Of hundreds of different strains of E. coli, only those possessing certain virulence factors cause gastrointestinal disease. Some of these factors were discussed in the previous section; there is strong evidence that not all factors have been identified. Two important virulence factors, enterotoxin production and the ability to adhere to the small intestine, are coded by plasmids. These plasmids can be transferred to other E. coli organisms by conjugation, through which virulence is conferred on the recipient strain. Gastroenteritis-producing strains of E. coli often have more than one type of virulence plasmid. It is interesting that the heat-labile toxin of E. coli is antigenically closely related to the cholera toxin of Vibrio cholerae. This similarity suggests that the genes responsible for the two toxins have a common ancestry. ■ conjugation, p. 206 ■ plasmids, p. 66

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