MOLECULAR ANALYSIS OF GLYCOPEPTIDE-RESISTANT ENTEROCOCCUS-FAECIUM ISOLATES COLLECTED FROM MICHIGAN HOSPITALS OVER A 6-YEAR PERIOD

The purpose of this study was to evaluate the molecular relatedness of clinical isolates of glycopeptide-resistant Enterococcus faecium isol ates collected from hospitals in Michigan, A total of 379 isolates use d in this study were all vancomycin-resistant E, faecium isolates coll ected from 28 hospitals and three extended-care facilities over a 6-ye ar period from 1991 to 1996, For the 379 isolates, there were 73 pulse d-field gel electrophoresis (PFGE) strain types, Within strain types, there were as many as six restriction fragment differences. Most isola tes (70%) belonged to six strain types, which were designated M1 (36%) , M2 (3%), M3 (18%), M4 (6%), M10 (4%) and M11 (3%), PFGE strain M1 wa s cultured from 135 patients in 13 hospitals during the period 1993 to 1996. Strain type M2 was cultured from 11 patients in two hospitals d uring the period 1991 to 1992 and was not observed after 1992, Strain type M3 was cultured from 70 patients in 10 hospitals during the perio d of 1994 to 1996, Both M4 and M10 were cultured from 23 patients in t hree hospitals and from 15 patients in two hospitals, respectively, du ring 1995 to 1996, M11 was cultured from 13 patients in four hospitals during 1996, A total of 23 of 28 hospitals had evidence of clonal dis semination of some isolates. Plasmid content and hybridization analysi s done on 103 isolates from one hospital and two affiliated extended-c are facilities indicated that the strains contained from one to eight plasmids, Mating experiments indicated transfer of vancomycin resistan ce from 94 of these isolates into plasmid-free E, faecium GE-1 at tran sfer frequencies of < 10(-9) to 10(-4). Gentamicin resistance and eryt hromycin resistance were cotransferred at various frequencies, A probe for the vanA gene hybridized to the plasmids of 23 isolates and to th e chromosomes of 72 isolates, A probe for the vanB gene hybridized to the chromosomes of 8 isolates. The results of this study suggest inter - and intrahospital dissemination of vancomycin-resistant E. faecium s trains over a 6-year period in southeastern Michigan. The majority of isolates studied belonged to the same few PFGE strains, indicating tha t clonal dissemination was responsible for most of the spread of resis tance that occurred.