[In vitro bactericidal activity of tobramycin and amikacin alone or in combination against Pseudomonas aeruginosa isolated from patients with cystic fibrosis].

The bactericidal kinetic of 60 P. aeruginosa isolates (40 from cystic fibrosis sputum and 20 from various origins) was studied. Liquid medium micromethod was performed. Bacteria were incubated with tobramycin and amikacin alone at several concentrations and combined with piperacillin, cefsulodin, ceftazidim, imipenem, and ciprofloxacin at concentrations obtained in vivo. When used alone, tobramycin showed the most rapid bactericidal activity, whatever the concentration used. The bactericidal activity (greater than or equal to 99.99% killing of the inoculum) was obtained in 5 hours, with 1 or 2 x MIC of the majority of the strains, with the 2 aminoglycosides. No difference was found between tobramycin and amikacin, when combinated with an antibiotic which provides a notable increase of the rapidity of the bactericidal activity. The combination of amikacin plus imipenem was more rapidly bactericidal: 48% of strains; 26% were synergistically inhibited by amikacin plus ciprofloxacin. When correlated with the susceptibility patterns of studied micro-organisms, the results were rather unpredictive.