Detection of reduced glycopeptide susceptibility and heterogeneous resistance among Staphylococcus epidermidis isolated from blood cultures of neonates

Introduction

Coagulase-negative staphylococci (CoNS) are today the most common finding in blood cultures obtained from neonates. Since the prevalence of methicillin resistance as well as multi drug resistance is high among CoNS, vancomycin is one of few remaining therapeutic alternatives. During the 1990s staphylococci with decreased susceptibility to vancomycin has been recognized. However, these bacteria often constitute a limited fraction of the bacterial population (i.e. a heterogeneous bacterial population). In addition, slowly increasing minimum inhibitory concentration (MIC) values (MIC creep) for glycopeptides among S. aureus has been reported.

The aim of this study was to investigate if the MIC values of S. epidermidis isolated from patients with neonatal bacteraemia have changed (MIC creep) regarding glycopeptide resistance during the three last decades. In addition, the presence of heterogeneous glycopeptide resistance (HGR) was investigated.

Material & Methods

All isolates (n=313) of CoNS isolated from blood cultures obtained from neonates (< 28 days of age) from 1984 to 2008 that have been stored at -70°C were screened by TMB and in selected cases determined to species level by API32STAPH or by sequencing the rpoB gene.

Antibiotic susceptibility test of S. epidermidis isolates (n=236) was performed by Etest (bioMérieux, Sweden) by the standard method (Müller-Hinton agar, 0.5 McFarland) as well as by the macromethod (Brain heart infusion agar, 2.0 McFarland) and the newly developed glycopeptides resistance determination (GRD)-test (bioMérieux), Figure 1.

Results

All isolates were susceptible to vancomycin (VA) but 20 isolates showed decreased susceptibility to teicoplanin (TP). The mean values for the MIC (µg/ml) for VA and TP, respectively, for the three decades are shown in Table I and II. The percentage of isolates with MIC ≥ 1.5 for VA increased from 85% during the 1980s to 94% during the first decade of the 21st century and for TP with MIC ≥ 2 the percentage were 63% and 83%, respectively, Table II and III. HGR was shown among 45 isolates using the macromethod and among 128 by the GRD method, Table IV.

Conclusion

A MIC creep regarding glycopeptides among S. epidermidis isolated from blood cultures from neonates is present during the three last decades in Sweden. In addition, HGR among S. epidermidis was found. However, discordance between the two methods used was found, but the GRD test has not been evaluated for CoNS previously, only for S. aureus.