Antimicrobial activity and chemical composition of essential oil from Oliveria decumbens Vent

Oliveria decumbens Vent. (Umbelliferae) is a shrub commonly found in the South East of Iran. Its aerial part is extensively used in herbal medicine. In this study, the antimicrobial activity of O. decumbens essential oil extracted from aerial parts of plant against a panel of microorganisms including gram positive, gram negative bacteria, yeast and fungi were assessed by disc diffusion method and micro broth dilution assay. The chemical constitutes of this oil was analyzed by GC. The main components of essential oil are thymol (26.9%), carvacrol (0.25%), p-cymene (13.3%) and γ-terpinene (11%). This oil exhibited strong antifungal activity against filamentous fungi and yeast with average of inhibition zone (AIZ) 34.86 and MIC≤0.25µl ml -1. The effect of 2 µl of essential oil (IZ≥27.3 mm) is larger than Amphotricin B (IZ≤17) against fungi. The gram positive bacteria are more sensitive than gram-negative bacteria (21.9 Vs 18.4). Spore forming bacteria (Bacillus sp.) are resistant to essential oil and the effect of oil against Bacillus sp. had inhibitory effect (MIC>2 µl ml-1). Pseudomonas aeruginosa were more resistant than others (IZ<8 mm). Thus, microorganisms differ in their resistance to O. decumbens oil, i.e. bacteria are more resistant than fungi and gram negative bacteria are more resistant than gram positive bacteria. These effects are more concerned to phenol components especially thymol. Therefore, further studies are required to evaluate in vivo efficacy.