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

We devised an accurate procedure with which to measure the affinities of β-lactam antibiotics for penicillin-binding protein (PBP) 2' in methicillin-resistant Staphylococ-cus aureus (MRSA). In the present study, we usedtwo isogenic strains of MRSA, one heterogeneous and the other homogeneous, derived from the methicillin-susceptible strainFDA209P, 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 anincubation period of 30 min. According to these results, the precise affinities of β-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 β-lactams than the 'usual competition assay'. However, each β-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.