Polymorphisms in the type A blaZ gene as determinants of the cefazolin inoculum effect in Staphylococcus aureus

Plasmids containing the wild-type (WT) or mutant type A blaZ (blaZ_A) gene (mutations at codons 226, 229, or both 226 and 229) were constructed and transformed into Staphylococcus aureus RN4220, yielding the strains WT-blaZ, M226-blaZ, M229-blaZ, and MB-blaZ. The high-inoculum cefazolin MIC was significantly lower in MB-blaZ and M226-blaZ than in WT-blaZ but not in M229-blaZ, suggesting that the single nucleotide polymorphism at codon 226 in blaZ_A contributes to the cefazolin inoculum effect.