Methicillin-resistant Staphylococcus capitis with reduced vancomycin susceptibility causes late-onset sepsis in intensive care neonates

Abstract

Background: Coagulase-negative staphylococci, mainly Staphylococcus epidermidis, are the most frequent cause of late-onset sepsis (LOS) in the neonatal intensive care unit (NICU) setting. However, recent reports indicate that methicillin-resistant, vancomycin-heteroresistant Staphylococcus capitis could emerge as a significant pathogen in the NICU. We investigated the prevalence, clonality and vancomycin susceptibility of S. capitis isolated from the blood of NICU infants and compared these data to adult patients.

Methodology/principal findings: We conducted a retrospective laboratory-based survey of positive blood cultures in NICU infants ≥ 3 days of age (n = 527) and in adult ICU patients ≥ 18 years of age (n = 1473) who were hospitalized from 2004 to 2009 in two hospital centers in Lyon, France. S. capitis was the most frequent pathogen in NICU infants, ahead of S. epidermidis (39.1% vs. 23.5% of positive blood cultures, respectively). Conversely, S. capitis was rarely found in adult ICU patients (1.0%) compared to S. epidermidis (15.3%). S. capitis bloodstream isolates were more frequently resistant to methicillin when collected from NICU infants than from adult patients (95.6% vs. 53.3%, respectively). Furthermore, we collected and characterized 53 S. capitis bloodstream isolates from NICU infants and adult patients from six distant cities. All methicillin-resistant S. capitis isolates from NICU infants were clonally related as determined by pulsed-field gel electrophoresis. These isolates harbored a type V-related staphylococcal chromosomal cassette mec element, and constantly showed either vancomycin resistance (37.5%) or heteroresistance (62.5%). Conversely, the isolates that were collected outside of the NICU were genetically diverse and displayed much lower rates of vancomycin resistance and heteroresistance (7.7% and 23.1%, respectively).

Conclusions/significance: A clonal population of methicillin-resistant S. capitis strains has spread into several French NICUs. These isolates exhibit reduced susceptibility to vancomycin, which is the most widely used antimicrobial agent in the NICU setting.

Figure 1. Flow diagram of case selection.

(N)ICU, (neonatal) intensive care unit; CoNS, coagulase-negative staphylococci; BC, blood culture. ^aIn NICU infants, a single CoNS-positive blood culture was interpreted as possible bacteremia and included in the analysis. The CoNS-positive results that were excluded were explicitly recorded as a contaminant in the microbiology record. ^bIn adult patients, a single CoNS-positive blood culture was interpreted as a contaminant and excluded from the analysis. ^cRecords were excluded when antimicrobial susceptibility results were not available.

Figure 2. Staphylococcus capitis isolates from distant neonatal intensive care units (NICUs) are clonal.

Pulsed-field gel electrophoresis (PFGE) was applied to 53 bloodstream isolates of S. capitis that were collected from NICU infants and adult patients from cities spanning the French territory. The PFGE dendrogram was generated using the GelCompar software version 4.1. The isolates were assigned to pulsotypes using >80% similarity (vertical dashed line). Staphylococcal chromosomal cassette mec (SCCmec) typing was applied to 23 methicillin-resistant isolates representative of each pulsotype and geographic origin. All methicillin-resistant isolates from the different NICUs belonged to the same pulsotype and shared a type V-related SCCmec element, whereas methicillin-susceptible and/or non-NICU isolates were genetically diverse. ccr, chromosomal cassette recombinase; unkn., unknown (a combination of the mec complex and ccr genes has not been assigned to an SCCmec type so far).

Figure 3. Vancomycin MICs are higher in Staphylococcus capitis pulsotype NRCS-A than in other pulsotypes.

Vancomycin MICs were determined by the E-Test method. The mean vancomycin MIC was significantly higher in pulsotype NRCS-A isolates (2.8 mg/L) than in pulsotypes NRCS-B to –M isolates (1.7 mg/L), as illustrated by the shift toward the higher values of the MIC distribution curve (P<0.01, two-tailed Mann-Whitney U-test).