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EnterotoxigenicBacteroides fragilis (ETBF) strains are associated with diarrheal disease in children. These strains produce a zinc metalloprotease enterotoxin, or fragilysin, that can be detected by a cytotoxicity assay with HT-29 cells. Recently, three different isoforms or variants of the enterotoxin gene, designatedbft-1 ,bft-2 , andbft-3 , have been identified and sequenced. We used restriction fragment length polymorphism analysis of the PCR-amplified enterotoxin gene to detect the isoformsbft-1 andbft-2 orbft-3 borne by ETBF. By sequencing the portion of thebft gene corresponding to the mature toxin in some strains and applying allele-specific PCR for strains categorized asbft-2 orbft-3 , we found in our collection two strains harboringbft-3 , a variant that had been described for isolates from East Asia. Analysis of 66 ETBF strains from different sources showed thatbft-1 is the most frequent allele, being present in 65% of isolates; it is largely predominant in isolates from feces of adults, whilebft-2 is present in isolates from feces of children. This association is statistically significant (P , 0.0064). Sixteen strains were examined by Southern hybridization using, as probes, thebft and second metalloprotease genes, both included in a pathogenicity islet. Five strains were found to harbor double copies of both genes, suggesting that the whole islet was duplicated. Four of these strains, harboringbft-1 (three strains) orbft-2 (one strain), were found to produce a large amount of biologically active toxin, as determined by a cytotoxicity assay with HT-29 cells. The strains harboringbft-3 , either in a single copy or in double copies, produced the smallest amount of toxin in our collection.

journal of clinical microbiology

The Alleles of the <i>bft</i> Gene Are Distributed Differently among Enterotoxigenic <i>Bacteroides fragilis</i> Strains from Human Sources and Can Be Present in Double Copies

The Alleles of the bft Gene Are Distributed Differently among Enterotoxigenic Bacteroides fragilis Strains from Human Sources and Can Be Present in Double Copies