Conjugal transfer of the 5-nitroimidazole resistance plasmid pIP417 from Bacteroides vulgatus BV-17: characterization and nucleotide sequence analysis of the mobilization region

Abstract

Three small 5-nitroimidazole (5-Ni) resistance plasmids (pIP417, pIP419, and pIP421) from Bacteroides clinical isolates are transferable by a conjugative process during homologous or heterologous matings. The mobilization properties of pIP417 originated from strain BV-17 of Bacteroides vulgatus were studied. The plasmid was successfully introduced by in vitro conjugation into different strains of Bacteroides and Prevotella species and could be transferred back from these various strains to a plasmid-free 5-Ni-sensitive Bacteroides fragilis strain, indicating that in vivo spread of the resistance gene may occur. The transfer of plasmid pIP417 harbored by the Tc(r) strain BF-2 of B. fragilis was stimulated by low concentrations of tetracycline or chlorotetracycline. This suggests a possible role for coresident conjugative transposons in the dissemination of 5-Ni resistance among gram-negative anaerobes. The nucleotide sequence of the 2.1-kb DNA mobilization region was determined. It contains a putative origin of transfer (oriT) in an A+T-rich-region, including three inverted repeats, and two integration host factor binding sites. The two identified mobilization genes (mobA and mobB) are organized in one operon and were both required for efficient transfer. Southern blotting indicated that the mobilization region of plasmid pIP417 is closely related to that of both the erythromycin resistance plasmid pBFTM1O and the 5-Ni resistance plasmid pIP419 but not to that of the 5-Ni resistance plasmid pIP421.