[Comparison of the checkerboard and E-test methods used for the analysis of two antibiotics combination].

The aim of this study was to compare checkerboard method with E-test assay for interaction analysis of aminoglycosides in combination with other antibiotics on selected clinical bacterial strains. In the first step, MIC values of analysed antibiotics, against particular bacterial strain were established. In the next step, antibiotics interaction was analysed by checkerboard technique and E-test. We found some difficulties while comparing these two methods. The checkerboard and E-test results corresponded in about 55%. Twenty-one percentage of results obtained by both methods showed some discrepancies. In 15% of cases, because of high MICs values, comparison of the results was impossible. Some investigators declare FIC indexes, from over 0.5 up to 4, for neutral effects. Sharing this point of view, above 21% of discrepancies results agreed. In such situation, definite disagreement was observed only in 8% of obtained results. In this investigation, additive and neutral effects were dominant. The E-test technique is less-laborous than standard agar method. In this study the E-test assay indicated synergy in only one case. Because of the manner in which the E-test strips were placed on the agar (scales intersecting at the MICs) only dramatic cases of antagonism were detected. Mild cases were undetected because the inhibition zone run under the crossed strips and was therefore unreadable and interpreted as indifference. On the basis of these results, examination of interactions between antibiotics by E-test, appears to be possible alternative to checkerboard method with mentioned limitations.