Dissemination of an Enterococcus Inc18-Like vanA plasmid associated with vancomycin-resistant Staphylococcus aureus

Abstract

Of the 9 vancomycin-resistant Staphylococcus aureus (VRSA) cases reported to date in the literature, 7 occurred in Michigan. In 5 of the 7 Michigan VRSA cases, an Inc18-like vanA plasmid was identified in the VRSA isolate and/or an associated vancomycin-resistant Enterococcus (VRE) isolate from the same patient. This plasmid may play a critical role in the emergence of VRSA. We studied the geographical distribution of the plasmid by testing 1,641 VRE isolates from three separate collections by PCR for plasmid-specific genes traA, repR, and vanA. Isolates from one collection (phase 2) were recovered from surveillance cultures collected in 17 hospitals in 13 states. All VRE isolates from 2 Michigan institutions (n = 386) and between 60 and 70 VRE isolates (n = 883) from the other hospitals were tested. Fifteen VRE isolates (3.9%) from Michigan were positive for an Inc18-like vanA plasmid (9 E. faecalis [12.5%], 3 E. faecium [1.0%], 2 E. avium, and 1 E. raffinosus). Six VRE isolates (0.6%) from outside Michigan were positive (3 E. faecalis [2.7%] and 3 E. faecium [0.4%]). Of all E. faecalis isolates tested, 6.0% were positive for the plasmid, compared to 0.6% for E. faecium and 3.0% for other spp. Fourteen of the 15 plasmid-positive isolates from Michigan had the same Tn1546 insertion site location as the VRSA-associated Inc18-like plasmid, whereas 5 of 6 plasmid-positive isolates from outside Michigan differed in this characteristic. Most plasmid-positive E. faecalis isolates demonstrated diverse patterns by PFGE, with the exception of three pairs with indistinguishable patterns, suggesting that the plasmid is mobile in nature. Although VRE isolates with the VRSA-associated Inc18-like vanA plasmid were more common in Michigan, they remain rare. Periodic surveillance of VRE isolates for the plasmid may be useful in predicting the occurrence of VRSA.

FIG. 1.

(A) Circular map of the Inc18-like vanA plasmid in the E. faecalis isolate from VRSA case 4. Genes are shown as arrows; certain partial or nonfunctional genes are shown as boxes. The putative conjugative transfer region is in gold, the vancomycin resistance genes are noted in light green, transposon genes are noted in dark blue, and the putative replication genes are in red. ORF, open reading frame. (B) Comparison of genetic organization of pWZ909 with that of Inc18 plasmids pRE25 and pIP501. The conjugative transfer region of pWZ909 is very similar to that of plasmids pRE25 from E. faecalis and pIP501 from Streptococcus agalactiae (24). The putative transfer region is shown in red with black outline, antibiotic resistance genes and replication regions are in blue, and transposon Tn1546 containing vancomycin genes is in red without outline.

FIG. 2.

PFGE analysis of SmaI-digested whole-chromosomal DNA of E. faecalis (A) and E. faecium (B) isolates with the Inc18-like vanA plasmid. The location and study phase of strain isolation are listed on the right. Isolates VRE1 and VRE4 were associated with VRSA cases 1 and 4, respectively.