The Bacteroides fragilis pathogenicity island is contained in a putative novel conjugative transposon

The genetic element flanking the Bacteroides fragilis pathogenicity island (BfPAI) in enterotoxigenic B. fragilis (ETBF) strain 86-5443-2-2 and a related genetic element in NCTC 9343 were characterized. The results suggested that these genetic elements are members of a new family of conjugative transposons (CTns) not described previously. These putative CTns, designated CTn86 and CTn9343 for ETBF 86-5443-2-2 and NCTC 9343, respectively, differ from previously described Bacteroides species CTns in a number of ways. These new transposons do not carry tetQ, and the excision from the chromosome to form a circular intermediate is not regulated by tetracycline; they are predicted to differ in their mechanism of transposition; and their sequences have very limited similarity with CTnDOT or other described CTns. CTn9343 is 64,229 bp in length, contains 61 potential open reading frames, and both ends contain IS21 transposases. Colony blot hybridization, PCR, and sequence analysis indicated that CTn86 has the same structure as CTn9343 except that CTn86 lacks a ∼7-kb region containing truncated integrase (int2) and rteA genes and it contains the BfPAI integrated between the mob region and the bfmC gene. If these putative CTns were to be demonstrated to be transmissible, this would suggest that the bft gene can be transferred from ETBF to nontoxigenic B. fragilis strains by a mechanism similar to that for the spread of antibiotic resistance genes.