Escherichia coli is one of the most familiar and common bacteria. Millions live relatively benignly in the human lower gut and their remains form a significant volume of faecal matter. However, the bacterium is readily transmitted under unhygienic conditions and can be the cause of cross-infections between individuals and can cause illness under certain conditions, for example urinary infections in the bladder.
Researchers have mapped the full genetic sequence of one strain of E. coli 0157:H7, which has caused a steadily growing number of infections worldwide since 1982. Compared with the genome of the ordinary, benign strain of E. coli, the disease-causing strain contains nearly 1,400 additional genes. It is thought that some of these genes create the toxins that make people ill, causing severe cramping and diarrhoea and, in the young and elderly, potentially deadly complications.
One strain of E.coli is so virulent that only 10 bacteria can make a human ill. In 1996 the strain infected 8,5000 people in Japan, killing eight, and 400 people in Scotland, killing at least 16.
E. coli infects about 73,000 people in the USA each year, usually through undercooked contaminated ground beef. The bacteria are also found in unpasteurized milk and juices, contaminated water, and fruits and vegetables that have been exposed to disease-causing strains of E. coli through fertilizer.