Bacteriological analysis indicates faecal pollution of water but is also a valuable check on the efficiency of the protection or treatment measures, and is a complementary tool for the sanitary survey and sanitary inspections.
It is an indicator of water quality only at the time of sampling, and does not retain its value over time, so needs to be interpreted in the light of the sanitary survey.
Pathogenic organisms present in water.
Pathogenic organisms that may be present in the water are very numerous and varied.
The faecal-oral infections can be transmitted by either water-borne or water-washed mechanisms.
Truly water-borne transmission occurs when the pathogen is in water which is drunk by a person or animal that may become infected.
But all water-borne diseases can also be transmitted by any other route which permits faecal material to pass into the mouth ( for example via contaminated food ).
Bacteriological analysis.
There are very many pathogens that may be found in water.
It is not feasible ( or feasible only through expensive and time-consuming methods ) to test for pathogens directly.
As both pathogens in water are derived for faeces ( human and animal ), the approach is therefore to analyse the water for bacteria that show human and animal faecal contamination has occurred. These are called "indicator bacteria".
Their presence in water does not prove the presence of pathogenic organisms, but provides a strong indication of it.
The most definite indicator bacteria ( according to WHO ) used in routine assessment of the risk of faecal contamination is "Escherichia coli".
This species is present in the faeces of warm-blooded animals, including humans, and is part of the normal intestinal flora of healthy individuals.
It is abundant in human faeces ( up to 1 billion bacteria/gram of fresh matter ), and persistent enough to be detected ( its detections duration in water at 20°C varies from 1 week to 1 moth).
E.coli is the most common species in the group of "thermotolerant coliform", or "faecal coliforms" ( more than 99% of faecal coliforms in water are E.coli.
In practice , faecal coliforms are commonly used to identify faecal contamination.
Results of bacteriological analysis do not provide definite information on the presence or absence of pathogens.
Some virus and protozoa may be found in drinking-water supplies when faecal coliforms are absent, so water that contains no faecal coliforms should be seen as low risk, rather than safe.
On the other hand, faecal coliform presence indicates faecal contamination, but does not prove the presence of pathogenic organisms, even if this may be strongly suspected.
Generally, faecal coliforms are not pathogenic. However, some serotypes of E.coli ( O157:H7 ) may cause serious diseases : pathogenic strains of E.coli are responsible for urinary tract infection, bacteraemia, meningitis and diarrhoeal disease in humans.
Animals, primarily cattle and, to a leaser extent, chickens, goats and pigs, are the main reservoir for pathogenic E.coli,
Note : Total coliforms are a much larger group of coliform bacteria that are present in cold- and warm-blooded animal and human faeces, but not all total coliforms have a faecal origin, and may are present in most non-treated water sources in tropical regions, particularly in water enriched with organic matter.
They are not systematically identified, as their presence does not necessarily indicate a sanitary risk, but they can be used to measure the effectiveness of treatment processes.
Main faecal-oral ( water-borne or water-washed ) water-related infections.
( source, Cairncross & Freachem, 1990 ).