Molecular investigation of genetic elements contributing to metronidazole resistance in Bacteroides strains.

OBJECTIVES The aim of this study was to investigate the constitution of nim gene types, their activating insertion sequence (IS) element, their localization (plasmid or chromosome) and cfiA gene status in metronidazole-resistant Bacteroides strains (n=26) in order to examine their interchangeability. METHODS Southern hybridization and conjugative plasmid transfer were used to localize the nimA-E genes and plasmid functions. PCR was used to detect the IS elements and the cfiA genes. PCR-mapping was applied to detect the nim gene-associated IS elements. PCR-mapping products and a nimE gene-containing plasmid fragment were sequenced. RESULTS Nine of the nimA genes (12) were activated by IS1168 and nine were carried on plasmids, four of which were pIP417-like. The five nimB genes were chromosomal, and two of them were associated with IS1168 and one with IS612. Of the three nimC genes, two were activated by IS1170, and one was carried on a pIP419-like plasmid. The only nimD gene was chromosomal. The five nimE strains harboured the resistance genes on plasmids: one plasmid, pBF388c, 8.3 kb, was characterized, and a novel IS-like element was demonstrated upstream of all the nimE genes. The insertion events of some of these IS elements were restricted to certain nim gene-specific positions. The 11 chromosomal nim genes displayed a positive association with the cfiA gene-specific background. CONCLUSIONS Fourteen strains harboured the well-known genetic elements: pIP417- and pIP419-like plasmids, chromosomal nimB genes and a common nimE plasmid. However, a rate of interchangeability was also demonstrated, mostly due to combinations of nim genes and their associated IS elements harboured on different replicons.