Binding affinities of beta-lactam antibodies for penicillin-binding protein 2' in methicillin-resistant staphylococcus aureus.

We devised an accurate procedure with which to measure the affinities of beta-lactam antibiotics for penicillin-binding protein (PBP) 2' in methicillin-resistant Staphylococcus aureus (MRSA). In the present study, we used two isogenic strains of MRSA, on heterogeneous and the other homogeneous, derived from the methicillin-susceptible strain FDA209P, harbouring the mecA gene. In these MRSA strains, PBP2' was saturated by [14C]benzylpenicillin (PCG) at a concentration of 300 mg/L. In addition, the saturation of PBP2' by [14C]PCG required an incubation period of 30 min. According to these results, the precise affinities of beta-lactam antibiotics for PBP2' were determined by the 'accurate competition assay', using a high concentration of [14C]PCG and extending the reaction time. This procedure yielded lower IC50 values of beta-lactams than the 'usual competition assay'. However, each beta-lactam had almost the same affinity for PBP2' in heterogeneous and homogeneous strains. These results suggest there is a factor(s) other than PBP2' responsible for controlling resistance levels and the heterogeneity or homogeneity of MRSA strains.