Synergistic activities of fortimicin A and beta-lactam antibiotics against Pseudomonas aeruginosa.

The inhibitory and bactericidal activities of fortimicin A (FTM-A) alone against Pseudomonas aeruginosa were compared with those of FTM-A in combination with beta-lactam antibiotics. As tested by the checkerboard method, most beta-lactam antibiotics tested had synergistic effects on the inhibitory activity of FTM-A against one strain of P. aeruginosa. Addition of a sublethal concentration of carbenicillin resulted in a significant increase in the rate of bacterial killing of FTM-A against P. aeruginosa. The antibacterial activity of FTM-A against 50 gentamicin-susceptible and 50 gentamicin-resistant clinical isolates of P. aeruginosa was clearly enhanced by addition of a subinhibitory concentration of carbenicillin or piperacillin.