Enterococcus faecium BM4102 was resistant to macrolide-lincosamide-streptogramin B-type (MLS) antibiotics; tetracycline-minocycline; and high levels of kanamycin, neomycin, tobramycin, and dibekacin but not gentamicin. This aminoglycoside resistance phenotype is new in enterococci. The genes conferring resistance to aminoglycosides and MLS antibiotics in this strain were carried on a plasmid, pIP810, that was self-transferable to to other Enterococcus strains. Resistance to tobramycin and structurally related aminoglycosides, kanamycin, neomycin, and dibekacin, was due to synthesis of a 4',4"-aminoglycoside nucleotidyltransferase. Homology was detected by hybridization between pIP810 DNA and a probe specific for a gene encoding an enzyme with identical site specificity in staphylococci. The bacteriostatic activity of amikacin apparently was not affected by the presence of the enzyme, although it was modified in vitro. However, the bactericidal activity of amikacin and the synergism of this aminoglycoside with penicillin were abolished
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