Antibacterial effect of black seed oil on Listeria monocytogenes

Listeria monocytogenes is a major foodborne pathogen in the United States. Effective methods for reducing L. monocytogenes in foods would reduce the likelihood of foodborne outbreaks of listeriosis, and decrease economic losses to the food industry. Nigella sativa is a herbaceous plant, whose seeds (black seed) have been used as a spice and condiment in foods in the Middle East. The objective of this study was to determine the antibacterial effect of black seed oil on twenty strains of L. monocytogenes by disc diffusion method. A population of 7.0 log CFU of each strain of L. monocytogenes was inoculated on duplicate plates containing antibiotic medium one agar. The plates were allowed to dry at room temperature for 15 min. Three discs (6 mm diameter), each impregnated with 10 μl of black seed oil, vegetable oil (oil control), or gentamicin (positive control) were placed on each inoculated plate. The plates were incubated at 37 °C for 24 h, and were observed for zones of L. monocytogenes growth inhibition. Black seed oil exhibited a strong antibacterial activity against all the strains of L. monocytogenes, yielding a significantly (P<0.01) larger inhibition zone than that of gentamicin. The mean zones of inhibition produced by black seed oil and gentamicin were 31.50 ± 1.0 and 14.80 ± 0.50, respectively. The vegetable oil had no inhibitory effect on L. monocytogenes. Results indicate that black seed oil could potentially be used to inhibit L. monocytogenes, but appropriate applications in foods need to be validated.