The major demonstrated or putative virulence factors of Shiga-toxin-produci
ng Escherichia coli (STEC) are the Shiga toxins, products of the locus of e
nterocyte effacement, and products encoded by the EHEC-hemolysin plasmid. M
olecular analysis shows that STEC acquired the majority of these virulence
factors by horizontal transfer of genetic material. In the case of Shiga to
xins, the phages encoding them are probably responsible for this transfer.
For the locus of enterocyte effacement, however, it is not clear how often
this transfer took place and which parts of the locus were involved in this
transfer. The large EHEC-hemolysin plasmid is clearly a mosaic structure,
which arose from multiple recombination events with foreign DNA. Two lineag
es of this plasmid can be distinguished, one of which is associated with ch
romosomally encoded virulence factors. Despite the wealth of information av
ailable, further comparative studies are needed to decipher definitively th
e evolution of virulence in STEC.