In Vitro and In Vivo Antimicrobial Activity of an Oxidative Stress-Mediated Bicyclic Menaquinone Biosynthesis Inhibitor against MRSA.

Menaquinone (MK) is an essential component in the oxidative phosphorylation pathway of Gram-positive bacteria. Drugs targeting enzymes involved in MK biosynthesis can prevent electron transfer, which leads to ATP starvation and thereby death of microorganisms. Previously, we reported a series of MenA inhibitors and demonstrated their antimicrobial activity against Gram-positive bacteria, including Methicillin-resistant Staphylococcus aureus (MRSA) and mycobacteria. These inhibitors were developed by mimicking demethylmenaquinone, a product of MenA enzymatic reaction in MK biosynthesis. In this study, compound NM4, MK biosynthesis inhibitor, inhibited the formation of MRSA biofilm and it was screened against 1952 transposon mutants to elucidate mechanisms of action; however, no resistant mutants were found. Also, compound NM4 induced the production of reactive oxygen species (ROS) by blocking electron transfer in the oxidative phosphorylation pathway as observed by MRSA growth recovery using various ROS scavengers. An oxygen consumption assay also showed that NM4 blocks the oxygen consumption by MRSA, but the addition of menaquinone (MK) restores growth of MRSA. The NM4-treated MRSA induced the expression of catalase by more than 25%, as quantified by the native gel. A pulmonary murine model exhibited that NM4 significantly reduced bacterial lung load in mice without toxicity. An NM4-resistant USA300 strain was developed to attempt to identify the targets participating in the mechanism of resistance. Our results support that respiration and oxidative phosphorylation are potential targets for developing antimicrobial agents against MRSA. Altogether, our findings suggest the potential use of MK biosynthesis inhibitors as an effective antimicrobial agent against MRSA.