High-level resistance to gentamicin in clinical isolates of enterococci

Nine clinical isolates of Streptococcus faecalis from a university hospital were found to be resistant to high levels (minimal inhibitory concentration, greater than 2,000 micrograms/ml) of gentamicin, tobramycin, amikacin, streptomycin, and kanamycin. The high-level resistance to these aminoglycosides and resistance to penicillin-aminoglycoside synergism were transferable by conjugation to a susceptible recipient strain. In three strains tested, the aminoglycoside resistance was mediated by four aminoglycoside-modifying enzymes: a 3'-phosphotransferase transferred from two strains, a 2"-phosphotransferase and a 6'-acetyltransferase transferred from all three strains, and a streptomycin adenylyltransferase transferred from two strains. This is the first report of the occurrence of high-level gentamicin resistance in enterococcal isolates from the United States.