State of penicillin-binding proteins and requirements for their bactericidal interaction with β-lactam antibiotics in Serratia marcescens highly resistant to extended-spectrum β-lactams

The quantities of penicillin-binding proteins (PBPs), and sensitivity to extended-spectrum beta-lactams, were measured in isogenic strains of Serratia marcescens with high (HR) and low (LR) resistance to extended-spectrum beta-lactam antibiotics and with constitutively overproduced chromosomal beta-lactamase in the periplasm. The binding of structurally different beta-lactams to PBPs in growing resistant bacteria was determined quantitatively. In S. marcescens HR, the amounts of PBPs 3 and 6 were, respectively, 1.5 and 2 times those in strain LR and in sensitive reference strains. Sensitivities of the essential PBPs in S. marcescens LR and HR to the tested beta-lactams were identical. Only a single target, PBP 3, was highly sensitive to cefotaxime, ceftazidime and aztreonam. In contrast, three PBPs (2, 1A and 3) were highly sensitive to imipenem. In growing S. marcescens HR and LR, all antibiotics, even at fractions of their minimal growth inhibitory concentrations (MICs), bound extensively to those PBPs which were highly sensitive to them. Thus, overproduced beta-lactamase did not prevent PBP-beta-lactam interaction. Only at or above their (high) MICs did cefotaxime, ceftazidime and aztreonam bind to multiple targets. Growth inhibition of the otherwise highly resistant S. marcescens HR at the lower MIC of imipenem was correlated with the binding of this antibiotic to multiple, highly sensitive targets in the bacteria. Killing of the bacteria by inactivation of multiple targets was suggested. This assumption was supported by the synergistic killing of HR bacteria by combinations of the PBP-2-specific mecillinam with PBP-3-specific beta-lactams.