Inducing capacity of the combinations mecillinam-ampicillin and mecillinam-ceftazidime in comparison with the capacity of the compounds administered separately

The inducing capacity of mecillinam in combination with ampicillin or ceftazidime was compared with that of the compounds administered separately and related to the inducing capacity of cefoxitin (1 and 10 micrograms/ml) for the chromosomal beta-lactamases from Enterobacter cloacae, Serratia marcescens, Citrobacter freundii, indole-positive Proteus, and Acinetobacter strains. In the majority of the strains all compounds tested alone or in combination showed a lower inducing capacity than cefoxitin at both concentrations (less than twofold); in a few strains a moderate inducing capacity was observed. In general, the inducing capacity of cefoxitin 10 micrograms/ml was similar to that of 1 microgram/ml. Only in two E. cloacae, two S. marcescens and two indole-positive Proteus strains did 10 micrograms/ml cefoxitin show a distinctly higher inducing capacity. The variation in inducing capacity within one species and between the species was remarkable.