An unusual disorder but an important disorder to mention is something called the hemolytic-
uremic syndrome and this is caused by one of the toxin-producing <i>E. coli</i>, the Shiga toxin-
producing <i>E. coli</i> or STEC and the most common of the <i>E. coli</i> to do that is an organism <i>E. coli</i>
O157:H7 and this is the kind of minute point that you may want to study up on for step exams.
The incidence is, however, low. It's 5 to 10% of those who get infected with <i>E. coli</i> O157:H7
and some other <i>E. coli</i> can possess that Shiga toxin and they cause exactly the same disease.
You might also recognize since it's Shiga toxin that some <i>Shigella</i> can also cause this disease
and it turns out that Shiga toxin 2 is the most important and that's possessed by <i>Shigella</i>
<i>dysenteriae</i>. So what's the pathology of this disorder? Well, what happens is there is
deposition of complement along the endothelium of the blood stream and especially intimal
hyperplasia of the small renal arteries. I think you can see where we're going with this uremia-like
syndrome. You get occlusion of blood vessels fibrin deposits and along with this a hemorrhagic
colitis and not only that but there's a blood dyscrasia as well, a microangiopathic hemolytic
anemia. What that means is that with this endothelial problem in all the clots as blood passes
through these abnormal blood vessels, the shearing force increases and so there are
schistocytes, helmet cells and other types of red cell distraction that you can see on a
peripheral blood film. So the way it presents is usually in infancy or early childhood and on the
first day of this the patient will have abdominal cramps and non-bloody diarrhea but by day 2
they start having high volume bloody stools and interestingly enough you don't usually find that
much in the way of white blood cells in the stool. Many of the patients have severe nausea
and vomiting and test of renal function are markedly abnormal, severe renal failure.
Interestingly enough, this usually resolves completely. They will have this microangiopathic
hemolytic anemia. High fever is not characteristic and as I mention very few fecal white cells
and thankfully it has a very good prognosis. So how do you identify this particular brand of
<i>E. coli</i>? Well, <i>E. coli</i> O157:H7, the way they test for it is they use Sorbitol-MacConkey agar.
Now, first of all MacConkey agar usually has lactose in it, not the sugar sorbitol. So normally
MacConkey agar is used to differentiate between lactose-fermenting and non-lactose-
fermenting organisms. Well if you use that for this O157:H7, it would be a lactose fermenter
but it was discovered that if you substitute sorbitol instead of lactose, no lactose in this
MacConkey agar, this organism is sorbitol negative. In other words, it does not turn red and
I'm showing you that on the right of a sorbitol negative MacConkey agar to be contrasted with
the rest of the <i>E. coli</i> which are easily able to ferment sorbitol and produce a change in the
color of the dye. You then have to confirm the serotype that's causing this problem.