Final answer:
E. coli will form bright pink colonies on MacConkey Agar, metallic green or deep purple colonies on EMB Agar, and pink colonies with a black center on XLD Agar. These agars are used not only to cultivate these bacteria but also to differentiate them based on their ability to ferment lactose.
Step-by-step explanation:
Escherichia coli (E. coli), which is a Gram-negative bacterial species, exhibits varied reactions on different types of agar plates, based on its ability to ferment lactose:
- Nutrient Agar: E. coli typically forms colorless or creamy colonies, as this medium does not differentiate based on lactose fermentation.
- MacConkey Agar: E. coli, being a strong lactose fermenter, will appear as bright pink colonies as lactose fermentation leads to acid production, changing the pH and, in turn, the color of pH indicators in the medium.
- EMB Agar (Eosin Methylene Blue): E. coli, due to lactose fermentation, forms metallic green or deep purple colonies. EMB Agar is a selective medium for Gram-negative organisms and differentiates lactose fermenters from non-fermenters through color changes.
- XLD Agar (Xylose Lysine Deoxycholate): E. coli colonies may appear pink with a black center due to fermentation of lactose and/or xylose and production of hydrogen sulfide.
MacConkey Agar and EMB Agar serve as both selective and differentiation media, allowing for the growth of certain bacteria while inhibiting others, and indicating if an organism can ferment lactose.