•Escherichia coli O157: H7 strain causes bloody diarrhea (dysentery) and kidney failure caused by eating undercooked, contaminated hamburger, radish sprouts, and even apple juice (lack of pasteurization). 

•There are two surface structures which form the basis for serological classification system

-O-antigen of LPS : identifies serogroup of a strain

-H-flagella: identifies serotype of a strain 

•Members of serotype 086 are members of the normal micro flora while the 055 are not in the normal micro flora and are associated with disease.

•There are 6 types of E. coli virotypes

(1) Enterotoxigenic E. coli [ETEG]

(2) Enteroaggregate E. coli [EAggEC]

(3) Diffusely adhering E. coli [DAEC]

(4) Enteropathogenic E. coli [EPEC]

(5) Enterrohemorrhagic E. coli [EHEC]

(6) Enteroinvasive E. coli [EIEC]

•Escherichia coli O157: H7 is an EHEC strain which have two main virulence factors that make this strain so pathogenic.

(1) Adhesins

(2) Toxins

•Adhesins- EHEC strains have an eaeA gene which encodes for intimin (94-kDa) which helps the bacteria bind tightly to the host cells.  These are three steps involved in the process.

(1) The first stage is called non-intimate binding,  This binding is mediated by pili called bundle-forming pili (Bfp)

(2) The second stage: The bacteria attach to the host cell which causes a signal transduction event.  This activates the host cell tyrosine kinase which causes intracellular Ca ²⁺  levels to increase.

(3) The third stage is called intimate binding because pedestal-like structures from the bacteria which are composed of actin fibers form inside the host cell.  The intimate binding is mediated by intimin. 

*Mutants that do not have intimin are less virulent than the wild type.  This is an indication that intimin is an extremely important virulence factor for E. Coli O157: H7.

•The toxin that EHEC strains produce is called the Shiga toxin [Stx].

•There are two types of EHEC toxin-Stx 1 and Stx 2

These two toxins differ at the amino acid level because antibodies are able to differentiate them.

•Receptors for the toxin are found in kidney and intestinal cells.  The spreading of the Stx from the intestinal mucosa to the kidney is responsible for the kidney failure which causes at times kidney hemorrhages.  The gene that encodes for the Stx is found on a temperate bacteriophage. 

•Stx is a major virulence factor because the strain can change to a more lethal one.  It is believed that EHEC strains arose from EPEC that were able to acquire more DNA. 

•Another important characteristic of the O157: H7 strain is that it colonizes more the intestinal tracts of farm animals including cattle.  Contamination with the intestinal substances during the killing of the animal is the first step in contamination.  Then, the grinding of the meat causes further contamination leading to a contaminated hamburger because it was not cooked enough.   

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