Chemical composition and antimicrobial properties of different basil essential oils chemotypes from Togo

Materials and Methods

Plant material sampling and volatile oils isolation

Leaves and inflorescences of O. basilicum used in this work were harvested from plants at full flowering stage in vegetable gardens in five various locations of Togo from May to October 2007.

Plant specimen was identified by Prof. Akpagana, Departement de Botanique, Facult des Sciences at the Universit de Lom (Togo), where voucher specimens were deposited in the herbarium.

A sample (50 g) of air-dried plant material was extracted by the hydrodistillation technique during 2 hours in a modified Clevenger-type glass apparatus (Craveiro et al., 1976). The extracted crude essential oils were stored in hermetically sealed dark glass flasks with rubber lids, covered with aluminium foil to protect the contents from light and kept under refrigeration at 4°C until use without any prior purification.

Essential oil analyses by gas chromatography

Gas chromatographic analysis was carried out on a Varian 3300 type gas chromatograph equipped with FID detector. An apolar capillary column DB-5 (30 m x 0.25 mm i.d.; film thickness 0.25 µm) and on a polar column supelcowax 10 with the same characteristics as above mentioned were used. DB-5 column operating conditions were as follows: From 50°C (5 min), 50 to 250°C at the rate of 2°C/min and supelcowax 10 from 50°C (5 min), 50 to 200°C at 2°C/min. The injector and detector temperatures were respectively 250 and 300°C. The carrier gas was helium at a flow rate of 1.5 mL/min. Samples (0.2 µL) of undiluted essential oil were injected manually.

Gas chromatography-mass spectrometry analysis

The GC/MS analysis was carried out on a Hewlett Packard 5890 series II chromatograph, coupled with a mass spectrometer of the Hewlett Packard 5971 series type operating in the EI mode at 70 eV. The capillary column type was DB5-MS (30 m x 0.25 mm i.d.; film thickness 0.25 µm). The amount of sample injected and GC/MS parameters were the same as above.

Identification of components

The components of oils samples were identified by their retention time, retention indices relative to C₈-C₂₄ n-alkanes, computer matching with Wiley 275L library and as well as by comparison of their mass spectra with the authentic samples  or with data already available in the literature (Kondjoyan and Berdagu, 1996; Adams, 2001).

The percentage of composition of the identified compounds was computed from the GC peak area without any correction factor and was calculated relatively.

In vitro antifungal testing

Fungal strains used are listed in Table I. They were supplied by Institut Pasteur de Paris and H'pital Saint Jacques de Besanon, France. The fungi were cultivated on a sabouraud agar medium in which was added chloramphenicol 1%, all purchased from BioMerieux Co. (Paris, France). Pure estragole, linalool and methyleugenol commercial standards were also purchased from BioMerieux Co. (Paris, France).

Table I: Antifungal activities of different O. basilicum essential oils chemotypes from Togo

The antimicrobial activities of the essential oils were assessed according to agar dilution method (Benjilali et al., 1986; Griffin et al., 2000). The tested essential oil and its pure major components from commercial origin were diluted in a minimal quantity of ethanol 95% 1/10 v/v to which was added an aqueous solution of Tween 80 (final concentration of 1% v/v) in order to obtain a homogeneous mixture. The later was incorporated as appropriate to the microbiological culture medium under solidification to obtain final concentrations of the active ingredient that ranged from 10 to 500 µL/L. The mixture was then poured into 3 cm diameter petri dishes.

After solidification fungal strains were respectively seeded as described below:

(i) dermatophytes were seeded with a disc of approximately 2 mm, from a mycelia carpet of pre-culture, laid in the middle area of a new petri dishes, upper side against the new culture medium;

(ii) 1 mL of a suspension of 10⁵ conidia per mL of Aspergillus fumigatus or 10⁵ blastospores per mL of yeast was poured on the surface of the culture medium. Incubation time and temperatures depended on the fungal strains: 24 hours at 37°C for Candida albicans and Aspergillus, 48 hours at 37°C for Cryptococcus, 14 days at 24°C for the dermatophytes and Scopulariopsis brevicaulis.

Antibacterial testing

Bacterial strains were supplied by Institut Pasteur Collection de Paris and Leeds University Microbiology Laboratory, United Kingdom. They were cultivated on 1.3% (m/v) nutrient broth-agar-granulated (NB-AG) culture medium (AES Labortories), in 9 cm diameter petri dishes. After cooling and solidification, the petri dishes were automatically inoculated with the bacterial suspensions 10⁵ CFU/mL with STEERS apparatus. The petri dishes were incubated at 37°C for 48 hours under aerobic conditions.

Major constituents: Others meaningful constituents were estragole (10.0%), (E)-alpha-bergamotene (3.6%) and linalool (2.9%). v) t-Anethole chemotype (sample from Bassar): This sample contained mainly t-anethole (74.6%), linalool (17.3%) and carvacrol (2.7%).

To our knowledge, the last three basil oil chemotypes have not yet been reported. O. basilicum has been thoroughly investigated with regards to volatile oil composition but some of our findings are supportive of the idea that continuing chemical inventory on this species is still of scientific interest to support action aimed at plant biodiversity knowledge, conservation and sustainable exploitation.

The findings on the chemical composition of basil oils in Togo were very instructive with regard to the possible applications. Hence, the estragole chemotype found here resembled that described by Ekundayo et al. (1987) and  Baritaux et al. (1992), which  is well-known for its valuable usage in perfumery and food (Kayibou, 1992). In addition, this chemotype has been recently indicated as a possible pesticide against larva and adult of Callosobruchus maculatus known as a major post-harvest pest of Vigna unguiculata in storage (Keita et al., 2001; Ketoh et al., 2002).

