MULTILABORATORY EVALUATION OF SCREENING METHODS FOR DETECTION OF HIGH-LEVEL AMINOGLYCOSIDE RESISTANCE IN ENTEROCOCCI

Since the early 1970s, the synergistic activity of an aminoglycoside w ith a cell wall-active agent has been predicted by determining the abi lity of an enterococcus to grow in the presence of high levels of the aminoglycoside (usually greater than or equal to 2,000 mu g/ml). Howev er, a variety of media and concentrations of aminoglycosides has been used for this screening procedure, In the present study, we sought to optimize the agar dilution, broth microdilution, and disk diffusion te sts used to detect high-level gentamicin and streptomycin resistance i n enterococci. For dilution tests, brain heart infusion agar or broth gave the best growth and performance, For agar dilution, 500 mu g of g entamicin per ml, 2,000 mu g of streptomycin per ml, and an inoculum o f 1 x 10(6) CFU/ml were optimal, while for broth microdilution, 500 mu g of gentamicin per mi, 1,000 mu g of streptomycin per ml, and an ino culum of 5 x 10(5) CFU/ml were best, Growth of more than one colony in the agar dilution test was determined to be the best indicator of hig h-level resistance, For disk diffusion, Mueller-Hinton agar, 120-mu g gentamicin disks, and 300-mu g streptomycin disks with breakpoints of no zone for resistance and greater than or equal to 10 mm for suscepti bility gave the best sensitivity and specificity if results for strain s with zones of 7 to 9 mm are considered inconclusive, indicating that a broth or agar test should be performed to determine susceptibility or resistance.