Detection of imipenem-resistant and metronidazole-resistant Bacteroides fragilis group strains in fecal samples

Objective: To investigate the imipenem and metronidazole resistance profiles of Bacteroides fragilis group strains in fecal samples and to detect the resistance genes ( ccrA and nim ) coding for imipenem and metronidazole resistance in B. fragilis group strains.
Methods: In total, 925 fecal samples, 729 from consecutive diarrhea patients and 196 from healthy controls, were collected at Huddinge University Hospital in 1997. A modified disk diffusion method was employed to screen for imipenem-resistant and metronidazole-resistant B. fragilis group strains. In strains considered resistant by the modified disk diffusion method, the minimum inhibitory concentrations (MICs) were further determined by the agar dilution method. PCR assays were used to detect the carbapenem-hydrolyzing metallo-β-lactamase gene ( ccrA ) and the 5-nitroimidazole resistance genes ( nim ) in pure cultures (purePCR), directly from fecal samples through direct broth enrichment (dirPCR) and by immunomagnetic separation (imsPCR).
Results: Two imipenem-resistant B. fragilis strains, one of which was simultaneously resistant to metronidazole, and two B. fragilis group strains with MICs near the breakpoint for metronidazole resistance, were isolated from the fecal samples of diarrhea patients. The ccrA gene was identified in all the imipenem-resistant B. fragilis strains by purePCR, dirPCR and imsPCR. The nim genes were also detectable by these PCR assays.
Conclusions: The incidences of imipenem-resistant and metronidazole-resistant B. fragilis group strains were low in the investigated diarrhea patients. Simultaneous resistance to imipenem and metronidazole is of great concern in clinical medicine, and the proposed PCR assays may be useful in epidemiologic studies of distribution of resistance genes in the fecal microflora.