Effect of combination of cefsulodin and beta-lactam antibiotics against Serratia marcescens.

The effect of cefsulodin in combination with various beta-lactam antibiotics was examined against Serratia marcescens. In vitro, the optimum ratio for all combinations tested was almost the same (cefsulodin - other antibiotic = 1:1 approximately 1:4). The combinations of cefsulodin-cefazolin and cefsulodin-cefotiam were found to have a synergistic effect and other combinations, such as cefsulodin-cefmenoxime, -ampicillin and -sulbenicillin, an additive effect with the checkerboard dilution and the fixed combination methods. The synergistic effect of cefsulodin-cefotiam was more potent than that of cefsulodin-cefazolin and the effect of both combinations was clearer with heavy than with light inoculum size. With the killing kinetic method, all combinations tested showed a synergistic effect. In vivo, the optimum combination ratios of cefsulodin-cefazolin and cefsulodin-cefotiam were 1:2 and 1:1, respectively, the protective effect of the latter combination being much stronger than that of the former. With the fixed combination method (cefsulodin - other antibiotic = 1:1 approximately 1:4), the effect of the combination of cefsulodin with all antibiotics except cefazolin and cefotiam was additive.