Genetic Diversity among Staphylococcus aureus Isolates Showing Oxacillin and/or Cefoxitin Resistance Not Linked to the Presence of mec Genes

Methicillin-resistant Staphylococcus aureus isolates lacking mec genes (n = 32), collected from Belgian hospitals, were characterized for their beta-lactamase production and the presence of mutations in pbp genes, the pbp4 promoter, and genes involved in penicillin-binding protein 4 overproduction (gdpP and yjbH). Twelve isolates were beta-lactamase hyperproducers (BHPs), while 12 non-BHP isolates might produce an incomplete GdpP protein. Most isolates showed nucleotide missense mutations in pbp genes. A few isolates also showed mutations in the pbp4 promoter.