inhibit the vasodilatation and the release of inflammatory mediators. In conclusion, T. algeriensis extracts exhibited prominent anti-inflammatory activities both in vitro and in vivo by inhibiting the inflammatory mediators such as COX, LOX, PGE2, LT, IL-1β, TNF-α, and IL-6 and by increasing the levels of Nrf–2 which controls the intracellular redox environment and redox homeostasis genes. The in vivo effects corroborate the anti-inflammatory activities reported in vitro and open promising prospects in using T. algeriensis-based compounds and derivatives in treating inflammatory related disorders in humans.
7.4. Leukocyte Recruitment
The effect of T. algeriensis leaf extracts on carrageenan-induced leukocyte migration into the peritoneal cavity in mice was also evaluated by Sobeh et al. [22] (Table 7). Following intraperitoneal injection of carrageenan, mice pretreated with T. algeriensis extract doses (200, 400, and 600 mg/kg) showed a dose-dependent decrease in the number of total leukocytes with up to 62%. This effect was more important that those observed using diclofenac and dexamethasone, which induced 39 and 30% decrease in leucocyte number, respectively. The observed reduction in leucocyte number has been explained by the suppression of inflammatory mediators such as prostaglandins and proinflammatory cytokines. It was also shown that the inhibition of leukocyte migration could be the result of the suppression of adhesion molecule and/or inhibition of the chemotactic of substances’ expression [123]. This investigation confirms the in vitro anti-inflammatory activity of T. algeriensis extracts and highlights its positive effect on leukocyte migration and inflammation mediation during the immune response process.
7.5. Antinociceptive Activity
The central and peripheral analgesic activity of T. algeriensis leaf extract was studied in mice using the hot plate test and acetic acid-induced abdominal writhing assay, respectively [22] (Table 7). The study showed that oral pretreatment of mice with T. algeriensis extracts at 200 and 400 mg/kg prior to acetic acid injection induced a significant peripheral analgesic activity with a dose-dependent reduction in induced writhes. The effect observed by the dose 400 mg/kg (94% reduction) was higher than that achieved by dexamethasone (2 mg/kg) or diclofenac (20 mg/kg). Moreover, central antinociceptive activity was also shown using the extract’s doses represented by a longer response latency. In addition, the extract at 400 mg/kg showed similar activity to the reference narcotic analgesic (nalbuphine). It was presumed that the leaf hydroalcoholic extract of T. algeriensis could act on the opioid system and/or other central nervous system (CNS) receptors involved in the antinociception. To sum up, T. algeriensis extracts exerted an antinociceptive effect in vivo by targeting the opioid system receptors (i.e., mu, delta, and kappa) and CNS nociceptors. This prominent analgesic effect of T. algeriensis extracts arouses a lot of interest in the search for metabolites endowed with antinociceptive activity which will certainly lead to good research and application perspectives of plant-based painkillers from T. algeriensis.
7.6. Gastroprotective and Antiulcerogenic Activity
The effect of T. algeriensis EO was investigated on Tunisian species using induced gastric lesions in adult male and female Wistar rats by 0.3 M HCl/60% ethanol [20] (Table 7). The results showed that oral treatment of rats by the EO (54–180 mg/kg, p.o.) induced a dose-dependent decrease in gastric lesions and reduced the ulcer index and percentage of inhibition mainly at doses of 180 mg/kg for male rats (88%) and between 117 and 180 mg/kg for female rats (96.25 and 98.85%). In addition, the gastric acidity of female rats pretreated with EO was significantly reduced comparatively to the ulcer control rats. The mucosal damage induced by HCl/ethanol in the control group was alleviated using the T. algeriensis EO. In fact, mucus production increased significantly in both male and female rats treated with 180 mg/kg. As compared to the acidified group in which visible change in the gross appearance of the gastric mucosa was seen, stomachs of male and female rats treated with the dose of 180 mg/kg showed a normal appearance. The authors suggested that the antiulceration effect of EO from T. algeriensis is attributed to their antioxidant effect and inhibition of neutrophil infiltration into ulcerated tissues. Nevertheless, the molecules and their structural aspects determining the observed effect are not yet established. The gastroprotective and antiulcerogenic activities of T. algeriensis EO seen in animal models explain its uses in traditional medicine for this purpose and open a great potential of using this plant’s volatile constituents in the treatment of peptic ulcers and gastric troubles.
