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Microbiological Guidelines for Drinking Water Quality |
Infectious diseases caused by human pathogens (viruses, protozoa and parasites) are the most common and widespread health risk associated with drinking water.
Infectious diseases are transmitted primarily through human and animal excreta, particularly faeces. If there are active cases or carriers in the community, then faecal contamination of water sources will result in the causative organisms being present in the water. The use of such water for drinking or for preparing food, contact during washing or bathing and even inhalation of water vapour or aerosols may then result in infection.
There are three categories of microorganisms that have to be controlled:
 | Most important human pathogens |
| Opportunistic and other water-associated pathogens |
| Nuisance organisms |
Water intended for consumption, for preparing food and drink, or for personal hygiene should thus contain no pathogenic microorganisms for humans.
They are set by the World Health Organisation (WHO). Bacteriological quality of drinking water is controlled through the presence or not of bacteria that specifically indicate the presence of faecal pollution
Although many pathogens can be detected by suitable methods, it is easier to test for bacteria that specifically indicate the presence of faecal pollution
(E. coli, thermotolerant coliform bacteria and total coliforms) or the efficiency of water treatment and disinfection. It follows that water intended for human consumption should contain none of these bacteria. The search for microbial indicators of faecal pollution is a "fail-safe" concept; in other words, if faecal indicators are shown to be present, then it must be assumed that pathogens could also be present. For this reason, faecal indicator bacteria must never be present in treated water delivered to the consumer, and any detection should prompt immediate action to discover the cause and to take remedial action.
The most specific of the readily detectable faecal indicator bacteria and the one present in greatest numbers in faeces is Escherichia coli and it is therefore recommended as the indicator of choice for drinking-water. The thermotolerant coliform test can be used as an alternative to the test for
E.coli. Thermotolerant coliform bacteria are also recommended as indicators of the efficiency of water treatment processes in removing enteric pathogens and faecal bacteria, and for grading the quality of source waters in order to select the intensity of treatment needed. Total coliform bacteria should not be present in treated water supplies and, if found, suggest inadequate treatment, post-treatment contamination, or excessive nutrients.
| Organisms | Guideline value |
| All water intended for drinking | |
|
E. Coli or thermotolerant coliform bacteria |
Must not be detectable in any 100-ml sample |
| Treated water entering the distribution system | |
| E. Coli or thermotolerant coliform bacteria | Must not be detectable in any 100-ml sample |
| Total coliform bacteria | Must not be detectable in any 100-ml sample |
| Treated water in the distribution system | |
| E. Coli or thermotolerant coliform bacteria | Must not be detectable in any 100-ml sample |
| Total coliform bacteria | Must not be detectable in any 100-ml sample. In the case of large supplies, where sufficient samples are examined, must not be present in 95% of samples taken throughout any 12-month period |
Pathogenic agents have several properties that distinguish them from chemical pollutants:
| Pathogens are discrete and not in solution. |
| Pathogens are often clumped or adherent to suspended solids in water, so that the likelihood of acquiring an ineffective dose cannot be predicted from their average concentration in water. |
| The likelihood of a successful challenge by a pathogen, resulting in infection, depends upon the invasiveness and virulence of the pathogen, as well as upon the immunity of the individual. |
| If infection is established, pathogens multiply in their host. Certain pathogenic bacteria are also able to multiply in food or beverages, thereby perpetuating or even increasing the chances of infection. |
| Unlike many chemical agents, the dose response of pathogens is not cumulative. |
Because of these properties there is no tolerable lower limit for pathogens, and water intended for consumption, for preparing food and drink, or for personal hygiene should thus contain no agents pathogenic for humans. Pathogen-free water is attainable by selection of high-quality uncontaminated sources of water, by efficient treatment and disinfection of water known to be contaminated with human or animal faeces, and by ensuring that such water remains free from contamination during distribution to the user.
