Phylogenetic diversity and similarity of active sites of Shiga toxin (Stx)in Shiga toxin-producing Escherichia coli (STEC) isolates from humans and animals

Nucleotide sequences of Shiga toxin (Stx) genes in STEC from various origin s were determined and characterized by phylogenetic analysis based on Shiga toxin (Stx) with those deposited in GenBank. The phylogenetic trees placed Stx1 and Stx2 into two and five groups respectively, and indicated that St x1 in sheep-origin STEC were placed into a different group from those in ot her STEC, and that Stx2 of deer-origin STEC also belonged to the unique gro up and appeared to be distantly related to human-origin STEC. On the other hand, Stx of STEC isolated from cattle, seagulls and flies were closely rel ated to those of human-origin STEC. Such a diversity of Stx suggested that STEC might be widely disseminated in many animal species, and be dependent on their host species or their habitat. In addition, the active sites in bo th toxins were compared; the active sites in both subunits of Stx in all th e animal-origin STEC were identical to those in human-origin STEC, suggesti ng that all the toxin of STEC from animals might be also cytotoxic, and the refore, such animal-origin STEC might have potential pathogenicity for huma ns.