E.coli

Escherichia coli (abbreviated as E. coli) are a large and diverse group of bacteria. Although most strains of E. coli are harmless, others can make you sick. Some kinds of E. coli can cause diarrhea, while others cause urinary tract infections, respiratory illness and pneumonia, and other illnesses. Still other kinds of E. coli are used as markers for water contamination—so you might hear about E. coli being found in drinking water, which are not themselves harmful, but indicate the water is contaminated. It does get a bit confusing—even to microbiologists.

Some kinds of E. coli cause disease by making a toxin called Shiga toxin. The bacteria that make these toxins are called “Shiga toxin-producing” E. coli, or STEC for short. You might hear them called verocytotoxic E. coli (VTEC) or enterohemorrhagic E. coli (EHEC); these all refer generally to the same group of bacteria. The most commonly identified STEC in North America is E. coli O157:H7 (often shortened to E. coli O157 or even just “O157”). When you hear news reports about outbreaks of “E. coli” infections, they are usually talking about E. coli O157.

In addition to E. coli O157, many other kinds (called serogroups) of STEC cause disease. These other kinds are sometimes called “non-O157 STEC.” E. coli serogroups O26, O111, and O103 are the non-O157 serogroups that most often cause illness in people in the United States.

Most of what we know about STEC comes from outbreak investigations and studies of E. coli O157 infection, which was first identified as a pathogen in 1982. The non-O157 STEC are not nearly as well understood, partly because outbreaks due to them are rarely identified. As a whole, the non-O157 serogroup is less likely to cause severe illness than E. coli O157; however, some non-O157 STEC serogroups can cause the most severe manifestations of STEC illness.

People of any age can become infected. Very young children and the elderly are more likely to develop severe illness and hemolytic uremic syndrome (HUS) than others, but even healthy older children and young adults can become seriously ill.

STEC live in the guts of ruminant animals, including cattle, goats, sheep, deer, and elk. The major source for human illnesses is cattle. STEC that cause human illness generally do not make animals sick. Other kinds of animals, including pigs and birds, sometimes pick up STEC from the environment and may spread it.

Infections start when you swallow STEC—in other words, when you get tiny (usually invisible) amounts of human or animal feces in your mouth. Unfortunately, this happens more often than we would like to think about. Exposures that result in illness include consumption of contaminated food, consumption of unpasteurized (raw) milk, consumption of water that has not been disinfected, contact with cattle, or contact with the feces of infected people. Some foods are considered to carry such a high risk of infection with E. coli O157 or another germ that health officials recommend that people avoid them completely. These foods include unpasteurized (raw) milk, unpasteurized apple cider, and soft cheeses made from raw milk. Sometimes the contact is pretty obvious (working with cows at a dairy or changing diapers, for example), but sometimes it is not (like eating an undercooked hamburger or a contaminated piece of lettuce). People have gotten infected by swallowing lake water while swimming, touching the environment in petting zoos and other animal exhibits, and by eating food prepared by people who did not wash their hands well after using the toilet. Almost everyone has some risk of infection.

About the E. coli outbreak in Germany

It appears that the Escherichia coli (abbreviated as E. coli) O104:H4 strain in Germany was spread mostly by contaminated sprouts, and in just a few cases, from close contact with a sick person. E. coli are a large and diverse group of bacteria, and not all strains of E. coli bacteria are harmful. However, E. coli O104:H4 has several genetic factors that contribute to its ability to cause sickness. These factors are already well known to scientists and occur in various strains of E. coli in nature. There is no reason to think that this strain was modified intentionally.  Because of minimal person-to-person spread of this strain, there is also no reason to think it will cause a pandemic. 

E. coli O104:H4, can make a potent Shiga toxin, as can most strains in the serotype O157:H7, and some other E. coli strains. This toxin is dangerous as it can attack the body in several areas: the gut (causing bloody diarrhea), the kidneys (causing kidney failure), and sometimes the nervous system.  The toxin can cause clots to form in small blood vessels. As red blood cells try to pass through the clots they get damaged (causing anemia). To make matters worse, this E. coli O104:H4 strain also has some genes that are found in another group of E. coli called enteroaggregative E. coli. On its own, diarrhea from enteroaggregative E. coli is not usually serious, but combined with Shiga toxin it can make people very sick. More research on enteroaggregative E. coli needs to be done, but so far, what we do know comes from studies of humans with diarrhea. Little work has been done to look for it in animals or in the environment. It is possible that there are animal reservoirs of enteroaggregative E. coli, and scientists haven’t discovered them yet.

The strain of E. coli O104:H4 causing the outbreak is resistant to many antibiotics, because it has many genes that code for that resistance. Since CDC doesn’t recommend that STEC infections be treated with antibiotics, the fact that this strain is antibiotic resistant is not likely to impact the care that sick people receive. The presence of these genes simply means that the bacteria were likely, at some point in the past, to have been in environment with antibiotics in it.  Other bacteria occur with this level of resistance.  There is no reason to think that this strain has been modified intentionally to be antibiotic resistant.

• 2011 CDC