Lec (6): Indicator Microorganisms
The routine examination of environmental samples for the presence of intestinal
pathogens is often difficult and time-consuming . Thus, it has been customary to
tackle such examinations by looking first for certain indicator microorganisms
indicator microorganisms :
It's nonpathogenic bacteria occur in the feces of all warm-blooded animals.
whose presence indicates that pathogenic microorganisms may also be
present .
These bacteria can easily be isolated and quantified by simple
bacteriological methods.
Detection of these bacteria in water means that fecal contamination has
occurred and suggests that enteric pathogens may also be present.
For example, coliform bacteria, which normally occur in the intestines of all
warm-blooded animals, are excreted in great numbers in feces. In polluted water.,
coliform bacteria are found in densities roughly proportional to the degree of fecal
pollution.??? Because coliform bacteria are generally hardier than disease-causing
bacteria, their absence from water is an indication that the water is bacteriologically
safe for human consumption.
Many countries have adopted coliforms and other groups of bacteria as official
standards for drinking water, recreational bathing waters, wastewater discharges,
and various foods. Indicator microorganisms have also been used to assess the
efficacy of food processing and water and wastewater treatment processes.
Table 6.1 The Criteria for an Ideal Indicator Organismn
1. The organism should be useful for all types of water.
2. The organism should be present whenever enteric pathogens are present
3. The organism should have a reasonably longer survival time than the
hardiest enteric pathogen.
4. The organism should not grow in water.
5. The testing method should be casy to perform.
6. The density of the indicator organism should have some direct relationship
to the degree of fecal pollution.
7. The organism should be a member of the intestinal microfl ora of warm
blooded animals.
Type of Indicater Organisms
1. Total Coliforms
The coliform group, which includes Escherichia, Citrobacter, Enterobacter , and
Klebsiella species, is relatively easy to detect , this group includes all aerobic and
facultatively anaerobic, gram-negative, non spore-forming, rod-shaped bacteria that
produce gas after lactose fermentation. The coliform group has been used as the
standard for assessing fecal contamination of recreational and drinking waters since
early in the twentieth century. Through experience it has been learned that absence
of this organism in 100 ml of drinking water ensures the prevention of bacterial
waterborne disease outbreaks.
Three methods are commonly used to identify coliforms in water. These are
" the most probable number (MPN)
" the membrane filter (MF)
the presence-absence (P-A) tests.
2. Fecal Coliforms and Escherichia Coli
Although the total coliform group has served as the main indicator of water
pollution for many years, but many of the organisms in this group are not limited to
fecal sources. Thus, methods have been developed to restrict the enumeration to
coliforms that are more clearly of fecal origin ,that is, the fecal coliforms. These
organisms, which include the genera Escherichia and Klebsiella , ecal coliforms
may be detected by methods similar to those used for coliform bacteria.
