The antibiotic resistance of penicillin-binding protein 2A of methicillin-resistant Staphylococcus aureus.

摘要

Emergence of methicillin-resistant Staphylococcus aureus (MRSA) has created challenges in treatment of nosocomial infections. The recent emergence of vancomycin-resistant MRSA (VRSA) is a disconcerting chapter in the evolution of these strains. S. aureus normally produces four PBPs, which are susceptible to modification by beta-lactam antibiotics, an event that leads to bacterial death. The gene product of mecA from MRSA is a penicillin-binding protein (PBP) designated PBP 2a. PBP 2a is refractory to inhibition by all commercially available beta-lactam antibiotics. Furthermore, PBP 2a is capable of taking over the functions of the other PBPs of S. aureus in the face of the challenge by beta-lactam antibiotics. PBP 2a was cloned and purified to homogeneity and the kinetic parameters for interactions with several beta-lactam antibiotics evaluated. The enzyme manifests resistance to beta-lactams through unfavorable changes in the microscopic rate constants for acylation (k2 ) and elevated dissociation constants (Kd). Both factors are believed to work in concert to prevent enzyme acylation by antibiotics in vivo. We demonstrated that synthetic fragments of the bacterial cell wall bind to PBP 2a in a saturable manner, causing a conformational change that makes the protein's active site more accessible to bind beta-lactam antibiotics. These observations and measurements point to a novel strategy by nature to keep the active site of PBP 2a sheltered from the inhibitory activity of the antibiotics, yet it becomes available to the polymeric cell wall by a requisite conformational change for the critical cell wall cross-linking reaction. Our incremental understanding of the interactions of PBP 2a with beta-lactams and cell wall was used to elucidate the mechanism of action for three cephaloporins active against MRSA and VRSA. We also applied our understanding of PBP 2a to study the mecA gene of S. sciuri, a gene believed to be the origin of mecA of MRSA based on epidemiological and genetic evidence. The biochemical properties of the MecA proteins from both S. aureus and S. sciuri are uncannily similar in regard to biochemical behavior in inhibition by beta-lactam antibiotics and the existence of the allosteric site for binding of peptidoglycan.