Isolation and identification of antimicrobial compound from Mentha longifolia L. leaves grown wild in Iraq.

BACKGROUND: Mentha longifolia L. (Lamiaceae) leaves have been traditionally implemented in the treatment of minor sore throat and minor mouth or throat irritation by the indigenous people of Iraq, although the compounds responsible for the medicinal properties have not been identified. In the present study, an antimicrobial compound was isolated and characterized, and its biological activity was assessed.

METHODS: The compound was isolated and characterized from the extracted essential oil using different spectral techniques: TLC, FTIR spectra and HPLC. Antimicrobial activity of the compound was assessed using both disc diffusion and microdilution method in 96 multi-well microtiter plates.

RESULTS: A known compound was isolated from the essential oil of the plant and was identified as (-) menthol. The isolated compound was investigated for its antimicrobial activity against seven selected pathogenic and non-pathogenic microorganisms: Staphylococcus aureus, Streptococcus mutans, Streptococcus faecalis, Streptococcus pyogenis, Lactobacillus acidophilus, Pseudomonas aeruginosa and the yeast Candida albicans. Menthol at different concentrations (1:1, 1:5, 1:10, 1:20) was active against all tested bacteria except for P. aeruginosa, and the highest inhibitory effect was observed against S. mutans (zone of inhibition: 25.3 mm) using the disc diffusion method. Minimal inhibitory concentration MIC values ranged from 15.6-125.0 microg/ml, and the most promising results were observed against S. aureus and S. mutans (MIC 15.6 microg/ml) while, S. faecalis, S. pyogenis and L. acidophilus ranked next (MIC 31.2 microg/ml). Furthermore, menthol achieved considerable antifungal activity against the yeast C. albicans (zone of inhibition range: 7.1-18.5 mm; MIC: 125.0).

CONCLUSION: The isolation of an antimicrobial compound from M. longifolia leaves validates the use of this plant in the treatment of minor sore throat and minor mouth or throat irritation.