The pesticidal activity of the volatile oil of Clausena anisata, containing mainly estragole and t-anethole (Moudachirou et al., 1997; Okunde and Olaifa, 1987). Such indications on the pesticidal properties of this essential oil containing estragole and t-anethole is certainly indicative of the biocide potential of these natural molecules which might be endowed with antimicrobial properties (Friedman et al., 2002), our focus in this investigation. 

The experimental data in Table II show that all fungal strains tested were sensitive to the metyleugenol type of basil oil, but dermatophytes and pathogenic yeasts were more particularly affected. Hence, markedly low MICs (80 µL/L) were recorded with T. mentagrophytes var mentagrophytes. Along with dermatophytes, imperfect filamentous fungi were also highly sensitive to the test volatile oil; MICs were in the range of 100-150 µL/L. Likewise, the methyleugenol oil chemotype appeared very toxic to pathogenic yeast strains including Candida, Cryptococcus, Aspergillus, and Scopulariopsis,  MICs varying from 80 to 100 µL/L. Also noticeable was the antifungal effect (MIC: 150 µL/L and 200 µL/L) of this oil chemotype on both strains of Scytalidium sp. The later, which often resist conventional antibiotics, are usually reported in human dermatomycosis in tropical and subtropical countries (Alvarez et al., 2000). The African Trichophyton soudanense, a parasite frequent in school environment (Ouaffak et al., 2001; Vandemeulebroucke et al., 1999) was interestingly also sensitive to the tested essential oils (MICs 100 and 200 µL/L). It was also the case for Cryptococcus neoformans, a hazardous opportunist yeast, which is a resistant germ usually infecting humans affected by HIV/AIDS, which group of patients is known to be generally at high-risk with regard to mycosis opportune affections.

The high antifungal activity of this basil oil chemotype on those pathogenic fungi like dermatophytes, filamentous fungi and yeasts confirmed the excellent fungal growth inhibition properties previously reported as a characteristic of essential oils rich in methyleugenol and/or other phenol derivatives (Chaumont and Leger, 1989). In this work there is no doubt that the antifungal activity of O. basilicum chemotype methyleugenol against tested fungi is a predictable consequence of its high content in methyleugenol known as one of the phenolic volatile molecules endowed with antimicrobial properties (Chaumont and Leger, 1989; Viollon and Chaumont, 1994). In comparison, commercial linalool and estragole tested as standards in this study were found non-effective unlike pure methyleugenol standard.

The methyleugenol/t-anethole chemotype showed a moderate antifungal activity only on dermatophytes with MICs ranged from 250-300 µL/L but not filamentous fungi and yeasts were not affected. The estragole type, the linalool/estragole type, and the t-anethole type were ineffective on all fungal strain tested in this work. A lesson learned from the findings mentioned above was that estragole and t-anethole, which are reported as possible natural pesticides, could not serve as natural fungicidal ingredients unlike methyleugenol against the fungal strains tested in this work. This simply indicates the broader understanding of what is generically termed to as the biological activity of essential oils.

Table III shows that all feet microflora bacterial strains tested were only sensitive to methyleugenol and methyleugenol/t-anethole oil chemotypes.

The MICs, which were in the range of 200-500 µL/L were slightly higher than those recorded on fungal strains in this study, indicating a lesser activity. The results recorded against common armpit microflora bacterial strains, were especially promising for a possible use against Corybacterium xerosis, the principal one of the bacteria responsible for bad odors. Its growth was interestingly inhibited with at a dose of 100 µL/L antibacterial test suspension. Note that this bacteria metabolizes human steroids produced via transpiration to yield small and bad-smelling volatile molecules (Rennie et al., 1991).

The findings in this study indicated bacteriostatic effects against feet microflora germs like Staphylococcus epidermidis and Staphylococcus hominis (MIC: 200 and 300 µL/L) partly responsible for feet bad odors (Marshall et al., 1987; Marshall et al., 1988). Methyleugenol and methyleugenol/t-anethole oil chemotypes samples were also very active against the Coryneform (Corynebacterium gr B, C and D2) (MIC: From 100-300 µL/L) which produce proteinase and lipase, both enzymes that cause feet surface skin bad odors (Marshall et al., 1987; Marshall et al., 1988). Both oil chemotypes also showed a moderate antibacterial activity against gram negative bacteria (Acinetobacter sp, Moraxella sp: MIC from 250- 500 µL/L) and gram positve bacteria (Micrococcus luteus, Staphylococcus epidermidis: MIC: From 200-500 µL/L). The later are involved in nosocomial infections (Bergogne-Berezin, 1995; CDC, 1999).

The proven antibacterial activities of the methyleugenol and metyleugenol/t-anethole types of basil oil found in this study are quite indicative of their potential as possible active ingredients for use in the formulation of deodorants for armpits or feet. Developing sustainable large scale consumer goods like natural drugs and cosmetics based on basil oil as well as on other essential oils bearing aromatic plants growing in Togo is of an obvious economic interest while helping preserve plant biodiversity.

Culture media with or without ethanol 95% and Tween 80 were used as controls.

The antimicrobial activities were evaluated by the determination of the minimum inhibitory concentration (MIC). The MIC of a tested active ingredient was determined as the lowest concentration of the test antimicrobial ingredient sample that resulted in a complete inhibition of visible growth of the microorganisms. All tests were carried out in triplicate.