FECAL INDICATOR BACTERIA
Fecal indicator bacteria, which are directly associated with fecal contamination, are used to detect the possible presence of waterborne pathogens by assessing the microbiological quality of water.
- Fecal material from warm-blooded animals may contain a variety of intestinal microorganisms (viruses, bacteria, and protozoa) that are pathogenic to humans. For example, bacterial pathogens of the Salmonella, Shigella and Vibrio can result in gastroenteritis and bacillary dysentery, typhoid fever, cholera, etc.
- The presence of E. coli in water is direct evidence of fecal contamination from warmblooded animals.
- A few strains of E. coli are pathogenic, such as E. coli O157:H7, but most strains are not.
- Densities of other indicator bacteria (total coliforms, fecal coliforms, and fecal streptococci) can be, but are not necessarily, associated with fecal contamination.
- Despite this limitation, total coliforms are used to indicate ground-water susceptibility to fecal contamination. Fecal coliforms also are used as a measure of water safety for bodycontact recreation or for consumption .
Usually, five types of fecal indicator bacteria i.e. total coliform bacteria, fecal coliform bacteria, Escherichia coli (E. coli), fecal streptococci, and enterococci are identified and quantified.
Following methods can be used to test for indicator bacteria:
- Total count of bacteria. In this method, total number of bacteria present in a milliliter of water is counted. The sample of water is diluted; 1 mL of sample water is diluted in 99 mL of sterilized water. Then 1 mL of diluted water is mixed with 10 mL of agar or gelatin (culture medium). This mixture is then kept in incubator at 37° C for 24 h or at 20°C for 48 h. After that, the sample is taken out from incubator and colonies of bacteria are counted by means of microscope. The product of the number of colonies and the dilution factor gives the total number of bacteria per mL of undiluted water sample.
- Membrane-filtration method: In this method, the sample is filtered through a sterilized membrane of special design due to which all bacteria get stained on the membrane. The member is then put in contact of culture medium in the incubator for 24 hours at 37°C.
The membrane after incubation is taken out and the colonies of bacteria are counted by means of microscope.
- Liquid broth method, using the presence-absence format or the most-probablenumber (MPN) format: In this method, the detection is done by mixing dilutions of a sample of water with lactose broth and keeping it in the incubator at for 48 h. The presence of acid or carbon dioxide gas in the test tube indicates presence of E-coli. After this, the standard statistical tables (Maccardy’s) are referred and the ‘Most Probable Number’ (MPN) of E-coli per 100 mL of water is determined. MPN is the number which represents the bacterial density which is most likely to be present.
COMPLETE ASSESSMENT OF THE QUALITY OF THE AQUATIC ENVIRONMENT
- Chemical analyses of water and aquatic organisms
- Biological tests such as toxicity tests and measurements of enzyme activities
- Descriptions of aquatic organisms including their occurrence, density, biomass, physiology and diversity
- Physical measurements of water temperature, pH, conductivity, light penetration, particle size of suspended and dissolved material, flow velocity, hydrological balance, etc. .
Following water quality parameters need to be determined to assess quality of water:
|Dissolved oxygen || Usually decreases as discharge increases. Used as a water quality indicator in most water quality models. |
|Biochemical oxygen demand (BOD) ||A measure of oxygen-reducing potential for waterborne discharges. Used in most water quality models.|
|Temperature||Often increased by discharges, especially from electric power plants. Relatively easy to model. |
|Ammonia nitrogen ||Reduces dissolved oxygen concentrations and adds nitrate to water. Can be predicted by most water quality models. |
|Algal concentration || Increases with pollution, especially nitrates and phosphates. Predicted by moderately complex models. |
|Coliform bacteria|| An indicator of contamination from sewage and animal waste |
|Nitrates|| A nutrient for algal growth and a health hazard at very high concentrations in drinking water. Predicted by moderately complex models. |
|Phosphates ||Nutrient for algal growth. Predicted by moderately complex models.|
|Toxic organic compounds|| A wide variety of organic (carbon-based) compounds can affect aquatic life and may be directly hazardous to humans. Usually very difficult to model. |
|Heavy metals||Substances containing lead, mercury, cadmium, and other metals can cause both ecological and human health problems. Difficult to model in detail. |
Table 1.7.1. Monitoring systems used to determine the quality of water in water bodies and liquid effluents