E. coli (Escherichia coli) is a commonly used indicator organism to determine pollution of water with faecal matter. Here's an explanation of the answer:

Indicator Organism:
- An indicator organism is a type of organism that is used to assess the presence or level of contamination in an environment.
- Indicator organisms are chosen based on specific characteristics that make them suitable for indicating the presence of certain types of contaminants.

Escherichia coli (E. coli):
- E. coli is a type of bacteria that naturally inhabits the intestines of warm-blooded animals, including humans.
- Most strains of E. coli are harmless and even beneficial, aiding in digestion and producing certain vitamins.
- However, some strains of E. coli can cause severe illness, particularly when they are present in high numbers and originate from faecal matter.

Water Pollution:
- Water pollution refers to the contamination of water bodies, such as rivers, lakes, and groundwater, with various substances that can be harmful to human health and the environment.
- Sources of water pollution include industrial activities, agricultural runoff, sewage discharge, and improper waste disposal.

Role of E. coli as an Indicator Organism:
- E. coli is used as an indicator organism in water quality testing because it is commonly found in the intestines of warm-blooded animals, including humans.
- The presence of E. coli in water indicates the potential presence of faecal matter, which may contain harmful pathogens and other contaminants.
- The detection of E. coli in water samples suggests that the water has been contaminated with sewage or other sources of faecal pollution.

Advantages of Using E. coli as an Indicator:
- E. coli is relatively easy to detect and identify in water samples using specific laboratory techniques.
- It is an indicator of recent contamination, as the bacteria do not survive for long periods outside the host organism.
- E. coli is more specific to faecal contamination compared to other indicator organisms like total coliforms, which can also originate from other sources.

Conclusion:
- In summary, E. coli is used as an indicator organism to determine pollution of water with faecal matter.
- Its presence suggests the potential presence of harmful pathogens and other contaminants originating from sewage or other sources of faecal pollution.