Escherichia coli

Escherichia coli is one of the most well-known and intensively studied bacteria. Often shortened to E. coli, the bacterium was discovered in 1885 by the German bacteriologist Dr. Theodor Escherich. Initially, the bacterium was termed Bacterium coli, but later the name was changed to honor Dr. Escherich.

Escherichia coli inhabits the intestinal tract of humans and other warm-blooded mammals. It constitutes approximately 0.1% of the total bacteria in the adult intestinal tract.

Escherich was also the first person to demonstrate that the bacterium could cause diarrhea and gastroenteritis (an inflammation of the intestinal tract) in infants. That the bacterium could cause intestinal maladies was at first confusing, given that it exists naturally in the intestinal tract. However, it is now known that certain types of E. coli exist that are more capable of causing disease than other types. If these types are present in water or food that is ingested, then an infection can result. The vast majority of the many types of E. coli are harmless to humans.

E. coli has been a popular bacterium for laboratory research almost since its discovery. This is because the organism can be grown quickly on simple and inexpensive lab growth media. Also, the organism can be used to study bacterial growth in the presence of oxygen (aerobic growth) and in the absence of oxygen (anaerobic growth). The ability of E. coli to grow aerobically and anaerobically classifies the bacterium as a facultative anaerobe.

A type (or strain) of E. coli that has been the workhouse of research is designated K12. K12's biochemical behavior and structure are well known. The huge amount of structural, biochemical, genetic, and behavioral information has made E. coli indispensable as a bacterial model system for numerous studies. Hospital laboratory scientists are also concerned with E. coli, as the bacterium is the primary cause of human urinary tract infections, as well as pneumonia, and traveler's diarrhea.

In its normal habitat of the intestinal tract, E. coli is beneficial. The bacteria participate in the digestion of food, and produce vitamin K and B—complex vitamins.

When E. coli is excreted from the intestinal tract, the bacteria are able to survive only a few hours. This characteristic of rapid death was recognized at the beginning of the twentieth century, when the bacterium began to be used as an indicator of fecal pollution of water. The presence of large numbers of E. coli in water is a strong indicator of recent fecal pollution, and so the possible presence of other intestinal bacteria that cause serious disease (i.e., Vibrio, Salmonella, Shigella). Even today, E. coli remains one of the important tests of water quality

In 1975, the United States Centers for Disease Control and prevention identified a new strain of E. coli that A culture of E. coli. © Lester V. Bergman/Corbis. Reproduced by permission. was designated O157:H7. Strain O157:H7 was first linked to human disease in 1983, when it was shown to have caused two outbreaks of a severe gastrointestinal illness in the Unites States. This strain is capable of causing severe, even lethal infection. Those who recover sometimes have permanent kidney damage.

The origin of O157:H7 is not known for certain. The consensus among researchers, however, is that O157:H7 arose when a strain of E. coli living in the intestine and which was not disease causing became infected by a virus. The virus carried the genes coding for a powerful toxin called Shiga-like toxin. Thus, the E. coli acquired the ability to produce the toxin.

The toxin can destroy the cells that line the intestinal tract and can enter the bloodstream and migrate to the kidneys and liver. Severe damage to these organs can occur. The intestinal damage causes severe bleeding, which can be lethal in children and elderly people. During the summer of 2000, E. coli O157:H7 contaminated the drinking water of the town of Walkerton, Ontario, Canada. Over 2,000 people became ill and seven people died. The source of the strain was the intestinal tract of cattle, a known natural habitat of O157:H7.

E. coli can be spread to food by handling of the food with unwashed hands, particularly after using the bathroom. The solution to this spread of the bacterium is proper hand washing with soap. Other preventative measures include avoiding unpasteurized milk or apple cider, washing raw foods before consumption, and thorough cooking of ground meat (cattle carcasses can become contaminated with feces during slaughter and the bacterium can be passed on to the ground beef).

The genome sequences of several strains of E. coli have been obtained. The sequence of strain K12 has approximately 4300 protein coding regions making up about 88% of the bacterial chromosome. Most of these proteins function in getting nutrients into the cell. Much of the remainder of the genome is devoted to coding for proteins involved in processing of the nutrients to produce the energy needed for cell survival, growth, and division. The genome sequence of O157:H7 is very different from that of K12. Much of the genome of O157:H7 codes for unique proteins, over 1,300, some of which are necessary for infection. Many of these genes are thought to have been acquired from other microorganisms. Strain O157:H7 is designed to acquire genes and change quickly.