Susceptibility of Enterococcus faecalis to twelve antibiotics, time-kill assays, and high-level aminoglycoside resistance in a university hospital in Argentina.
A total of 201 Enterococcus faecalis strains isolated from different body sites were tested to (i) establish their antibiotic susceptibility pattern; (ii) determine the percentage of strains highly resistant (MIC greater than 2,000 micrograms/ml) to five aminoglycosides and (iii) know if the combination of penicillin or ampicillin plus an aminoglycoside is reliably synergistic for the strains with low-level resistance (MICs ranged from the break point of susceptibility for each aminoglycoside to 2,000 micrograms/ml). Erythromycin exhibited very poor activity with MIC90 greater than 128 micrograms/ml. Pefloxacin and norfloxacin had intermediate activity, inhibiting 50% of isolates at 4 micrograms/ml and 90% at 8 micrograms/ml. Trimethoprim-sulfamethoxazole (1:20) inhibited 94% of isolates at less than or equal to 2 micrograms/ml and 87.6% at less than or equal to 0.5 microgram/ml. Ampicillin, penicillin and piperacillin were the most potent agents studied. Ninety percent of strains were inhibited at 1 microgram/ml of ampicillin and 4 micrograms/ml of penicillin and piperacillin. The E. faecalis isolates were relatively or totally resistant to the aminoglycosides. Ninety six (47.8%) were highly resistant at least to one of them. High level resistance to streptomycin was found in 47.3% of all strains and was the most frequent resistance encountered; amikacin highly resistant strains were the less common and accounted for 4.5%. Low-level resistance to the aminoglycosides ranged from 50.2% (for streptomycin) to 94.5% (for amikacin). Thirty one E. faecalis isolates were selected for 24-time kill-assays. There was a good correlation between resistance to penicillin or ampicillin aminoglycoside synergy in all but 3 strains which were highly resistant. Among the strains with low-level resistance to the aminoglycosides, there was no synergy in 37 (63.8%) of 58 killing assays with each of the aminoglycosides combined with penicillin. These results demonstrate that the susceptibility to 2,000 micrograms/ml of the aminoglycoside does not assure the penicillin or ampicillin aminoglycoside synergism.
Predari
S C
SC
Departamento de Microbiología, Facultad de Medicina, Universidad de Buenos Aires, Argentina.
Gutiérrez
M A
MA
Ribas
C
C
Molinari
G S
GS
Santoianni
J E
JE
D000617:Aminoglycosides; D000900:Anti-Bacterial Agents / Q000494:pharmacology; D001118:Argentina; D003428:Cross Infection / Q000382:microbiology; D004352:Drug Resistance, Microbial; D004357:Drug Synergism; D004359:Drug Therapy, Combination / Q000494:pharmacology; D013293:Enterococcus faecalis / Q000187:drug effects* / Q000302:isolation & purification; D016908:Gram-Positive Bacterial Infections / Q000382:microbiology*; D006785:Hospitals, University; D006801:Humans; D008826:Microbial Sensitivity Tests
D000617:Aminoglycosides; D000900:Anti-Bacterial Agents