Final answer:
E. coli is used as an indicator organism in experiments because it is a common gut resident, with certain pathogenic strains crucial to identify for health reasons, and it is easily distinguishable on selective media due to lactose fermentation.
Step-by-step explanation:
E. coli is used as an indicator organism in confirmation tests for several key reasons. First, E. coli is a common inhabitant of the human gut, making it a relevant organism to monitor for potential fecal contamination in food and water. Second, certain strains, like EHEC (enterohemorrhagic E. coli), are known pathogens, making their detection critical for public health. Finally, E. coli can be easily identified and differentiated from other bacteria due to its characteristic growth on media like EMB agar, where it forms distinct metallic or purple colonies indicative of lactose fermentation.
Lactose fermenters like E. coli turn EMB agar into distinctive colors, which helps in quick identification amidst other non-lactose fermenting organisms that form colorless colonies. This, along with E. coli's rapid growth rate and well-understood genetics, makes it a prime choice for indicator tests.