Coliforms are a large group of bacteria, often of intestinal origin. The coliform Escherichia coli (Figure 8) is present in the intestines of humans and other mammals. Its presence in water implies that human pathogens from faeces may also be present, and it is therefore a useful indicator organism of faecal contamination.
E. coli is a rod-shaped faecal coliform about 0.5 μm by 2–3 μm in size. The strain of E. coli usually present in humans is harmless and occurs consistently in faeces in far greater numbers than pathogenic bacteria. However, there are other strains – such as E. coli O157:H7 – that are pathogenic. These have been found in partially cooked meat and have led to deaths (Rangel et al., 2004).
Concentrations of E. coli as low as 10 cells per litre can be detected. The presence or absence of E. coli in a water sample provides an important indicator of pollution and possible risk to public health.