Synergy with cefsulodin or piperacillin and three aminoglycosides or aztreonam against aminoglycoside resistant strains of Pseudomonas aeruginosa.

Fifty-one strains of Pseudomonas aeruginosa, with resistance to one or more amino-glycosides, were tested for synergy with cefsulodin or piperacillin plus amikacin, tobramycin, gentamicin or aztreonam by the agar dilution technique. Cefsulodin plus any one of the three aminoglycosides regardless of the degree of resistance to the aminoglycoside was synergistic against P. aeruginosa for two thirds of the isolates. In contrast, synergy rates with piperacillin were much less uniform. The highest rate of synergy with piperacillin (90.0%) was observed with gentamicin for the gentamicin resistant strains. The lowest rate of synergy was observed with piperacillin plus amikacin (32.2%) for isolates with moderate resistance to amikacin. Synergy for strains with moderate resistance to amikacin was observed more commonly with cefsulodin than with piperacillin. Synergy for strains with a known mechanism of resistance to amikacin was more common with cefsulodin regardless of the mechanism of resistance. Cefsulodin or piperacillin in combination with aztreonam was rarely synergistic (less than 12%).