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Indicator organisms are specifically chosen microorganisms that serve as a signal for the potential presence of pathogens in water. These organisms are not necessarily harmful themselves, but their presence indicates that conditions may be favorable for pathogens that can cause disease. By detecting these indicator organisms, water quality professionals can assess the safety of water for human consumption and use.

This approach relies on the understanding that indicator organisms, often a type of coliform bacteria, are found in higher numbers than the actual pathogens. If these indicators are present in significant quantities, it suggests a possible contamination of the water source and prompts further testing for harmful pathogens. Thus, the selection of these microorganisms as indicators is based on their correlation with waterborne pathogens, making them a key focus in water quality testing and management.