Assessment of In vitro Antibacterial Activity and Cytotoxicity Effect of Nigella sativa Oil

BACKGROUND: Methicillin resistance is a serious health concern since it has spread among Staphylococcus aureus and coagulase-negative Staphylococci (CoNS) that are frequent community and nosocomial pathogens worldwide. Methicillin-resistant strains are often resistant to other classes of antibiotics, making their treatment difficult. Nigella sativa oil is known to be active against Gram-positive cocci, yet its in vitro cytotoxicity is rarely investigated, is a proper and powerful candidate for treatment of methicillin-resistant isolates. OBJECTIVES: The aim of this study is to evaluate the in vitro antibacterial activity and cytotoxicity effect of N. sativa oil. MATERIALS AND METHODS: The minimal inhibitory concentrations (MICs) of N. sativa oil were determined by broth microdilution method against four different American Type Culture Collection strains, 45 clinical isolates of methicillin-resistant S. aureus (MRSA), and 77 methicillin-resistant CoNS (MRCoNS). The effects of different dilutions (0.25 μg/mL, 0.5 μg/mL, and 1 μg/mL) of N. sativa oil on the proliferation of gingival fibroblasts were evaluated. RESULTS: The MIC values of N. sativa oil against clinical isolates of Staphylococci were between <0.25 μg/mL and 1.0 μg/mL. Compared to the control group, there was no cytotoxic effect on the proliferation of the gingival fibroblasts. CONCLUSION: In the present study, the oil of N. sativa was very active against MRSA and MRCoNS and had no in vitro cytotoxicity at relevant concentrations. These findings emphasize that there is a requirement for further clinical trials on N. sativa oil for “safe” medical management of infections caused by methicillin-resistant Staphylococci. SUMMARY: The minimal inhibitory concentration (MIC) values of Nigella sativa oil against Staphylococcus aureus American Type Culture Collection (ATCC) 29213, Enterococcus faecalis ATCC 29212, Escherichia coli ATCC 25922, and Pseudomonas aeruginosa ATCC 27853 standard strains were 0.5 μg/mL, 2 μg/mL, 64 μg/mL, and 64 μg/mL, respectively. The N. sativa oil showed an excellent antibacterial activity against clinical isolates of methicillin-resistant S. aureus and methicillin-resistant coagulase-negative Staphylococci with very low MIC range of <0.25–1.0 µg/mL. The N. sativa oil exhibited no cytotoxic effect on the proliferation of the gingival fibroblasts. Abbreviation used: ATCC: American Type Culture Collection; CLSI: Clinical and Laboratory Standards Institute; CoNS: Coagulase-negative Staphylococci; DMEM: Dulbecco's modified Eagle's medium; DMSO: Dimethyl sulfoxide; FBS: Fetal bovine serum; HGF: Human gingival fibroblast; MIC: Minimal inhibitory concentration; MRCoNS: Methicillin-resistant CoNS; MRSA: Methicillin-resistant S. aureus