The bug of the week is....

once again, everybody's favorite E. coli! This time the culprit is once again an item of fresh produce: green onions.

More than you ever wanted to know about E. coli
E. coli is a normal part of your normal intestinal flora. In fact, E. coli produces vitamins for you in your gut. Some strains which cause disease in humans are normal organisms in the gut of other animals. E. coli O157:H7, which is the bacteria involved in the current green onion outbreak and the spinach outbreak, is a normal resident in the guts of bovines, swine and deer, to name a few hosts. If that doesn't cause you to think twice about letting your animal friends lick your face, I can tell you all about other germs and parasites you can get, like hookworm, whipworm and Salmonella.

There are two parts to the classification system for E. coli. These two parts correspond to two antigens on the surface of the bacteria and are designated by the letters "O" and "H". The O antigen is part of the bacteria's lipopolysaccharide (LPS). LPS is a part of the membrane of gram-negative bacteria. The O antigen is a carbohydrate complex that branches out of the membrane and into the space around the bacteria. The H antigen is the flagella (the German word for flagella is "hauch").

The O antigen defines the serogroup of an E. coli strain while the H antigen identifies the serotype. There are over 170 O antigens and 56 H antigens at this time. So, E. coli O157:H7 is one specific strain of E. coli. It is not, however, the only strain which causes disease.

Which brings us to the virotypes of E. coli. E. coli strains can be grouped based on their virulence factors--on the features of the bacteria which make you sick and *how* they cause illness. There are five virotypes which cause gastrointestinal illness: enteroaggregative E. coli (EAggEC or EAEC), enteroinvasive E. coli (EIEC), enteropathogenic E. coli (EPEC), enterotoxigenic E. coli (ETEC), and enterohemorrhagic E. coli (EHEC). To complicate things, EHEC are also sometimes called Shiga-toxin producing E. coli or STEC.

Enteroaggregative E. coli (EAEC)
These bacteria will bind to the mucosa of the small intestine, but do so in clumps, hence the aggregative part of the name. Other E. coli will bind uniformly. EAEC can produce a shiga-toxin like toxin and are known to cause persistent diarrhea in children. This virotype does not invade intestinal cells.

Enteroinvasive E. coli (EIEC)
As you might expect, this virotype does not just bind to intestinal cells, but actually invades the cells. After invading, the bacteria spreads between cells directly, instead of leaving one cell before entering the next. EIEC causes severe diarrhea. Although they behave virtually identically to Shigella species, EIEC does not produce Shiga toxin.

Enteropathogenic E. coli (EPEC)
This virotype also binds to the cells of the small intestine in clumps, but when EPEC binds to the cells, they cause dramatic changes in the cells. These changes in the intestinal mucosa also result in severe diarrhea, typically in children and sometimes fatal.

Enterotoxigenic E. coli (ETEC)
ETEC also adheres to the intestinal mucosa, but does not invade. It causes disease by releasing toxins which act on the mucosal cells to cause diarrhea. ETEC also causes vomiting and diarrhea. ETEC is a leading cause of severe diarrhea in infants and children in developing nations. Adults in areas where ETEC is common are less susceptible to disease due to partial immunity. Travellers' diarrhea, aka "Montezuma's Revenge", is caused by the ETEC virotype.

Enterohemorrhagic E. coli (EHEC)
These are the bugs that get everybody's attention. There are over 40 strains of EHEC, of which E. coli O157:H7 is one strain. As you probably already know, EHEC causes diarrhea as well as nausea and vomiting. In binding to the intestinal cells, EHEC causes cellular changes very like those seen in EPEC infections. EHEC produces a toxin which is virtually identical to the toxin produced by Shigella species. This is why EHEC are sometimes known as STEC (Shiga-toxin producing E. coli) or Shiga-like E. coli. One of the unique characteristics of EHEC is their association with hemolytic uremic syndrome (HUS). HUS results in acute kidney failure and can develop in children infected with EHEC. Antibiotic treatment is a major risk factor for the development of HUS in EHEC infections. Antibiotics appear to increase toxin production and thus increase disease severity.

Now you know about the various E. coli varieties that cause GI upsets. There are others which cause urinary tract infections and other diseases, but those rarely make the news headlines. Just remember, not all E. coli is bad and nearly any food product can be contaminated either in its production, processing, shipping or preparation for consumption. Room for improvement exists in most aspects of food production. Don't just blame large farms or producers. The little guys have contamination problems too. They just result in smaller outbreaks that don't make CNN and FoxNews headlines.

Re-launch Saturday
NASA is hoping to launch Discovery on Saturday evening. I'll be glued to the computer again, watching a live stream of the NASA channel. With any luck the weather will cooperate this time.


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