Transferable resistance and aminoglycoside-modifying enzymes in enterococci.

Ten isolates of Streptococcus faecalis and two isolates of S. faecium were studied together with an NCTC strain of each species. Antibiotic susceptibility tests showed different patterns of high-level resistance to aminoglycosides. Three S. faecalis strains were highly resistant to gentamicin and other aminoglycosides. By means of a filter membrane technique, transfer of high-level resistance to aminoglycosides was demonstrated from S. faecalis to S. faecalis, from S. faecium to S. faecalis and from S. faecalis to S. faecium, the last of which has not previously been described. Aminoglycoside-modifying enzymes were assayed with radiolabelled cofactors. High-level resistance to gentamicin and other aminoglycosides could be attributed to the production of 2''-O-phosphotransferases, 3'-O-phosphotransferases, 6-O-adenylyltransferases and 6'-N-acetyltransferases. Other resistance mechanisms accounted for resistance in two strains of S. faecalis and one strain of S. faecium that were highly resistant only to streptomycin and one S. faecalis strain that was moderately resistant to all aminoglycosides. A low level of 6'-N-acetyltransferases was detected in the three strains of S. faecium but this did not confer high-level resistance to aminoglycosides and this trait could not be transferred.