Effect of metronidazole on the pathogenicity of resistant Bacteroides strains in gnotobiotic mice.

Metronidazole is widely used to treat protozoan and fungal infections. As an antibacterial drug, it is used mainly against anaerobes. Among anaerobes, the Bacteroides fragilis group is the most relevant in terms of frequency of recovery and antimicrobial resistance patterns. The use of metronidazole and other antimicrobial drugs induces morphological changes in this bacterial group. The present study investigated in vivo if these morphological modifications were accompanied by changes in virulence patterns by using germfree mice experimentally challenged with metronidazole-resistant Bacteroides strains before and after exposure to metronidazole. It was observed that metronidazole-resistant strains were more virulent after contact with the drug, as demonstrated by anatomicopathologic data for spleen, liver, and small intestine samples. These results suggest that long-term therapy and high metronidazole concentrations could interfere with microbial pathogenicity, resulting in changes to host-bacterium relationships.