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Bacteriological testing of water [Quality assurance: Margaret Ince]

Author(s): Cairncross, Sandy  |  Ince, Margaret

Publisher: WEDC
Place of publication: Loughborough University, UK
Year: 1993

Series: WELL Fact Sheet
Collection(s): WELL

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Adapted, with permission, from: Cairncross & Feachem (1993) Environmental Health Engineering in the Tropics: an Introductory Text 2nd edition. London: John Wiley & Sons.

The potential for water-borne disease arises when water is polluted with faecal matter. Polluted water may contain pathogenic (disease-causing) faecal bacteria, viruses, or other micro-organisms. It would be far too complex to try and detect all of these on a routine basis, and in any case many of the pathogens may only be present in very small numbers or not at all. It is therefore normal practice to look for 'indicator bacteria". These are species which are always excreted in large numbers in the faeces of warm-blooded animals, whether they are healthy or sick. Their presence indicates faecal contamination; it does not prove that water-borne disease is occurring.

The convention is to use faecal coliform bacteria for this purpose. Faecal coliforms, mainly comprising Escherichia coli, are a subgroup of the total coliform group and they occur almost entirely in faeces. By contrast, other members of the coliform group can be free-living in nature and therefore their presence in water is not necessarily evidence of faecal contamination. Escherichia coli are always present in faeces; the majority are not pathogenic, although some strains can cause diarrhoea. Differentiation can be made between faecal and total coliforms by the temperature of the test. All colifors will be detected at 37oC but only faecal coliforms at 44oC.

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