A Molecular Insight into the Synergistic Mechanism of Nigella sativa (Black Cumin) with β-Lactam Antibiotics against Clinical Isolates of Methicillin-Resistant Staphylococcus aureus

Abstract

Methicillin-resistant Staphylococcus aureus (MRSA) infection is detrimental to hospitalized patients. With diminishing choices of antibiotics and the worry about resistance to colistin in synergistic combined therapy, there are suggestions for the use of herbal derivatives. This investigation evaluated the synergistic effects of Nigella sativa (NS) in combination with beta-lactam (β-lactam) antibiotics on extreme drug-resistant (XDR) MRSA isolates. NS concentrations of 10, 7.5, 5.0, 2.5, 1.0, and 0.1 µg/mL, alone and in combination with β-lactam antibiotics, were used to determine the antimicrobial susceptibility of MRSA isolates by the well diffusion method. Time–kill assays were performed using a spectrophotometer, with time–kill curves plotted and synergism ascertained by the fractional inhibitory concentration (FIC). Scanning and transmission electron microscopy were used to gain insight into the mechanism of action of treated groups. Isolates were inhibited by the NS concentrations, with differences in the zones of inhibition being statistically insignificant at p < 0.05. There were statistically significant differences in the time–kill assay for the MRSA isolates. In addition, NS combined with augmentin showed better killing than oxacillin and cefuroxime. The mechanism of action shown by the SEM and TEM results revealed cell wall disruption, which probably created interference that led to bacterial lysis.