Restoration of Oxacillin susceptibility in MRSA strains by NPPB

Methicillin-resistant Staphylococcus aureus (MRSA) is a critical public health issue because of its resistance to multiple antibiotics, including β-lactams such as methicillin and oxacillin. To search for antibacterial sensitizers, we have investigated the synergistic antibacterial effects of the combination of the β-lactam antibiotic oxacillin and 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB) against the MRSA strains. Checkerboard assays showed the synergistic antibacterial efficacy of NPPB combined with oxacillin against five MRSA strains. In addition, the combined effect of NPPB and oxacillin was evaluated by disc diffusion assay and time-dependent growth assay in MRSA strain ATCC 33591, and real-time PCR results showed that the combined treatment decreased the expression levels of abcA and norA genes encoding efflux transporters. NPPB also decreased the oxacillin-induced biofilm formation of ATCC 33591. Field emission transmission electron microscopy showed that the combination treatment induced extensive damage to the bacterial cell wall and membrane, leading to cell lysis. NPPB and oxacillin treated to mice for one week did not cause toxicity on the liver and kidney functions. These findings highlighted the potential of combination therapy of oxacillin and NPPB to treat MRSA infection by enhancing antibiotic efficacy and overcoming resistance.