The human pathogens that can be transmitted orally by drinking water are listed in the following table, together with a summary of their health significance and main properties. Most of these pathogens are distributed worldwide. The elimination of all these agents from water intended for drinking has high priority
| Pathogen | Health significance | Persistence in water supplies | Resistance to chlorine | Relative infective dose | Important animal reservoir |
| Bacteria | |||||
| Campylobacter jejuni, C. coli | High | Moderate | Low | Moderate | Yes |
| Pathogenic Escherichia coli | High | Moderate | Low | High | Yes |
| Salmonella typhi | High | Moderate | Low | High | No |
| Other salmonellae | High | Long | Low | High | Yes |
| Shigella spp. | High | Short | Low | Moderate | No |
| Vibrio cholerae | High | Short | Low | High | No |
| Yersinia enterocolitica | High | Long | Low | High (?) | No |
| Pseudomona saeruginosae | Moderate | May multiply | Moderate | High (?) | No |
| Aeromonas spp | Moderate | May multiply | Low | High (?) | No |
| Viruses | |||||
| Adenoviruses | High | ? | Moderate | Low | No |
| Enteroviruses | High | Long | Moderate | Low | No |
| Hepatitis A | High | ? | Moderate | Low | No |
| Enterically transmitted non-A, non-B hepatitis viruses, hepatitis E | High | ? | ? | Low | No |
| Norwalk virus | High | ? | ? | Low | No |
| Rotavirus | High | ? | ? | Moderate | No (?) |
| Small round viruses | Moderate | ? | ? | Low | No |
| Protozoa | |||||
| Entamoeba histolytica | High | Moderate | High | Low | No |
| Giardia intestinalis | High | Moderate | High | Low | Yes |
| Cryptosporidium parvum | High | Long | Moderate | Low | Yes |
| Helminths | |||||
| Dracunculus medinensis | High | Moderate | Moderate | Low | Yes |
Opportunistic pathogens are naturally present in the environment and are not formally regarded as pathogens. They are able to cause disease in people with impaired local or general defence mechanisms, such as the elderly or the very young, patients with burns or extensive wounds, those undergoing immunosuppressive therapy, or those with acquired immunodeficiency syndrome (AIDS). Water used by such patients for drinking or bathing, if it contains large numbers of these organisms, can produce various infections of the skin and the mucous membranes of the eye, ear, nose, and throat. Examples of such agents are Pseudomonas aeruginosa and species of Flavobacterium, Acinetobacter, Klebsiella, Serratia, Aeromonas, and certain "slow-growing" mycobacteria.
Certain serious illnesses result from inhalation of water in which the causative organisms have multiplied because of warm temperatures and the presence of nutrients. These include Legionnaires' disease
(Legionella spp.) and those caused by the amoebae Naegleria fowleri (primary amoebic meningoencephalitis) and
Acanthamoeba spp. (amoebic meningitis, pulmonary infections).
Schistosomiasis (bilharziasis) is a major parasitic disease of tropical and sub-tropical regions, and is primarily spread by contact with water during bathing or washing. The larval stage (cercariae) released by infected aquatic snails penetrate the skin.
It is conceivable that unsafe drinking-water contaminated with soil or faeces could act as a carrier of other parasitic infections, such as balantidiasis
(Balantidium coli), and certain helminths (species of Fasciola, Fasciolopsis, Echinococcus, Spirometra, Ascaris, Trichuris, Toxocara, Necator, Ancylostoma, Strongyloides and
Taenia solum).
There are a number of unconfirmed reports of adverse health effects caused by algal toxins in drinking water:
| Hepatotoxins, produced by species of Microcystis, Osculatoria, Anabaena, and Nodularia, |
| Neurotoxins, produced by species of Anabaena, Oscillatoria, Nostoc, Cylindrospermum, and Aphanizomenon, lipopolysaccharides. |
There are a number of diverse organisms that have no public health significance but which are undesirable because they produce turbidity, taste and odour, or because they appear as visible animal life in water. As well as being aesthetically objectionable, they indicate that water treatment and the state of maintenance and repair of the system are defective. Examples include:
| seasonal blooms of cyanobacteria and other algae in reservoirs and in river waters, impeding coagulation and filtration and causing coloration and turbidity of water after filtration; |
| in waters containing ferrous and manganous salts, oxidation by iron bacteria, causing rust-coloured deposits on the walls of tanks, pipes and channels, and carry-over of deposits in the water; |
| microbial corrosion of iron and steel pipes by iron and sulfur bacteria; |
| production of objectionable tastes and odours; |
| colonization of unsuitable non-metallic fittings, pipes, jointing compounds and lining materials by microorganisms able to utilise leached organic compounds; |
| microbial growth in distribution systems encouraged by the presence of biodegradable and assimilable organic carbon in water, often released by oxidative disinfectants (chlorine, ozone); this growth may include Aeromonas spp., which can produce false positive reactions in the coliform test; |
| infestation of water mains by animal life, feeding on microbial growth in the water or on slimes, for example crustacea (Gammarus pullex, Crangonyx pseudogracilis, Cyclops spp., and Chydorus sphaericus), Asellus aquaticus, snails, mussels (Dreissena polymorpba), bryozoa (Plumatella), Nails worms, nematodes, and larvae of chironomids (Chironomus spp.) and mosquitos (Culex spp.); in warm weather, slow sand filters can sometimes discharge chironomid larvae by draw-down into the filtered water .The only positively identified health hazard from animal life in drinking-water arises with the intermediate stage of the guinea worm, Dracunculus medinensis, which parasitizes the water flea, Cyclops. |
