[Comparison of ATB STREP, AMS-Vitek GPS-TA, and disk diffusion for the detection of a high level of aminoglycoside resistance in Enterococcus spp].

BACKGROUND

The absence of the synergic effect between beta-lactam and aminoglucoside versus Enterococcus is predicted by the presence of a high level of resistance to aminoglucoside. There are several commercial systems which include wells for the performance of this determination for streptomycin and gentamicin.

METHODS

A comparative study of two commercial systems, Vitek GPS-TA and ATB STREP (Biomérieux) and disk diffusion versus the Mueller-Hinton agar dilution method used as a method of reference. Two hundred ninety strains of Enterococcus spp. from blood, wounds, pus, fluids from paracentesis and urine were studied.

RESULTS

By the reference method, 17 presented high level of resistance only to gentamicin, 45 only to streptomycin, 46 to both and 182 were sensitive to the two antibiotics. The sensitivity of ATB STREP, Vitek GPS-TA and disk diffusion was respectively for high level of resistance to gentamicin of 96.8, 100 and 100% and specificity of 99.5, 99.1 and 96.5%. For high level of resistance to streptomycin sensitivity was 78.2, 100 and 100% and specificity 96, 79.4 and 72.4% respectively.

CONCLUSIONS

The authors consider the three methods valid for the determination of high level of resistance to gentamicin, presenting excellent correlation between the results of the reference methods. However, for high level of resistance to streptomycin with ATB STREP 1/5 of the resistant strains would not be detected due to low sensitivity. With Vitek GPS-TA and disk diffusion, although excellent sensitivity was presented, a high percentage of false positives were obtained due to low specificity, thus the authors recommend that before a results of resistance be decided the same should be confirmed by another alternative method (agar dilution, macrodilution in tubes or death-time curve).