Nigella sativa L. seed extracts and oils have been embraced by traditional medicine of cultures inhabiting Middle East and North Africa for centuries. Among other uses, it has been applied against dermatitis and eczema often worsened by staphylococcal colonization of the skin.
The study was conducted to evaluate applicability of N. sativa seed extract in antibacterial skin formulations by examination of its activity against methicillin-resistant Staphylococcus aureus as well as cytotoxicity against human dermal fibroblasts.
Two variants of N. sativa seed extract containing 9.91 and 2.10 % of thymoquinone were prepared by supercritical carbon dioxide extraction. The extracts and standards of their major volatile ingredients; thymoquinone, thymol, p-cymene alongside with the reference antiseptics; chlorquinaldol and a combination of amylmetacresol with 2,4-dichlorobenzyl alcohol were subjected to evaluation of antibacterial efficacy against a collection of Staphylococcus aureus strains. The preparation based on Vaseline containing 1% of N. sativa extract was applied on Mueller-Hinton agar plates and its ability to inhibit S. aureus growth was examined. The MTT assay was employed to study cytotoxic effects of the thymoquinone-rich N. sativa seed extract against HDFa fibroblasts.
N. sativa seed extract and thymoquinone have shown potent bacteriostatic and bactericidal effect against Staphylococcus aureus, including methicillin-resistant strains (MRSA) isolated in Poland. Results suggest that N. sativa seed extract activity against S. aureus should mainly be attributed to thymoquinone, which was effective in concentrations of 4-16 μg/ml. Regarding the activity against S. aureus, thymoquinone was more efficient than a combination of amylmetacresol with 2,4-dichlorobenzyl alcohol and comparable to chlorquinaldol. The Vaseline-based preparation containing N. sativa extract caused growth inhibition comparable to an equally concentrated DMSO solution of the extract. The IC50 of N. sativa extract against HDFa fibroblast was determined at 0.2 mg/ml, which was 2-fold higher than the average MIC and MBC of the extract against S. aureus.
The observed effectiveness of N. sativa seed extracts against bacteria was found to be dominantly dependent on concentration of thymoquinone. Its efficiency against S. aureus isolates as well as results of cytotoxicity examination against human dermal fibroblasts indicate on its applicability as an antibacterial agent for topical use and motivates further research in this area.