Methicillin-resistant Staphylococcus epidermidis isolates with reduced vancomycin susceptibility from bloodstream infections in a neonatal intensive care unit

Abstract

<b>Introduction.</b> Vancomycin has become the first-line therapy for most infections caused by methicillin-resistant staphylococci.<b>Aim.</b> To evaluate the vancomycin MIC, staphylococcal cassette chromosome <i>mec</i> (SCC<i>mec</i>) types and clonality of coagulase-negative staphylococci (CoNS) isolates recovered from neonates with true primary bloodstream infections (BSI).<b>Methodology.</b> CoNS isolates were prospectively recovered from blood cultures of non-repetitive patients admitted to a neonatal intensive care unit (NICU) in a tertiary-care hospital during a 3-year period. BSI was defined based on established criteria. Micro-organisms were identified phenotypically and by PCR. MIC-values for vancomycin and oxacillin were determined by broth dilution method and E-test. The SCC<i>mec</i> type conferring methicillin resistance was determined by multiplex PCR. The heterogeneous vancomycin (hV) resistance phenotype was screened on brain heart infusion agar containing 4 µg ml^-1 of vancomycin. The clonality was investigated by PFGE.<b>Results.</b> Seventy-four CoNS isolates were recovered from blood cultures of neonates during the study period but only 40 (54 %) were associated with true primary BSI. Nine (22.5%) babies died. <i>Staphylococcus epidermidis</i> was the most prevalent species (95 %; 38/40). All <i>S. epidermidis</i> isolates were methicillin-resistant (MR). SCC<i>mec</i> type IV was predominant (55.3 %; 21/38). Most (80.0 %; 32/38) isolates exhibited vancomycin MIC-values of 2-4 µg ml^-1 not associated with the SCC<i>mec</i> type or clonality. Sixteen (42.1%) isolates displayed hV resistance. All babies who died were harbouring MR-<i>S. epidermidis</i> exhibiting vancomycin MICs of 2-4 µg ml^-1.<b>Conclusion.</b> The findings of this study demonstrated that blood invasive MR-<i>S. epidermidis</i> isolates recovered at NICU tend to show decreased vancomycin susceptibility making therapy of those fragile patients difficult.