7.7. Neuroprotective Activity
The effects of T. algeriensis extracts on chronic neuropathic pain and the underpinning mechanisms were investigated by Rezq et al. [47] (Table 7). Using the chronic constriction injury (CCI) model in male Wistar rats, 80% MeOH leaf extracts of Algerian T. algeriensis (200 mg/kg and 400 mg/kg) prominently attenuated hyperalgesia and allodynia at day 14 of surgery. Noteworthy, the extracts showed a higher effect than the standard drug (pregabalin) towards both stimuli. In addition, they restored the normal appearance of the nerve fascicle which regained its regular form. Finally, the degeneration of most neurons caused by CCI was restored by the T. algeriensis extract at 400 mg/kg with few pyknotic neurons and low and deeply stained nuclei. The extracts also mitigated the increase of caspase 3 and synaptophysin. The underlying mechanisms of the observed neuroprotective activity were (i) inhibition of NOX–1, iNOS; (ii) increase of catalase activity; and (iii) inhibition of inflammatory mediators such as TNF–α, NF–κB, COX–2, and PGE2. These effects suggest the substantial potential of T. algeriensis extracts in improving painful peripheral neuropathy. Furthermore, another earlier study on hydrophobic fractions of T. algeriensis (HFTS) growing in Tunisia showed that HFTS mitigates neuroinflammation via AChE inhibition and attenuates H2O2-induced brain toxicity [118]. In conclusion, T. algeriensis attenuated neuropathic pains and degeneration of neurons via antioxidant and anti-inflammatory properties. This opens new avenues for research of molecules exhibiting neuronal activities. Hence, bio-guided assays on T. algeriensis extracts can be envisaged with the aim of isolating molecules with neuroprotector power, unraveling their functional features, identifying their targets, and understanding their mode of actions.
7.8. Hot Topic Analysis of T. algeriensis
In this study, the bibliometric data used for the mapping tool is from the Web of Science database. The cooccurrence analysis of more than 50 high frequency keywords has been made to construct a knowledge map of the main strong domain of research related to T. algeriensis. In the visualization results, each keyword is represented by a circle. From the analysis of Figure 7, four topic groups are identified, which are “the different chemotypes of the plant,” “the chemical composition of the essential oil,” “the biological activities,” and the” in vivo activities.” Among them, the field of biological activities is the link between the field of chemical composition and the chemotype of the plant as there is a close relationship with these fields. After further domain structure detection as shown in Figure 8, the main topics in the field of biological activities of T. algeriensis were the antibacterial and antioxidant activities, directly related to the richness of the essential oil of the aerial parts of the plant in thymol, carvacrol, and p-cymene. The occurrences of these compounds, their absence, or their presence determine the chemotype of the plant.
8. Discussion
To the best of our knowledge, this is the first review to summarize and critically evaluate the chemical composition, biological and pharmacological activities, efficacy, and safety of T. algeriensis extracts and essential oils. T. algeriensis is one of the most well-known Thymus species in northern Africa. Most of the prior research on this plant species has focused on its biological activities and pharmacological and preclinical characteristics. Many chemical constituents have been identified from T. algeriensis extracts such as flavonoids, mainly kaempferol and rutin, besides luteolin, apigenin, hesperidin, and neohesperidin among others. They also contain phenolic acids such as rosmarinic acid and vanillic acid. Other constituents include sterols (e.g., β-sitosterol) and triterpenoid (e.g., oleanolic acid) [22, 23].
Current evidence revealed that the chemical composition of T. algeriensis essential oils (EO) differs according to its source. Its EO contains 40 volatile compounds represented mainly by monoterpenes and sesquiterpenoids. EO from T. algeriensis growing in Tunisia contains mainly 1,8-cineole and 4-terpineol [9, 15], while borneol, camphene, camphor, and carvacrol were listed from the Algerian and Moroccan plants. p-Cymene and thymol were detected in all flora. β-Myrcene (20.22%) has only been identified in the aerial parts of the Libyan plants [124], while myrcene characterized the EO of the Moroccan plants [53].
Free radicals are unstable and reactive because they have an unpaired electron. Superoxide, nitric oxide, and hydroxyl radicals, the most reactive and poisonous ROS, are among them. Hydrogen peroxide, singlet oxygen, and ozone are nonradical oxidants that produce free radicals in tissues through a variety of chemical processes. ROS are produced normally in mitochondria during aerobic respiration, by macrophages to fight infection, during induction of cytochrome p450, and by peroxisomes in the cells. Many exogenous factors can stimulate ROS production which damages cells and causes cellular injury and disease development such as tobacco smoke, ionizing radiation, UV light, industrial toxins such as carbon-tetrachloride, drugs, and charcoal-broiled foods [125]. Natural products with antioxidant properties such as T. algeriensis are beneficial in disease prevention and treatment. Antioxidant effects of T. algeriensis were investigated using different methods including DPPH, ABTS, FRAP, TBRS, TAC, HRS, and β-carotene bleaching assays [42, 126–128]. These studies showed that T. algeriensis extracts and EO had antioxidant properties. Furthermore, in vivo studies confirmed these in vitro tests. Although the in vitro studies investigated the antioxidant effects of this species, more comparative studies between extracts and EO of T. algeriensis obtained from various geographical areas are required. Moreover, in vivo antioxidant effects of T. algeriensis extracts and EO are not sufficient, and more studies are required.
