A statistical evaluation of the bactericidal effects of ceftibuten in combination with aminoglycosides and ciprofloxacin.

The bactericidal effects of ceftibuten in combination with netilmicin, isepamicin or ciprofloxacin against two strains of Escherichia coli and three of Klebsiella pneumoniae were studied by the killing curve method. Interpretation of the results was made on a statistical basis by comparing the bactericidal effects observed when the antibiotics were tested alone with those when they were tested in combination. The results were also assessed by survival probabilities according to the ratio of the number of viable bacteria at time t(N(t)) to the initial number of viable bacteria (N(0)). The effects of the combinations were defined in terms of antagonism, indifference, single agonism, addition and synergy by taking account of the confidence intervals of the survival probabilities. All of the combinations exhibited bactericidal activities which were time-dependent. Synergy was demonstrated when ceftibuten was combined with the aminoglycosides, except against the two extended-spectrum beta-lactamase-producing strains of K. pneumoniae, but not when ceftibuten and ciprofloxacin were combined. Antagonism was not demonstrated with any of the combinations.