Paradoxical antibacterial activity of cefmenoxime against Proteus vulgaris

The growth-inhibitory effect of cefmenoxime against Proteus vulgaris was studied by using the broth dilution and paper disk diffusion methods. Cefmenoxime showed growth-inhibitory activity against Proteus vulgaris at low concentrations but not at high concentrations up to a certain limit. This paradoxical antibacterial activity was not observed with cefoperazone and cefbuperazone. The induction of beta-lactamase by cefmenoxime and the rate of hydrolysis of cefmenoxime in the culture broth were proportional to the initial concentration of this antibiotic. At high initial concentrations, cefmenoxime was rapidly inactivated. On the other hand, neither cefoperazone nor cefbuperazone was inactivated irrespective of concentration. We conclude that cefmenoxime induces beta-lactamase in P. vulgaris, perhaps accounting for its paradoxical antibacterial effect.