Staphylococcus aureus PBP4 Is Essential for β-Lactam Resistance in Community-Acquired Methicillin-Resistant Strains

Recent cases of infections caused by community-acquired methicillin-resistant Staphylococcus aureus (MRSA) (CA-MRSA) strains in healthy individuals have raised concerns worldwide. CA-MRSA strains differ from hospital-acquired MRSAs by virtue of their genomic background and increased virulence in animal models. Here, we show that in two common CA-MRSA isolates, USA300 and MW2 (USA400), a loss of penicillin binding protein 4 (PBP4) is sufficient to cause a 16-fold reduction in oxacillin and nafcillin resistance, thus demonstrating that mecA, encoding PBP2A, is not the sole determinant of methicillin resistance in CA-MRSA. The loss of PBP4 was also found to severely affect the transcription of PBP2 in cells after challenge with oxacillin, thus leading to a significant decrease in peptidoglycan cross-linking. Autolysis, which is commonly associated with the killing mechanism of penicillin and beta-lactams, does not play a role in the reduced resistance phenotype associated with the loss of PBP4. We also showed that cefoxitin, a semisynthetic beta-lactam that binds irreversibly to PBP4, is synergistic with oxacillin in killing CA-MRSA strains, including clinical CA-MRSA isolates. Thus, PBP4 represents a major target for drug rediscovery against CA-MRSA, and a combination of cefoxitin and synthetic penicillins may be an effective therapy for CA-MRSA infections.