Most probably because e. coli is the most harmful pathogen and appears most often in water. These contaminated water mostly compose of the bacteria which can lead to bad health.
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As it indicates the levels of the water contamination.
Explanation:
As the e-coil can survive for short periods of time in the environments and hence it has a universal use in finding out the levels of the water contamination of the waterways.
Other methods are the levels of the dissolved oxygen and the water transparency and other visual tests.
