Genomic determinants of penicillin G susceptibility and cefazolin inoculum effect in methicillin-susceptible Staphylococcus aureus bloodstream isolates

Abstract Background This study aimed to describe penicillin G susceptibility, blaZ diversity, and genetic determinants of the cefazolin inoculum effect (CfzIE) among MSSA bloodstream isolates. Methods The first three non-duplicate MSSA isolates recovered each month from blood cultures at our French university hospital between January 2010 and December 2023 were selected. Whole-genome sequencing was performed using the Illumina NextSeq platform to characterize blaZ and its genetic context. Susceptibility to penicillin G and the CfzIE were assessed using phenotypic methods. Results Among 488 MSSA isolates, 373 (76.5%) carried blaZ, of which 119 (31.9%) exhibited CfzIE. Thirty BlaZ allotypes were identified overall; 26 were included in the genetic determinant analysis after exclusion of isolates with multiple or truncated blaZ. In multivariable analysis, BlaZA3 was strongly associated with CfzIE (aOR, 15.72; 95% CI, 3.33–74.24; P  Conclusion CfzIE was mainly associated with a BlaZA3-related genetic background, particularly in the presence of complete blaR1, rather than with blaZ type alone. Approximately one-quarter of MSSA bloodstream isolates were susceptible to penicillin G, and rare phenotype–genotype discrepancies were associated with truncated blaZ sequences.