Aminoglycoside-resistant enterococci.

Abstract

Thirty-four recent clinical isolates of Streptococcus faecalis were tested for sensitivity to amoxycillin, benzylpenicillin, streptomycin, kanamycin, gentamicin, tobramycin and amikacin. Amoxycillin was 2- to 4-fold more active than benzylpenicillin and all strains were inhibited by low concentrations of the penicillins. The aminoglycosides were less active against the enterococci than were the penicillins and a significant number of strains were insensitive or relatively insensitive to one or more of the aminoglycosides. Eight (23%) strains showed a high level of resistance to streptomycin and kanamycin (MIC [minimum inhibitory concentration] > 5000 .mu.g/ml) but were sensitive to gentamicin, tobramycin and amikacin. Two strains of S. faecalis, isolated at different hospitals from patients who received topical gentamicin therapy, were relatively resistant to gentamicin (MIC 250-500 .mu.g/ml) and were less sensitive also to the other aminoglycosides. Bactericidal synergy was demonstrated by amoxycillin/aminoglycoside combinations against the enterococci, provided that the test strain of S. faecalis was sensitive to the aminoglycoside in the combination. An exception to this was the combination of amoxycillin plus amikacin which was not synergistic against kanamycin-resistant strains of S. faecalis although these organisms were sensitive to amikacin in the growth inhibition tests. The gentamicin-resistant strains showed variable responses to amoxycillin/aminoglycoside combinations in tests for bactericidal synergy and were generally less sensitive than typical strains of S. faecalis.