Experimental evidence for Moraxella-induced penicillin neutralization in pneumococcal pneumonia.

Resistance of microorganisms to antimicrobial agents is an increasing problem in the treatment of infectious diseases. In mixed infections, an interesting development can arise when one organism protects another from being killed by an antibiotic. Unfortunately, in the case of respiratory tract infections, experimental evidence of this development is poor. In this study, mice intranasally infected with a lethal number of pneumococci and treated with a curative dose of penicillin or amoxicillin died from pneumococcal pneumonia when they were coinoculated with beta-lactamase-producing Moraxella catarrhalis. beta-lactamase-negative M. catarrhalis did not show a similar indirect pathogenic effect. Treatment with a combination of amoxicillin and the beta-lactamase inhibitor clavulanic acid was not affected by beta-lactamase-producing M. catarrhalis. These findings help explain antibiotic failure in respiratory tract infections, even though the causative microorganism is sensitive to the antibiotic in vitro.