In a microbiology lab experiment, 2 different E. coli strains as a well as a untreated water sample were plated on an nutrient agar dish using aseptic techniques. The same bacterial solutions were plated on another nutrient agar dish, however this dish was treated with X-gal. What is the purpose of the agar dish not treated with X-gal? (X-gal shows the presence of an enzyme that metabolizes it, which yields a blue colour).