Reduced expression of PBP-2A by neonatal mecA-positive coagulase-negative staphylococci (CoNS) blood isolates: β-lactams are useful first-line agents for the treatment of neonatal CoNS sepsis, restricting the use of vancomycin

Vancomycin use for neonatal coagulase-negative staphylococci (CoNS) sepsis is based on a high CoNS carriage rate of mecA, encoding penicillin-binding protein (PBP)-2a, with low affinity for, and associated with resistance to, β-lactam antibiotics. The relationship between mecA gene carriage, phenotypic expression of the gene by PBP-2a production and in vitro resistance to the β-lactam antibiotics oxacillin, cefazolin and amoxicillin/clavulanate was determined for 85 CoNS blood isolates randomly obtained from our collection of isolates from neonates with CoNS sepsis. The relationship between mecA gene carriage, phenotypic expression of the gene by PBP-2a production and in vitro resistance to the β-lactam antibiotics oxacillin, cefazolin and amoxicillin/clavulanate was determined for randomly obtained CoNS blood isolates from our collection of isolates from neonates with CoNS sepsis. The mecA gene was detected using multiplex PCR, and PBP-2a expression was determined using a latex agglutination (LA) test (Oxoid). β-Lactam susceptibility was determined using the Phoenix automated system and, in addition, by Etest with interpretation of MIC values according to the reference MIC breakpoints adopted from the CLSI guidelines M100-S20, Infobase™. Among 85 CoNS blood isolates, 73 (86%) were mecA positive and 12 (14%) were mecA negative. None of the mecA-negative isolates expressed PBP-2a and all were β-lactam susceptible. All mecA-positive CoNS isolates were oxacillin resistant, although most oxacillin MICs were not very high, ranging from 2 to 8 mg/L for the majority of isolates. Only 8/73 (11%) mecA-positive CoNS isolates had oxacillin MICs ≥32 mg/L (range 32 to >256 mg/L). mecA-positive CoNS blood isolates, although fully resistant to oxacillin, were almost universally susceptible to cefazolin and amoxicillin/clavulanate, which was associated with a low expression rate of PBP-2a. β-Lactam antibiotics are useful for the treatment of neonatal CoNS sepsis, reserving vancomycin for selected cases.