Molecular Epidemiology of Vancomycin-Resistant Staphylococcus aureus

Clinical Center

Investigators

Abstract

Of the nine vancomycin-resistant S. aureus (VRSA) cases reported in the literature, seven occurred in Michigan (MI). An Inc18-like vanA plasmid (VA-vanA plasmid) was identified in the VRSA isolate and/or VRSA-associated vancomycin-resistant Enterococcus (VRE) for five of seven MI VRSA cases. Plasmid association could not be determined in the remaining 2 cases. We undertook a study to determine if VRE carrying the VA-vanA plasmid are more common in MI compared with 14 other states.[unreadable] [unreadable] Between 2004-07 the NIH Clinical Center Microbiology Lab served as a core lab for detecting methicillin-resistant Staphylococcus aureus (MRSA) and VRE colonization in patients in 19 Intensive Care Units (ICUs; STAR-ICU study) in 15 states. Nasal swabs for MRSA and rectal swabs for VRE were collected from ICU patients on admission and weekly thereafter and were processed in the Microbiology Lab. To determine the prevalence of VA-vanA plasmid in VRE strains, all VREs from the two Michigan healthcare facilities (HCF) (n = 386) and randomly chosen VREs from 17 HCF in 14 states (between 47 and 69 isolates per HCF; N = 884) were analyzed. Plasmids were detected by PCR amplification of traA, repR, vanA, and Tn1546 - plasmid junction sequences. Species was assigned by PCR amplification of ligase sequences (Enterococcus faecalis or Enterococcus faecium) or by 16S rRNA gene sequence analysis (other species). Plasmid-positive isolates were typed by pulsed-field gel electrophoresis (PFGE) of SmaI-digested cellular DNA.[unreadable] [unreadable] Of the 386 Michigan VREs, 291 (75%) were E. faecium, 72 (19%) were E. faecalis, and 23 (6%) were VRE other species. 14 isolates were positive for the plasmid; 8 E. faecalis, 3 E. faecium, 2 E. avium, and 1 E. raffinosus. Two plasmid-positive E. faecalis isolates demonstrated indistinguishable PFGE patterns, all other isolates demonstrated unique patterns. Of the 884 VREs from outside of MI, 704 (80%) were E. faecium, 112 (12%) were E. faecalis, and 67 (8%) were VRE other species. One E. faecium isolate was positive for the plasmid and it had the same PFGE pattern as a plasmid-positive E. faecium isolate from Michigan. Thus, VRE carrying the VA-vanA plasmid are more common in Michigan HCF than in HCF from 14 other states. This may be an explanation for more frequent occurrence of VRSA in MI.