Heterogeneity of resistance elements in clinical isolates of enterococci with high-level gentamicin resistance

High-level resistance (minimum inhibitory concentration, MIC > 1,000 mu g/m l) to gentamicin (HLGR) in enterococci is common in Taiwan. In this study, we investigated the distribution of gentamicin resistance elements in enter ococci isolated at National Taiwan University Hospital in a 1-year period, and also examined the transfer and the genetic variability of the resistanc e elements of different isolates. Among 109 isolates tested, 43 (39%) HLGR isolates were identified. HLGR was most common in Enterococcus faecium isol ates (7/15, 47%), followed by Enterococcus faecalis (34/80, 43%),Enterococc us avium (1/5, 20%), and Enterococcus casseliflavus (1/9, 11%). To understa nd the mechanism of resistance transfer, four isolates of E. faecalis and f ive isolates of E. faecium showing HLGR were studied. Transfer of resistanc e markers to a plasmid-free recipient strain of E. faecalis JH2-7 was obser ved, with transfer frequencies ranging from 10(-2) to 10(-8). All of the tr ansconjugants contained plasmids, with sizes ranging from 45 kb to larger t han 70 kb. At least three plasmid patterns were observed on digestion with HaeIII. Hybridization with a probe specific for the aac6'aph2 '' gentamicin resistance gene confirmed that all of these HLGR isolates carried a Gm(r) determinant, though the hybridization patterns of the plasmids from E. faec alis and E. faecium were different. Although many similarities exist among enterococcal Gm(r) determinants, the results suggest heterogeneity may occu r in the flanking regions of resistance elements.