Comparison of five antimicrobial susceptibility tests in detecting high level aminoglycoside and vancomycin resistances in hospital acquired Enterococcus isolates.

BACKGROUND

Multiple antibiotic resistance is increasing in the genus Enterococcus. The aim of this research is to compare five different antibiotic susceptibility test methods used in enterococcal isolates.

METHODS

Disc diffusion, agar dilution, Etest, and API Enteroc 5 tests were compared with the standard antimicrobial susceptibility test (SAST) (broth microdilution) in 100 Enterococcus strains isolated from various clinical specimens.

RESULTS

The resistance rates of the isolates to antibiotics were as follows: 51% resistance to tetracycline, 38% to erythromycin, 28% to penicillin and ampicillin, 23% to high level gentamicin, 16% to high level streptomycin, 14% to chloramphenicol, 9% to ciprofloxacin, and 1% to nitrofurantoine, as determined by SAST. Except a moderate-susceptible vancomycin strain, all strains were found to be susceptible to vancomycin and teicoplanin. None of the strains examined had beta-lactamase enzyme activity. Of significance, one major error in detecting high level gentamicin and two serious errors in detecting high level streptomycin resistances were detected when API was compared with the SAST method. Major errors were also detected in penicillin, erythromycin, tetracycline, ciprofloxacin, and nitrofurantoin with the API method.

CONCLUSIONS

Disc diffusion, agar dilution, SAST, and Etest methods were equally reliable for the detection of all antimicrobials studied and the disc diffusion method is considered easy to perform and inexpensive method. The API method is considered unreliable in detecting high level aminoglycoside resistance.