An alanine permease mutation or exposure to D-cycloserine re-sensitizes MRSA to beta-lactam antibiotics

Prolonging the clinical effectiveness of β-lactams, which remain first-line antibiotics for many infections, is an important part of efforts to address antimicrobial resistance. We report here that inactivation of the predicted D-cycloserine (DCS) transporter gene cycA gene re-sensitizes MRSA to β-lactam antibiotics. The cycA mutation also resulted in hyper-susceptibility to DCS, an alanine analogue antibiotic that inhibits the alanine racemase and D-Alanine ligase enzymes required for D-Alanine incorporation into peptidoglycan. Amino acid transport studies showed that CycA functions as an alanine permease, and the cycA mutation was accompanied by increased accumulation of muropeptides with tripeptide stems lacking the terminal D-Ala-D-Ala, and reduced peptidoglycan cross linking. Exposure of MRSA to DCS was also associated with a dose-dependent accumulation of muropeptides with tripeptide stems and reduced PG crosslinking. Because impaired alanine transport or DCS-treatment have similar effects on PG structure, synergism between β-lactams and DCS was investigated. Therapeutically achievable concentrations of DCS re-sensitised MRSA to β-lactam antibiotics in vitro and significantly enhanced MRSA eradication in a mouse bacteraemia model. The susceptibility of several other ESKAPE group pathogens to β-lactams was also increased by DCS. These data reveal the potential of DCS, or new drugs targeting CycA, to facilitate the reintroduction of β-lactam antibiotics for the treatment of infections caused by MRSA.