1. What are the indicator organisms detected by bacteriological analysis of water?
How are they being identified in the laboratory?
Indicator organisms are microorganisms that are nonpathogenic in nature such as
bacteria and viruses that are used as biological indicators or markers representing habitat
types or a combination of habitats. They are only utilized as surrogate to evaluate the
presence of pathogens in water analysis as they possess the same survival rate. The
indicator organisms usually detected by bacteriological analysis of water are total
coliforms, thermotolerant coliforms such as Escherichia coli and fecal streptococci.
Principle analytical techniques used for the detection of indicator organisms in
water include the Presence-Absence Tests, Membrane-Filtration Method (MF) and
Multiple-Tube or Most Probable Number Method (MPN). Each test have different
sensitivities to different indicator organisms and thus are modified based on the principle
that they follow. For the presence-absence test, coliforms are presumptively detected
based on the fact that coliforms and other pollutants should not be present in a water
sample of 100mL. Culture with lactose fermentation is observed for the latter. Conversely,
membrane filtration technique uses a vacuum to filter and obtain a sample for culture.
Identifiable colonies are then counted and expressed in numbers or colony-forming units
(CFU). Lastly, the MPN method employs a selective broth culture medium placed in
several tubes and inoculated with the water sample. The presence of coliforms will
present with lactose fermentation present in the medium, producing acid and gas which
manifests as a color change in the medium.
2. Suppose that, after confirmation of the presence of thermotolerant coliforms, a
water sample tested shows the following results:
1 × 50 mL positive
3 × 10 mL positive
Give the MPN value and interpret it according to the standard chart.
The presence of thermotolerant coliforms on the tested water sample revealed one
positive tube reaction for 50 mL of sample and three positive tube reactions for 10 mL of
the sample. Based on the standard chart, this gives a value of 9 MPN which can be
directly interpreted that the water sample contains an estimated 9 coliforms per 100 mL.
