Application of DNA probes for rRNA and vanA genes to investigation of a nosocomial cluster of vancomycin-resistant enterococci.

DNA probes specific for genes encoding rRNA and the glycopeptide resistance gene vanA were used to investigate a cluster of vancomycin-resistant (MICs, > 512 mg/liter) Enterococcus faecalis and Enterococcus faecium isolated from separate patients in a renal unit in a London hospital. When digested with BamHI, 12 of 13 vancomycin-resistant E. faecalis isolates exhibited a common restriction fragment length polymorphism pattern of rRNA genes (ribotype). A vanA probe hybridized with chromosomal DNA in these 12 isolates. The other isolate of vancomycin-resistant E. faecalis had a different ribotype and the vanA gene was located on plasmid DNA. These data suggest that cross-infection with a single strain of vancomycin-resistant E. faecalis occurred in most instances. In contrast, 23 vancomycin-resistant E. faecium isolates showed greater heterogeneity, comprising 8 ribotypes, suggesting that multiple strains were present in the unit. Twenty-one of these 23 isolates harbored a 24-MDa plasmid which hybridized with the vanA probe, implying that interstrain dissemination of a vancomycin resistance plasmid may have occurred among E. faecium isolates in the renal unit.