Interactions of ceftazidime and amikacin on multiresistant Pseudomonas aeruginosa. Have they altered in the six-year period from 1988 to 1994?

Objective: The wide clinical application of the combination of ceftazidime and amikacin over recent years for infections caused by multiresistant isolates may render the combination useless in the future because of the development of resistance. The present study aimed to investigate whether the in vitro susceptibility of multiresistant Pseudomonas aeruginosa isolated in 1994 to this drug combination had changed compared with 1988 isolates. Design: Twenty-three P. aeruginosa strains isolated in 1988 and 30 in 1994, all resistant to ceftazidime, imipenem, ciprofloxacin and amikacin, were exposed in vitro to 16 mg/L of ceftazidime and 16 mg/L of amikacin, i.e. a concentration within their serum levels. Results: The enhanced killing effect of the tested combination remained stable through 1988 to 1994 involving at least half of the isolates, a finding that was independent of the minimum inhibitory concentration of both agents. However, the tested combination produced a statistically superior decrease in baseline viable cell counts of strains isolated in 1988 compared with those isolated in 1994. Conclusion: These results confirm the adequacy of the tested combination on multiresistant Pseudomonas aeruginosa despite its wide clinical application over the last 6 years, and the need for a strict antibiotic policy to prevent resistance development.