E. coli is important to human health because it is a source of vitamins B12 and K, which it manufactures from undigested food in the large intestine. Unlike many other intestinal bacteria, E. coli can survive and grow in the presence of oxygen (although it can also grow without oxygen), which makes it a useful experimental model organism in the laboratory.
Even though E. coli is a single species of bacteria, many different varieties (called strains) of the species exist. Each has different characteristics, and while some are safe model organisms, others can cause potentially deadly disease. This is the case with E. coli 0157:H7, which is considered a dangerous pathogen which can infect humans. This strain is significantly different from the commonly used laboratory strains, which do not cause disease.
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