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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.

Activity interaction analysis of two antibiotics by two methods: checkerboard and "time-kill" was compared during this study. Combinations of procaine penicillin, polymyxin B and bacitracin with neomycin and procaine penicillin with dihydrostreptomycin were examined. Checkerboard method is the most widely used technique for antimicrobials interactions analyses. The "time-kill" method, performed by the broth macrodilution technique, provides a dynamic picture of antimicrobial action and interaction over time (based on serial colony counts). Differences of "time-kill" method and the checkerboard technique, allow single visual examination (after 16 to 24 hours of incubation). Additive and inhibition effects were observed in combinations of neomycin with beta-lactam antibiotic (procaine penicillin) and peptide antibiotics (bacitracin and polymyxin B) on clinical strain S. Enteritidis IL 35 "Time-kill" method also confirmed observations mentioned above. In combinations of procaine penicillin with dihydrostreptomycin on strains E. coli IL 531 and E. coli IL 256 synergy effects on checkerboard technique were noticed. Such observation was not confirmed by the "time-kill" method. The methodologies and definitions of synergism are variable and not standardized. This situation should be improved, because comparison of the results obtained by different methods becomes a very difficult task.

The aim of this study was to analyse the activity interaction of aminoglycosides (gentamicin, kanamycin, streptomycin and dihydrostreptomycin) when combined with other antibiotics (lincomycin, benzylpenicillin, amoxicillin, cephalexin, spectinomycin and erythromycin), on selected clinical bacterial strains. The checkerboard method has been selected from the traditional assays for the measurement of antibiotic interaction. Checkerboard results for all strains demonstrated synergism for nine cases (9/112--8%). Additive effects were predominant--73/112--65.2%. In 12.5% neutral effects were shown, but in 11.6% of combinations FIC indexes were not possible to calculate, because of the resistance of clinical strains to the highest concentration of at least one antibiotic. The best results were achieved for combinations of dihydrostreptomycin with procaine penicillin because of higher number of cases synergy effect was observed. Antagonism of aminoglycosides and beta-lactams in case of gentamicin and amoxicillin for E. coli and E. cloacea strains were shown. Potential activity for combination of streptomycin and erythromycin was shown.