Heterogeneously vancomycin-resistant Staphylococcus epidermidis strain causing recurrent peritonitis in a dialysis patient during vancomycin therapy

Abstract

Methicillin-resistant Staphylococcus epidermidis (MRSE) was recovered over a 2-month period from the dialysis fluid of a peritoneal dialysis (PD) patient who experienced recurrent episodes of peritonitis during therapeutic and prophylactic use of vancomycin. Characterization of five consecutive MRSE isolates by molecular and microbiological methods showed that they were representatives of a single strain, had reduced susceptibility to vancomycin, did not react with DNA probes specific for the enterococcal vanA or vanB gene, and showed characteristics reminiscent of the properties of a recently described vancomycin-resistant laboratory mutant of Staphylococcus aureus. Cultures of these MRSE isolates were heterogeneous: they contained-with a frequency of 10(-4) to 10(-5)-bacteria for which vancomycin MICs were high (25 to 50 microg/ml) which could easily be selected to "take over" the cultures by using vancomycin selection in the laboratory. In contrast, the five consecutive MRSE isolates recovered from the PD patient during virtually continuous vancomycin therapy showed no indication for a similar enrichment of more resistant subpopulations, suggesting the existence of an "occult" infection site in the patient (presumably at the catheter exit site) which was not accessible to the antibiotic.

FIG. 1

PFGE patterns of isolates recovered from the PD patient. Chromosomal DNA was prepared, SmaI restricted, and separated by PFGE (see Materials and Methods). The bottom panel shows results of hybridization with a probe for vanA. E. faecium strains EFSK2 (vanB) and EFSK33 (vanA) were used as controls. l.m.w., low molecular weight.

FIG. 2

Phenotypic expression of methicillin (A), vancomycin (B), and teicoplanin (C) resistance of consecutive isolates of S. epidermidis strains recovered from the PD patient.

FIG. 3

Enrichment of S. epidermidis cultures for subpopulations of bacteria for which the vancomycin MICs were elevated. Isolate RR4 was plated for population analysis on agar containing increasing concentrations of vancomycin (○). Bacterial cells capable of forming colonies on the agar containing 6 and 25 μg of vancomycin/ml (A, 6∗ and 25∗, respectively) were picked, diluted in drug-free TSB, grown overnight, and subsequently plated for population analysis. Symbols indicate cultures grown from a colony picked from the 6-μg/ml plate (░⃞) or from the 25-μg/ml plate (■). A culture of RR4 was diluted into TSB con- taining increasing concentrations of vancomycin (eventually 25 μg/ml), grown overnight, and then plated for population analysis on agar plates containing vancomycin (B) or teicoplanin (C). Open circles (○) represent population profiles of the original culture.

FIG. 4

Selection for bacteria with increased vancomycin resistance during the bactericidal action of vancomycin. (Top panel) Effect of vancomycin (30 μg/ml added at the time indicated by arrow) on the viable titers of strains RR4 (○) and RR8 (■). (Bottom panels) Population analysis of the two strains before (○) and after (■) the massive population shift caused by vancomycin treatment.