CoNS blood culture isolates from very preterm infants: antibiotic susceptibility, synergy testing and mecA gene expression

Objectives Previous reports suggest that neonatal sepsis with methicillin-resistant (MR) CoNS can be treated with first-generation cephalosporins. We have evaluated the in vitro effect of cefazolin and other antibiotics, synergy between β-lactams and aminoglycosides, and mecA expression in MR-CoNS isolates. Patients and methods Antimicrobial susceptibility testing was performed in 167 CoNS isolates from 135 very preterm infants (<32 weeks gestation) with late-onset sepsis. Microbroth dilution chequerboard assay explored antibiotic synergy. PCR detected mecA and three aminoglycoside-modifying enzyme genes. Induction experiments evaluated if exposure to cefazolin induces resistance. RT-PCR determined mecA expression in selected MR-CoNS isolates with different cefazolin/oxacillin MICs. Results All CoNS isolates were susceptible to amikacin and vancomycin. Most were phenotypically resistant to gentamicin (92%) and cefoxitin (93%), and 89% carried the mecA gene. MIC90 values of cefazolin and oxacillin were 16 mg/L and >256 mg/L, respectively. At the cefazolin breakpoint of 8 mg/L, 83% of isolates were susceptible in vitro. No antibiotic combinations showed synergy using the FIC index, but cefazolin combined with aminoglycosides had favourable susceptibility breakpoint index values. In a simplified synergy screen of ‘highly resistant’ isolates, growth was inhibited with a combination of gentamicin and cefazolin at sub-MIC concentrations in 8/42 (19%) isolates. Exposure to cefazolin sub-MICs caused reduced susceptibility in 1/30 (3%) tested isolates. The highest mecA expression was observed in isolates with high oxacillin/cefazolin MICs. Conclusions CoNS isolates had lower MICs for cefazolin than oxacillin, and 83% were in vitro susceptible to cefazolin. A clinical trial comparing cefazolin with vancomycin as empirical therapy for suspected neonatal sepsis is warranted.