Susceptibilities of gram-negative bacteria to combinations of antimicrobial agents in vitro.

Combinations of either clindamycin or erythromycin with gentamicin, colimycin, or carbenicillin were studied for their antimicrobial effects on clinical isolates of Escherichia coli, Klebsiella sp., Proteus mirabilis, Serratia marcescens, Enterobacter aerogenes, and Pseudomonas aeruginosa. No antagonistic effects of the antibiotic combinations were observed. Synergistic effects of the antibiotic combinations were noted against most strains of E. coli tested, especially when either clindamycin or erythromycin was combined with gentamicin or colimycin. Of 16 other combinations of antibiotics examined with strains of E. coli as the test organisms, only the combination of penicillin G with either gentamicin or colimycin showed consistent synergy. The marked synergy of clindamycin combined with either gentamicin or colimycin was further demonstrated by following the kinetics of the bactericidal action of the antibiotic combinations and the effects on bacterial protein synthesis.