Escherichia coli are one of many species of bacteria living in the lower intestines of mammals, known as gut flora. When located in the large intestine, it assists with waste processing, vitamin K production, and food absorption. Discovered in 1885 by Theodor Escherich, a German pediatrician and bacteriologist, E. coli are abundant: the number of individual E. coli bacteria in the faeces that a human defecates in one day averages between 100 billion and 10 trillion. However, the bacteria are not confined to the environment, and specimens have also been located, for example, on the edge of hot springs. The bacteria are Gram-negative, rodshaped, flagellated and non-spore forming. Most strains are non-pathogenic but some cause food poisoning in humans with transmission largely being through the faecal-oral route. E.coli have a circular, DNA genome of approximately 4.6 Mb but also carry plasmids.
Shiga toxin-producing E. coli (STEC) are a form of enterohaemorrhagic E.coli that cause illness ranging from mild intestinal disease to severe kidney disease. The shiga toxin can cause haemorrhagic colitis, the source of the bloody diarrhoea associated with E. coli O157: H7 infections, as well as being responsible for haemolytic uremic syndrome (HUS). Shiga toxins derive their name from the organism where they were first classified, Shigella dysenteriae. When the shiga toxin is released, it can translocate to organs other than the digestive tract such as the kidneys and central nervous system. The ability of the shiga toxins to pass through cell barriers is possibly due to the increased permeability of the intestinal epithelial cells resulting from effects of the body’s own immune system. The body increases permeability of cell barriers so that important cells of the immune system (neutrophils/PMN’s) can reach the E. coli infection. Shiga toxin may use this opportunity to break through the walls of the digestive tract, enter the blood stream, and bind white blood cells for transport to locations such as the kidney or brain.
Enterohaemorrhagic E. coli are found in humans, cattle, and goats. There are a number of E. coli serogroups that produce shiga toxin such as O157:H7, O26, O111, and O103. Typical symptoms include severe abdominal cramping, sudden onset of watery diarrhoea, frequently bloody, and sometimes vomiting and a low-grade fever. Most often the illness is mild and selflimited generally lasting 1-3 days. However, serious complications such as haemorrhagic colitis, haemolytic uremic syndrome (HUS), or post diarrhoeal thrombotic thrombocytopenic purpura (TTP) can occur in up to 10% of cases. The incubation period ranges from 1 to 8 days, and transmission is predominantly through consumption of contaminated foods.