Role of inhibition of penicillin binding proteins and cell wall cross-linking by beta-lactam antibiotics in low- and high-level methicillin resistance of Staphylococcus aureus.

We have investigated the relationships between the minimum inhibitory concentrations (MICs) of eight beta-lactams and their binding affinities of penicillin binding proteins in low- and high-level methicillin-resistant Staphylococcus aureus (MRSA). For high-level MRSA (H-MRSA), a significant correlation was found between IC50 for PBP2A and MIC, whilst no clear relationship was apparent for low-level MRSA (L-MRSA). However, the product of the IC50s for PBP2A and PBP4 significantly correlated with the MIC in L-MRSA. Since PBP4 is thought to mediate secondary cross-linking of the staphylococcal cell wall, we analyzed the effect on cell wall cross-linking of L- and H-MRSA of two representative drugs (cefoselis and flomoxef). Interestingly, the decrease in cell wall cross-linking, which was clearly observed in L-MRSA, was diminished with H-MRSA. It is concluded that for L-MRSA, a reduction in cell wall cross-linking caused by inhibiton of PBP4 may contribute to the antimicrobial activity of beta-lactam antibiotics, while for H-MRSA, inhibition of PBP2A mainly determines the antimicrobial activity. Since neither alteration of expression nor binding affinity of PBP4 were observed in these studies, unknown factors must operate to diminish the effect of PBP4 inhibition and contribute to the mechanism of high level resistance of MRSA.