Characterization of a Bacteroides mobilizable transposon, NBU2, which carries a functional lincomycin resistance gene.

The mobilizable Bacteroides element NBU2 (11 kbp) was found originally in two Bacteroides clinical isolates, Bacteroides fragilis ERL and B. thetaiotaomicron DOT. At first, NBU2 appeared to be very similar to another mobilizable Bacteroides element, NBU1, in a 2.5-kbp internal region, but further examination of the full DNA sequence of NBU2 now reveals that the region of near identity between NBU1 and NBU2 is limited to this small region and that, outside this region, there is little sequence similarity between the two elements. The integrase gene of NBU2, intN2, was located at one end of the element. This gene was necessary and sufficient for the integration of NBU2. The integrase of NBU2 has the conserved amino acids (R-H-R-Y) in the C-terminal end that are found in members of the lambda family of site-specific integrases. This was also the only region in which the NBU1 and NBU2 integrases shared any similarity (28% amino acid sequence identity and 49% sequence similarity). Integration of NBU2 was site specific in Bacteroides species. Integration occurred in two primary sites in B. thetaiotaomicron. Both of these sites were located in the 3* end of a serine-tRNA gene NBU2 also integrated in Escherichia coli, but integration was much less site specific than in B. thetaiotaomicron. Analysis of the sequence of NBU2 revealed two potential antibiotic resistance genes. The amino acid sequences of the putative proteins encoded by these genes had similarity to resistances found in gram-positive bacteria. Only one of these genes was expressed in B. thetaiotaomicron, the homolog of linA, a lincomycin resistance gene from Staphylococcus aureus. To determine how widespread elements related to NBU1 and NBU2 are in Bacteroides species, we screened 291 Bacteroides strains. Elements with some sequence similarity to NBU2 and NBU1 were widespread in Bacteroides strains, and the presence of linAN in Bacteroides strains was highly correlated with the presence of NBU2, suggesting that NBU2 has been responsible for the spread of this gene among Bacteroides strains. Our results suggest that the NBU-related elements form a large and heterogeneous family, whose members have similar integration mechanisms but have different target sites and differ in whether they carry resistance genes. Bacteroides spp. are gram-negative obligate anaerobes that comprise 20 to 30% of the normal microbiota of the human colon. Some Bacteroides species are opportunistic pathogens, and infections caused by them are becoming more difficult to treat successfully due to increasing antibiotic resistance in this genus. Bacteroides spp. have been shown to carry a plethora of self-transmissible and mobilizable elements, which are probably responsible for the spread of antibiotic resistance genes. Antibiotic resistance genes have been found on conjugative and mobilizable plasmids (21, 34, 43), conjugative transposons (CTns) and integrated elements that are mobilized by CTns (33, 35, 37). In particular, a family of CTns, exemplified by CTnDOT and CTnERL, appears to be playing an important role in transferring resistance genes among Bacteroides strains. CTns of this family not only transfer themselves but also mobilize coresident plasmids. In addition, proteins encoded on these CTns trigger in trans the excision and circularization of mobilizable integrated elements called NBUs, and they mobilize these circular forms to Bacteroides or Escherichia coli recipients (35, 49, 57). They may also mobilize other integrated elements that have been given transposon designations, such as Tn4399 (14), Tn4555 (51), and Tn5520 (60). To distinguish mobilizable elements that have been given a transposon designation from nonmobilizable Bacteroides transposons such as