Characterization of a Potential Pathogenicity Island of Enterotoxigenic Bacteroides fragilis (ETBF) Strains

ABSTRACT

Enterotoxigenic B. pagilis (ETBF) strains which produce a 20 kDa zinc metalloprotease toxin (BFI) have been associated with diarrheal disease of animals and young children. Using B. pngilis toxin gene (bfi) from strain 86-5443-2-2 (piglet isolate) as a probe, the gene was identified in 74/77 human and animal ETBF strains but only 2/97 non-toxigenic B. fragilis (NTBF) strains. The region flanking bp was mapped with several restriction enzymes and 8 resriction fragments aacent to bft were used to probe colony blots of 77 KTBF and 97 NTBF strains. All 74 bft-positive ETBF strains hybridized to the 8 probes spanning a ca. 18 kb chromosomal region; however, this 18 kb region was absent in the 3 ETBF strains lacking p, and 47 of the 97 (48%) NTBF strains lacked the entire 18 kb region. Of note, the 2 NTBF strains containing btf did not have a ca. 12 kb region upstream of btfp. A ca. 9 kb fragment flanking the btf gene has been sequenced. Analysis of this data revealed several open reading frames (ORF) of which 3 are of particular interest (ORFs 1, 2 and 3). ORF1 and ORF3 encode proteins with significant homology to mobilization proteins, and ORF2 encodes a protein with significant homology to metalloprotease proteins, but only 50% similarity and 30% identity to BFf. These results suggest 1) the btf genes are flanked by at least 18 kb of DNA largely unique to ETBF strains indicating a putative pathogenic island, 2) another metalloprotease protein present in ETBF strains may contribute to the pathogenicity and variable virulence of these diarrheagenic strains and 3) the pathogenic island may be mobiTized among different Bacteroides strains, and possibly among different species of intestinal bacteria.