Bacteriolytic combination effect of cefminox and piperacillin evaluated by turbidimetry

The bacteriolytic combination effect of cefminox (a potent bactericidal cephamycin) and piperacillin (a broad spectrum ureidopenicillin) was investigated using turbidimetry and expressed as a measure of the combination effect by the relative ratios of bacteriolytic area under the growth curves. Against 20 strains of Gram-positive and -negative bacteria, simultaneous treatment of both antibiotics showed synergy (five strains), indifference (15 strains) and no antagonistic effect. Pretreatment with piperacillin for 1 h followed by combined treatment with cefminox showed a profound enhancement of the bacteriolytic activity against 12 out of 20 strains, especially against Serratia, Enterobacter and Pseudomonas species. In contrast, pretreatment with cefminox against seven strains gave mainly an indifferent effect (four strains). The turbidimetric method gave results comparable with those obtained from the chequerboard method (FIC index), as far as Gram-positive and some of the Gram-negative bacteria (E. coli, K. pneumoniae, M. morganii) were concerned. For Serratia, Enterobacter and Pseudomonas sp., the turbidimetric method showed synergy or indifference in many cases, whereas the chequerboard method showed antagonism. Marked enhancement of lysis by the combination was ascribed at least partly to the D-amino acid side-chain of cefminox.