Genetic basis of 5-nitroimidazole resistance in Bacteroides spp.

The majority of the Bacteroides spp. strains with a reduced sensitivity to metronidazole harbor specific resistance genes (nim). The nim genes, either plasmid or chromosomally determined, are transferable to sensitive strains by a conjugative process. The transcription of the nim genes is provided by outward-oriented promoters carried by different insertion sequence elements located a few base pairs upstream of the genes. The nim genes likely encode 5-nitroimidazole reductase proteins. The enzymatic reduction of the 5-nitroimidazole leads to the formation of the 5- amino derivative with a by-pass of the toxic products. The electrons required for that enzymatic reduction could be supplied by the reduced ferredoxin, as in the case of non catalytic reaction occurring in sensitive strains. The clinical consequence of these resistance genes within the Bacteroides species remains questionable.