Comparison of Antibacterial Activity of Nigella sativa Oil Against Clinical Isolates Using a Controlled Disc Diffusion Method

Objective: To compare the antibacterial potency of cold press Nigella sativa oil against 49 clinical isolates. Method: A controlled disc diffusion method based on British Society of Antimicrobial Chemotherapy antibiotic susceptibility testing guidelines was used. Results: Reproducible results were obtained that allowed comparison of intra species sensitivity. Staphylococcus aureus isolates (18) from skin and soft tissue infections were highly susceptible (31 to 51mm zones of inhibition, ZOI), with significant differences (p<0.01) in sensitivity between many of the isolates, but no significant difference (p >0.05) between methicillin- sensitive and -resistant isolates. Streptococcus spp. isolates (6) from skin and soft tissue infections produced consistently lower mean ZOI (22 to 27mm). Haemophilus infuenzae isolates (4) from respiratory tract infections showed little variation in ZOI (22 to 23mm). Conversely, two respiratory tract Streptococcus pneumoniae isolates showed markedly different sensitivity (p<0.01) to the oil (24 and 40mm). For the first time, antibacterial activity of N. sativa oil against Neisseria gonorrhoeae is reported (29 to 38mm). All Gram-negative isolates (14) - Escherichia coli, Proteus mirabilis, Enterobacter cloacae, Klebsiella pneumoniae and Pseudomonas aeruginosa - were uninhibited by the oil, while ZOI for Grampositive isolates exceeded those obtained for first line antibiotics used to treat the relevant infections. There was no evident link between isolate antibiotic resistance profile and extent of growth inhibition by the oil. Conclusion: N. sativa oil has a potential for combatting antibiotic resistant Gram-positive infections at various body sites, including sexually transmitted disease. Significant strain variation in sensitivity to the oil within some species is revealed.