Ocimum gratissimum leaf oil, which has been reported to possess in vitro and in vivo efficacy against enteric bacteria was tested against forty six clinical isolates of Pseudomonas aeruginosa in Lagos, Nigeria. The effect of the essential oil (EO) on pyocyanin production among these strains was also investigated. Agar well diffusion assay revealed susceptibility in 40 (87%) of the 46 tested strains with inhibition zone diameter (20-36 mm) comparable with the effect of tobramycin. Of the 40 susceptible isolates, 34 strains were quantitatively demonstrated to show susceptibility when further tested with the essential oil in broth and on agar yielding MIC and MBC values of 36 – 54 mg/ml and 42 – 66 mg/ml respectively. The values were higher than the 12 mg/ml (MIC) and 24 mg/ml (MBC) observed in Escherichia coli ATCC 25922. Compared with the control, the essential oil was found to reduce pyocyanin production significantly (p < 0.01) at 15 mg/ml (30.1 – 30.5 vs 259.2 – 276. 7 μg/ml) and 75 mg/ml (2.5 – 3.5 vs. 259.2 – 276.7 μg/ml) in both sensitive and resistant strains, suggesting that Ocimum gratissimum leaf oil may inhibit expression of virulence factors and progression of Pseudomonas infections caused by the tested strains.
Key Words: Ocimum gratissimum leaf oil, Pseudomonas aeruginosa, Nigeria
Afr. J. Clin. Exper. Microbiol. 2004; 5 (3): 265-271