In vitro activity of fosfomycin-gentamicin, fosfomycin-ceftazidime, fosfomycin-imipenem and ceftazidime-gentamicin combinations against ceftazidime-resistant Pseudomonas aeruginosa.

The effects of antimicrobial combinations against ceftazidime-resistant Pseudomonas aeruginosa strains isolated from hospitalized patients were investigated. Using the checkerboard titration method, combination of fosfomycin-gentamicin, fosfomycin-ceftazidime, fosfomycin-imipenem and ceftazidime-gentamicin was synergistic against 4, 11, 38 and 39% of 22, 18, 29 and 18 strains tested respectively and additive effect of the combinations against the strains tested was 41, 33, 14 and 44%, respectively. Antagonistic effects against the isolates were noted when fosfomycin was combined with gentamicin (27%), ceftazidime (22%) and imipenem (7%). No antagonistic effect was observed in the ceftazidime-gentamicin combination.