Activity of gatifloxacin and ciprofloxacin in combination with other antimicrobial agents

The influence of non-quinolone antimicrobial agents on the antibacterial activities of gatifloxacin and ciprofloxacin was determined using chequerboard. fractional inhibitory concentration, (FIC) and time-kill analysis methods. In the chequerboard method, the quinolones were tested in combination with ten antimicrobial agents (macrolides, aminoglycosides, p-lactams, vancomycin, rifampicin and chloramphenicol) against five bacterial strains (one strain each of Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa, Enterococcus faecalis and Streptococcus pneumoniae). In no incidence was antagonism (FIC greater than or equal to 4) or synergy (FIC less than or equal to 0.5) observed; all dual drug combinations involving gatifloxaoin or ciprofloxacin showed additivity/indifference (FIC > 0.5, < 4). By time-kill analysis, the strains were tested at a quinolone concentration equal to 8 x MIC in combination with a second antibiotic at 0.5 x its MIG. These combinations killed non-enterococcal strains at rates similar to those with quinolones alone, However, rifampicin and chloramphenicol were often antagonistic (100-fold lesser killing) to the lethal action of gatifloxacin and ciprofloxacin against E. faecalis These findings indicate that, with the exception of E. faecalis, the antibacterial activities of quinolones are generally additive/indifferent to those of other antimicrobial agents. (C) 2001 Published by Elsevier Science B.V. and international Society of Chemotherapy. All rights reserved.