Antimicrobial potential of the Ethiopian Thymus schimperi essential oil in comparison with others against certain fungal and bacterial species

In the preliminary screening of the essential oils, T. schimperi at 5 ?l concentration caused higher inhibition zone with the range of 35.66 mm to
88.66 mm against tested fungi and 63 mm to 90 mm against bacteria. In similar study
antimicrobial activity of seven essential oils from three Thymus species against Salmonella enteritidis, Salmonella typhimurium, Eschericcha coli, Shigella flexneri and Listeria monocytogens11]. The result showed essential oil from T. hyemalis inhibited with the range of 19.6 mm to 45 mm. The existence of variations in the
inhibition zone can be assumed that due to differences in the number of molecules
and chemical type of molecules in the plant materials 12], 13].

In this study the essential oil extracted from C. zeylanicum bark demonstrated strong antifungal and antibacterial activity next to T. schimperi. The antifungal activity of C. zeylanicum exhibited 61 mm, 62.66 mm, 62 mm, 31.33 mm, 54.66 mm and 54 mm inhibition zone against
Tricophyton spp. (scalp isolate), Tricoophyton spp. (nail isolate), Microsporum spp., C.albicans, A. niger and R.rubra respectively. It also showed inhibition zone of 42.66 mm, 43.33 mm, 36.66 mm and
41.33 mm against E. coli, Bacillus spp., Shigella spp. and Streptococci respectively. The antimycotic activity of C. zeylanicum bark is due to presence of cinnamaldehyde 14].

In our work, the C. limon peel oil inhibited the tested fungi and bacteria in the range of 14.66 mm to 42 mm
and 20.33 mm to 27 mm respectively and the antimicrobial effect is taken as a moderate
activity when compared to Thymus and Cinnamon oils. However, the present study results were better than reported by (Gulay-Kirbasilar
et al., 15]). This may be due to variations in climate and soil composition in which the plant
growing 12], 13].

In this study, E. camaldulensis showed less inhibition zone against almost all the tested organisms. However, the
result of E. coli and Streptococci were 16 mm and 14.3 mm respectively and taken as a moderate activity. In contrast
to the present study, Babayi, et al., 16]) tested the efficacy of methanol extract of E. camaldulensis against Salmonella typhi, Staphylococcus aureus and Bacillus subtilis and reported a higher inhibition zone in a range of 15 mm-16 mm. In this case, the solvent
extract of E. camaldulensis is better than the hydrodistiled essential oil against tested organism. According
to Masotti et al., 13]) and Angioni et al., 12]), the extraction product can vary in quality, quantity and in composition according
to the type of extraction method.

Some oils appeared more active with respect to Gram reaction, exerting a greater inhibitory
activity against Gram positive bacteria 17]. However, the result obtained in the present study showed that the oil of T. schimperi did not show any selectivity towards the tested gram-positive and gram-negative bacteria.
The substances extracted from thyme especially the phenolic components thymol and carvacrol showed antibacterial activity against gram-positive and gram- negative bacteria due
to their effects on the bacterial membrane 7]. In this case, T. schimperi seems to better than other essential oils.

The MIC values of T. schimperi were with the range of 0.08 ?l/ml to 0.031 ?l/ml against

Tricophyton spp. (scalp and nail isolate); Microsporum spp. and R. rubra. These results are in line with the work of Eugenia et al., 6] who had reported that Thymys pulegioides essential oil showed the lowest MIC value in the range of 0.16 ?l/ml to 0.32 ? l/ml
for Dermatophyte and 0.32 ?l/ml to 0.64 ?l/ml for Candida. This indicates that plants rich in a wide variety of secondary metabolites such
as tannins, alkaloids, terpenoids which have been found invitro to have antimicrobial property 18]. C. zeylanicum oil also showed similar result with the same concentration on Microsporum spp., C. albicans, A.niger and R. rubra.

The MIC of C. limon peel oil was 1.25 ?l/ml against R. rubra and Microsporum spp. On the other hand, 2.5 ?L/ml was recorded for the rest of the tested organisms. Therefore,
Rhodotorula and Microsporum were more susceptible to C. limon peel oil than other tested organisms. However, the result obtained in limon oil was
not comparable from the result of T. schimperi and C. zeylanicum which showed better antimicrobial activity for all tested organisms. This variation
was due to the difference in plant type and part (organ) used in the experiment 12], 13]. In the present study, MIC of the essential oil of E. camaldulensis was in the range of 2.5 ?l/ml to 5 ?l/ml against all the tested organisms. The obtained
results were in contrast to Morteza et al., 19]) who had recorded 1.95 ?l/ml against Staphylococcus aureus. This variation may due to the difference in the tested organisms and the method
used to assess antimicrobial activity (Janssen et al., 20]).

Our study has several limitations. There were no controls for C. albicans, A. niger and R. rubra, because the purpose was not comparison of the standard antimicrobial agent with
plant materials. Moreover, all the tested organisms were not standards. However, the
organisms were identified by cellular, cultural and biochemical characteristics. Finally,
this study also did not include the chemical composition of plant materials due to
financial shortage and lack of materials.