Evaluation of ciprofloxacin's synergism with other agents by multiple in vitro methods

The efficacies of ciprofloxacin, ceftizoxime, azlocillin, mezlocillin, and amikacin (minimal inhibitory concentration and minimal bactericidal concentration) against six Pseudomonas aeruginosa, six Enterobacteriaceae, and six group D streptococcal strains were evaluated using both agar and broth susceptibility methods, two inoculum sizes (5.7 log10 colony-forming units (cfu)/ml and 7.7 log10 cfu/ml), and aerobic and anaerobic incubation conditions. The results showed agreement between broth and agar methods of susceptibility determination; inoculum effects with beta-lactam antimicrobials; and decreased susceptibility to amikacin under anaerobiasis. Ciprofloxacin combined with azlocillin, ceftizoxime, or aminoglycosides in broth microdilution checkerboards against 100 gram-negative bacilli and gram-positive cocci demonstrated that ciprofloxacin combined with azlocillin or ceftizoxime was synergistic against at least 50 percent of P. aeruginosa and Serratia marcescens isolates and that ciprofloxacin combined with amikacin was synergistic against at least 50 percent of S. marcescens and Staphylococcus aureus isolates. Ciprofloxacin and azlocillin in combination were evaluated by microdilution checkerboard, agar dilution, and broth macrodilution time-kill methods at two inoculum sizes to assess antibacterial activity. Comparison between in vitro combination methods showed the following: the presence or absence of checkerboard synergism (as defined by the fractional inhibitory concentration index and the fractional bactericidal concentration index) with ciprofloxacin and azlocillin did not correlate with time-kill results; and good agreement between methods when comparing broth macrodilution time-kill (3 log10 cfu/ml or more decrease) with antimicrobial combinations at a single concentration in both agar and microdilution broth for ciprofloxacin and azlocillin. Rabbit studies using subcutaneous dialysis membrane chambers inoculated with six P. aeruginosa, six Enterobacteriaceae, and six group D streptococcal strains were performed using ciprofloxacin, azlocillin, ceftizoxime, and amikacin alone and in combination as therapy. In vitro testing of antibiotic combinations that provided the best prediction of in vivo outcome were combination antibacterial activity (3 log10 cfu/ml or more decrease) at 24 hours using either broth macrodilution time-kill or antimicrobial combinations at a single concentration in either agar or broth (microdilution). For the most efficacious in vivo combination, ciprofloxacin plus azlocillin, there was in vitro correlation with in vivo outcome for 17 of 18 isolates.