Antibacterial activity of the methanol, aqueous and n-hexane extracts of Lippia adoensis

The fresh leaves of the wild plant (Lippia adoensis var. adoensis) are used for washing wooden and ceramic utensils to give fresh and clean smell. The dried leaves of the cultivated variety, L. adoensis var. koseret, are also used to flavor butter often added to flavor ‘kitfo’ (minced meat) dish. The dried leaves powdered together with barley are eaten to get relief from stomach complaints. These properties of the plant may be associated with its antibacterial and antioxidant activities. Leaves of L. adoensis collected from the wild habitats were shade dried and ground into fine powder. Powder was extracted using the solvents by mixing them in (1:10 w/v) ratio and shaking for 48 h. After filtering, the methanol and n-hexane extracts were dried over a rotary evaporator and the aqueous extract in a lyophilizer. The extracts were then made in concentrations of 25, 50 and 100 mg/mL by Tween 80 (T-80) (2%). Sterilized disks impregnated by these extracts were applied on cultures of bacteria (Shigella boydii, Escherichia coli, Pseudomonas aeruginosa and Staphylococcus aureus) incubated overnight and inhibition zones measured. The aqueous extract inhibited all the test bacteria at lower doses followed by the methanolic and n-hexane extracts and S. aureus was found to be more resistant to the plant extracts than the rest of the test bacteria. The methanol, aqueous and n-hexane extracts showed significantly higher levels of inhibition zones with increases in concentration when compared with the negative control (T-80). The positive controls (tetracycline and chloramphenicol) also showed significantly higher inhibition zones than the 100 mg/mL concentrations of the extracts and T-80 except that chloramphenicol failed to inhibit S. aureus and P. aeruginosa. However, the resistance of these bacteria against chloramphenicol was curved by mixing them with plant extracts. The aqueous extract was the best followed by the methanol and n-hexane extracts in decreasing order. Sh. boydii was the most, S. aureus the least sensitive bacteria towards leaf extracts of L. adoensis. Combination of leaf extracts to antibiotics increased the sensitivity of S. aureus and P.aeruginos which were more resistant to the antibiotics.