2.4.1 Microbiological tests
As you have been reading, there are many different types of pathogenic micro-organisms that may be present in water but it would be very difficult and time-consuming to test for all of them. The source of the pathogens is usually human faeces; therefore, tests have been devised that detect the presence of faecal contamination. If faecal contamination is found, this indicates that pathogenic organisms may be present. The most widely used tests for faecal contamination are for total coliforms, faecal coliforms and Escherichia coli (E. coli). Coliforms are a group of bacteria found in human and animal faeces and also in soil. ‘Total coliforms’ includes all bacteria in this group. The presence of ‘total coliforms’ indicates contamination of some sort but, because of their relatively wide distribution, they cannot be used to confirm if the contamination is from faeces. Faecal coliforms are a sub-set of total coliforms and, as the name suggests, are typically found in faeces. E. coli is a type of faecal coliform bacterium that is commonly found in the faeces of humans and other warm-blooded animals. If E. coli is present in a water sample this indicates faecal pollution and the possible presence of pathogenic organisms; the absence of E. coli from a sample shows that the chances of faecal contamination of the water, and therefore of pathogens being present, are negligible. Thus the presence of E. coli in a water sample provides an important indicator of pollution. An indicator in this context is a biological species that tells us something about the environment.
It is important to realise that E. coli is only an indicator and its absence cannot give complete assurance that the water is safe. Some pathogens – such as Giardia, Entamoeba histolytica and some viruses – can survive in waters long after E. coli has died; therefore, the absence of E. coli will not necessarily mean that water is totally free from other organisms.