Emergence of Streptococcus faecalis isolates with high-level resistance to multiple aminocyclitol aminoglycosides.

Isolates of Streptococcus faecalis were occasionally found to be resistant to gentamicin concentrations of 500 micrograms/ml in a microdilution tray. Additional tests were performed with 28 such isolates. They all demonstrated high-level resistance (minimum inhibitory concentration greater than 2000 micrograms/ml) to gentamicin, kanamycin, sisomicin, streptomycin, and tobramycin. This high-level aminoglycoside resistance was associated with total resistance to in vitro synergism when each aminoglycoside was combined with ampicillin. In addition to ampicillin-aminoglycoside combinations, ampicillin-vancomycin, ampicillin-rifampin, rifampin-vancomycin, and vancomycin-gentamicin combinations were also tested; all failed to exhibit an in vitro synergistic-bactericidal effect against these enterococci. The emergence of enterococci with high-level resistance to multiple aminoglycosides in this clinical population is a source of grave concern with obvious therapeutic implications in situations such as endocarditis, where an in vitro bactericidal effect is thought to be necessary for a cure. No drug or combination of drugs was found to be bactericidal for these isolates. It is suspected that similar isolates of group D streptococci with high-level resistance to gentamicin may emerge in other institutions but go unrecognized. Surveillance by others for identifying this newly emerging pathogen carrying multiple resistance and a continued search for a satisfactory chemotherapeutic agent(s) are encouraged.