Why coliforms are selected as indicator microorganisms? Discus briefly the membrane filter tests for their detection in water. What are advantages and disadvantages of this method over other methods?

Coliforms are selected as indicator microorganisms primarily due to their unique characteristics that make them suitable for indicating the potential presence of pathogenic (disease-causing) bacteria in water. The membrane filter test is a common method used for their detection.

Why Coliforms are Selected as Indicator Microorganisms

1. Presence in Intestinal Tract: Coliforms are commonly found in the intestinal tracts of humans and animals, where many pathogens also originate.
2. Survival Outside Host: They can survive outside the body, though they do not multiply in water, making them good indicators of fecal contamination.
3. Ease of Detection: Coliforms are relatively easy and inexpensive to detect compared to many pathogens.
4. Correlation with Pathogens: The presence of coliforms often correlates with the presence of more harmful microorganisms, indicating potential health risks.
5. Safety: They are not typically pathogenic, making them safe to work with in laboratory settings.

Membrane Filter Test for Coliform Detection

Procedure

1. Sample Filtration: A known volume of water is passed through a membrane filter which retains bacteria.
2. Incubation: The filter is placed on a selective growth medium and incubated.
3. Coliform Identification: After incubation, coliforms form characteristic colonies that can be counted.

Growth Medium

• Selective Medium: Typically, a lactose-based medium is used. Coliforms ferment lactose, producing acid and gas, which leads to the formation of distinctive colonies.
• Differential Aspects: The medium may contain indicators that change color in response to acid production, aiding in the identification of coliforms.

Advantages of the Membrane Filter Method

1. Sensitivity: Can detect low levels of coliforms due to the concentration of bacteria on the filter.
2. Speed: Faster than some traditional methods, such as the multiple-tube fermentation technique.
3. Direct Counting: Allows for direct counting of coliform colonies, providing quantitative results.
4. Less Labor-Intensive: Simplifies the testing process compared to more labor-intensive methods.
5. Specificity: The use of selective media enhances the specificity for coliforms.

Disadvantages of the Membrane Filter Method

1. Susceptibility to Interference: High levels of non-coliform bacteria can interfere with the isolation and counting of coliform colonies.
2. Limited to Water with Low Turbidity: Particulate matter in turbid water can clog the filter and interfere with the accuracy of the test.
3. Skill Required: Requires technical skill and understanding to interpret results correctly.
4. Equipment Dependency: Relies on the availability of incubation and filtration equipment.
5. Not Suitable for All Water Types: May not be suitable for waters with very high bacterial counts, as the filter can become overloaded.

Comparison with Other Methods

1. Multiple-Tube Fermentation (MTF): The MTF method is more labor-intensive and time-consuming than the membrane filter method. However, it is more suitable for turbid waters and does not require the same level of technical skill for interpretation.
2. Presence-Absence (P-A) Test: The P-A test is simpler but provides less quantitative information compared to the membrane filter method. It is useful for preliminary screening.
3. Enzyme-Linked Immunosorbent Assay (ELISA) and Polymerase Chain Reaction (PCR): These are more advanced and sensitive techniques but are also more expensive and require specialized equipment and expertise.

Conclusion

The selection of coliforms as indicator organisms is based on their prevalence in the gastrointestinal tract of warm-blooded animals, their survival in water, and their correlation with the presence of pathogens. The membrane filter method for detecting coliforms in water offers a balance of sensitivity, speed, and specificity, making it a popular choice in many water quality testing scenarios. However, its effectiveness can be limited by water turbidity and the presence of non-target bacteria. The choice of method for coliform detection often depends on the specific requirements of the water quality assessment, including the nature of the water source, the level of sensitivity required, and available resources.