Indicator organisms are organisms that are utilized to detect the presence of another organisms that cannot be easily identified.
We do not test for pathogens like salmonella directly but use an indicator organism like coliform because these pathogens are not easily isolated and they are much harder to identify.
Pathogens like salmonella are not tested for directly because they are much lower in numbers when found in wastes or waterbodies and to identify them directly is expensive.
Also testing directly for these micro-organisms is very expensive and non-viable. So it is better to use indicator organisms to detect their presence.
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