Inflammation is now recognized as a major factor in cellular and subcellular disorders. Few cells are engaged in this process, which boosts the production of proinflammatory chemical mediators (IL, TNF-α, NO, and PGs). Stimuli and tissue injuries influence mediator overproduction [129]. It was reported that phenolic compounds are similar to nonsteroidal anti-inflammatory drugs (NSAIDs) in their ability to decrease the chemical mediators of inflammation [130]. As mentioned previously in this review, T. algeriensis extracts and EO are rich in different phenolic compounds. In vitro studies showed their ability to inhibit inflammatory enzymes such as COX–1, COX–2, and 5-LOX [22, 44]. Besides, in vivo studies showed that the extracts suppressed the production of proinflammatory cytokines such as NF-κB, TNF-α, lipoxygenase, COX–2 enzymes, and PGE2 [47, 131]. Additionally, the administration of T. algeriensis extracts to animal models enhanced their enzymatic and nonenzymatic antioxidant machinery (CAT, SOD, GPx, and GSH) in response to stress induction (Figure 9). Nevertheless, more studies on the different extracts and EO from T. algeriensis on inflammatory cell lines and in vivo models are still needed.
Moreover, in vitro studies showed that T. algeriensis extracts and EO had anticholinesterase activities [132], while in vivo studies on their use in treatment of Myasthenia gravis, Alzheimer diseases, or other diseases treated with anticholinesterases are lacking. In the same context, in vitro studies on ACE inhibition are promising [133], but no in vivo studies to our knowledge studied the antihypertensive effects or compared the extract effects with known ACE inhibitors (ACEIs). Similarly, α-glucosidase inhibitory effects of T. algeriensis extracts were explored, but no in vivo study on their antidiabetic effects was carried out [134].
Although gastroprotective, antinociceptive, anti-inflammatory, antioxidant, neuroprotective, and anticancer properties have all been documented for T. algeriensis extracts and EO, several studies are required on other disease models.
9. Conclusions and Future Perspectives
This review profiles the chemical composition of T. algeriensis and synthesizes the biological and pharmacological activities of its extracts and EO. Based on this comprehensive overview, the use of this plant’s extracts in biotechnology seems to be promising because of their wealth in bioactive molecules mainly for antioxidant, antimicrobial, anticancer, anti-inflammatory, antinociceptive, gastro- and neuroprotective, and antiacetylcholinesterase purposes. However, as most of the biological activities have been evaluated in vitro, it would be interesting to extend their in vivo activities and uncover the underpinning mechanisms of action. In addition, the activities of T. algeriensis extracts vary depending on the solvents utilized. Moreover, environmental factors, geographical regions of growth, seasonal variations, phenological stages, degree of ripeness, plant part used, and harvesting time as well as postharvest treatment and processing could greatly influence the chemical composition of the plant and thus the related activities. This led to result inconsistency which represents a real issue for the development of bioactive preparations and their reliability. Thus, it could be interesting to study the effect of T. algeriensis domestication on its chemical composition and biological activities as it is a solution to conserve this plant species.
Since T. algeriensis was shown to improve sperm viability and mobility, further investigation could shed light on the potential estrogenic activity of this plant and its effect on reproductive hormones as Lamiaceae species including Thymus genus are traditionally used in Morocco to treat infertility disorders [135]. Additionally, as the revolution for patient safety has gained momentum importance, the safety of T. algeriensis cannot be definitively established from reported studies. More attention should still be given to the toxicity aspect in a more in-depth manner especially when using EO that could be harmful in some individuals. Although the preclinical investigations (in vitro, cell-based, and animal studies) on this plant species are available in a large quantity, they should be translated into evidence-based clinical progress using human trials which are completely missing. Having said that, there are many opportunities for the healthcare industry to explore the benefits of T. algeriensis because of the potential growth market of medicinal plants. Therefore, future prospective studies should screen fractions or individual constituents that exhibit health-protecting and curative activities from T. algeriensis.
Conflicts of Interest
The authors declare no competing interest.
Authors’ Contributions
I.M., W.B.B., and M.F. reviewed the literature and wrote the manuscript. G.T.M.B. and H.A. revised the manuscript. M.S. revised the manuscript and designed and conceived the work. All authors approved the final version.
Funding
The APC was funded by UM6P.