next section

Microbiological Analysis of Drinking Water

Drinking water supplies are prone to contamination with sewage and excreted matter which can cause outbreaks of intestinal infections such as typhoid fever. Regular bacteriological tests are crucial to ensure that drinking water supplies are microbiologically safe for human consumption. Monitoring and detecting the indicators and disease-causing microorganisms are the major parts of the sanitary microbiology. Most disease-causing bacteria present in the drinking water supplies can be significantly reduced by means of chlorination method.

The major concern is about the inability to consistently remove viruses and protozoa and to achieve quality standards for these microorganisms. The contamination of water with human faecal wastes would result in viral, bacterial, and protozoan diseases. Many of these pathogens can be detected directly. The environmental microbiologists usually examine the water by focusing on the indicator organisms which are considered as the index of possible water contamination developed by human pathogens.

Numerous criteria have been stated in various studies in order to establish the most effective indicator organism among which the following prevail:

1. “The indicator bacterium should be suitable for the analysis of all types of water: tap, river, ground, impounded, recreational, estuary, sea, and waste.”
2. “The indicator bacterium should be present whenever enteric pathogens are present.”
3. “The indicator bacterium should survive longer than the hardiest enteric pathogen.”
4. “The indicator bacterium should not reproduce in the contaminated water and produce an inflated value.”
5. “The detailed procedure for the indicator should have great specificity; i.e. other bacteria should not give positive results.”
6. “The testing method should be easy to perform.”
7. “The indicator should be harmless to humans.”
8. “The level of the indicator bacterium in contaminated water should have some direct relationship to the degree of faecal pollution.”

The following are usually analysed to determine the presence/absence of commensal bacteria of intestinal origin:– Coliform group- Streptococcus faecalis- Clostridium perfringens The presence of these bacteria in the water indicates that faecal matter has entered the water supply and the water is therefore liable to contamination with more dangerous organisms. The coliform bacilli are usually the most reliable indicators of faecal pollution. They make up approximately 10% of the intestinal microorganisms of humans. They lose their viability in fresh water slowly (compared to the majority of intestinal bacterial pathogens). Escherichia coli bacteria (members of the family Enterobacteriaceae are categorised as the main species of faecal coliform group.

Coliforms are defined as facultatively anaerobic, gram-negative, nonsporing, rod-shaped bacteria that ferment lactose with gas formation within 48 hours at 35°C. There are currently numerous techniques to test coliforms in water which include the multiple tube fermentation assay (the original method). This method is based on three basic steps consisting of presumptive, confirmed, and completed tests.
The presumptive test is implemented to encourage the development of the coliforms which have been potentially stressed in the sample; it gives an estimate of the most probable number (MPN) of the present coliforms in the water. The presence of coliforms (including E.coli Enterobacter aerogenes and
klebsiella pneumoniae) may be identified by presumptive coliform test when the production of gas and turbidity is detected.

The confirmed test provides an inhibiting activity against the growth of all present microorganisms except for the existing coliforms. The completed test is carried out (if positive results detected in the tubes of the confirmed test) to determine the presence of coliform organisms and to supply quality control record.
Coliforms consist of a wide range of bacteria and their primary sources may not be the intestinal tract. In this regard, specific tests have been developed that allow water to be tested for the presence of faecal coliforms. These characteristic coliforms are derived from the intestine of warm-blooded animals and are able to grow at the restrictive temperature of 44.5°C.

The multiple tube fermentation technique is effective in the elimination of most bacterial pathogens but it is not effective against viruses and protozoan parasites. These tolerant organisms are able to destroy coliform bacteria; thus their presence in the water samples can produce inaccurate results. For example, E. coli O157 may be present in the sample while the results display the absence of faecal coliforms (due to the suppressing effect of the present viruses and parasites against coliforms). Furthermore, viruses and most protozoan parasites such as Giardia and Cryptosporidium, are resistant to chlorination and filtration.

The presence of Faecal coliforms, faecal streptococci and Clostridium perfringens in water supplies necessitates immediate action to remove the source of faecal pollution. These organisms are normally controlled through the disinfection of water.

  • Hadish Bekuretsion says:

    I am a student in medical microbiology department Msc. program. I would like to do my thesis on the microbiological analysis of hand pump drinking water.But I can not get any formula how to determine sample please help me how I can determine my sample size.
    thank you

  • hanif says:

    TBC and TFC incubation time and temperature for microbiological analysis of potable water

  • Duah Boateng Eric says:

    it is necessary to perform serial dilution in membrane filtration method for microbial analysis of watera

  • kamatchi says:

    I want the simplified procedure for doing Microbiological analysis for drinking water at water plant.

Hi, This article is not intended to be considerd as an academic reference. For peer-reviewed articles of mine please see my ResearchGate page. Thank you.
Click here to see this article
next section