The effect of rifampicin on the in-vitro activity of cefpirome or ceftazidime in combination with aminoglycosides against Pseudomonas aeruginosa

The in-vitro activity of cefpirome and ceftazidime when combined with aminoglycosides (gentamicin, amikacin, and tobramycin) in the presence and in the absence of rifampicin was evaluated against 32 isolates of Pseudomonas aeruginosa by two methods. Agar dilution susceptibilities demonstrated a marked reduction in synergy (FIC less than or equal to 0.5) when rifampicin was added to the combination. Synergy rates decreased from 59.4-84.4% without to 3.1-9.4% with the addition of rifampicin. In contrast, kill curve tests performed on two P. aeruginosa strains demonstrated synergy at 24 h when rifampicin was added to cefpirome, ceftazidime, gentamicin or a beta-lactam agent plus gentamicin combination. The addition of rifampicin to the combinations of cefpirome or ceftazidime plus gentamicin achieved a 2-log10 lower bacterial count at 24 h than that of the beta-lactam and gentamicin combination alone. When rifampicin was added to the combination cefpirome or ceftazidime plus gentamicin at different times during incubation, a greater bactericidal effect was observed when rifampicin was added at 0 and 1 h of incubation than when added later. No antagonism was observed with rifampicin when used in combination with beta-lactam agents and/or aminoglycosides.