Different concentrations of metronidazole are used widely to treat protozoan and fungal infections. As an antibacterial drug, metronidazole is mainly used against anaerobes, of which the Bacteroides fragilis group is the most important in terms of the frequency of recovery and antimicrobial resistance patterns. The objective of this study was to investigate (1) in vivo metronidazole-induced modifications in the B. fragilis group reflected by altered virulence, and (2) the interference of metronidazole in cellular viability of these samples when subjected in vitro to human polymorphonuclear leukocytes (PMNs). Strains adapted to low metronidazole concentrations were observed to be more virulent, as demonstrated experimentally in mice by weight loss, quantitative evidence of tissue damage, hemorrhage and anatomopathology of spleen, liver and small intestine samples. A significant increase (P < 0.05) in mean bacterial viability rate of about 2.62-fold was observed for all the drug-adapted strains after contact with human PMNs. However, the level of this phenomenon was quite different among the tested species. These results draw attention to the risk that prolonged therapy, even with low concentrations of metronidazole, may affect the pathogenicity of Bacteroides strains, producing changes in host-bacteria relationships.