Response of high level gentamicin resistant enterococci to penicillin and gentamicin combination

ABSTRACT

In the present study the incidence of high level resistance [HLR] to gentamicin [MIC >/= 500 micro g/ml] was determined for 138 clinical enterococcal isolates, representing 108 E. faecalis, 26 E. faecium and 4 other Enterococcus spp. A 28.3% of the isolates, comprising 42.3% of E. faecium and 25.9% of E. faecalis isolates were found to be HLR to gentamicin. A 5% of the isolates were resistant to 2000 micro g/ml of gentamicin, representing one [0.9%] of E.faecalis isolates and 7[23%] of E. faecium isolates. Eight selected isolates were used to evaluate the bactericidal activity of penicillin-gentamicin combination by the time-kill curve method. Bactericidal synergy for the combination was obtained for two E. faecium isolates [No. 16 and 32] whereas a synergistic bacteriostatic activity was observed for a third E. faecium isolate [No.35], despite their resistance to penicillin alone. No synergy was obtained for the other two E.faecalis isolates [No.19 and 32]. No synergy was obtained for the other non-E. faecium isolates. Although penicillin alone was bactericidal, the two E. faecalis isolates [No.2 and 137] did not show synergistic bactericidal activity for the combination, irrespective of the level of resistance to gentamicin [No. 2 is not HLR but 137 is]. For the other Enterococcus raffinosus [isolate], although it was not HLR to gentamicin and being susceptible to penicillin neither bactericidal effect for penicillin nor for the combination was observed