Background: CoNS bacteraemia causes significant neonatal morbidity. Previous work has suggested that β-lactam antibiotics vary in their binding affinity to PBP2a (produced by the mecA gene) present in most CoNS.
Objectives: We evaluated cefazolin MICs for CoNS isolated in an Australian neonatal ICU (NICU) and correlated them with isolate genotype and phenotype.
Methods: Significant blood isolates from 2009 to 2017 were speciated and underwent broth microdilution testing for cefazolin, cefoxitin, oxacillin and flucloxacillin. Correlation with mecA presence and PBP2a expression was evaluated. A selection of Staphylococcus capitis isolates underwent WGS.
Results: The CoNS (n = 99) isolates were confirmed as S. capitis (n = 57), Staphylococcus epidermidis (n = 32), Staphylococcus haemolyticus (n = 2) and Staphylococcus warneri (n = 8). The MIC of cefazolin was ≤2 mg/L for 30% of isolates and 75% had an MIC of ≤8 mg/L (MIC90 = 16 mg/L). This contrasted with MIC90s of cefoxitin, oxacillin and flucloxacillin, which were all ≥32 mg/L. WGS found a number of S. capitis isolates closely related to the globally established NRCS-A clone.
Conclusions: CoNS displayed distinctly lower MIC values of cefazolin than of other agents tested. MIC variation may be related to binding affinity of PBP2a or regulation of expression of mecA by mecR1-mecI functional genes. Further, NRCS-A S. capitis strains were present in this Australian NICU before and after the unit underwent physical relocation, which raised questions about a common environmental source. It is considered justified to conduct a randomized clinical trial that assesses cefazolin versus vancomycin for management of late-onset neonatal sepsis.