Question

Background

Selective and differential media; EMB and Tsl agar bacill inhabit a wide range of environments. Many are significant pathogens, such a coli, Salmonella, Shigella, but many compose our normal flora and inhabit soil and water. Many gra negative bacilli that inhabit our large intestine are called enterics. Enteric pathogens are the most frequent cause of diarrheal illnesses and have become significant contaminants spread through foo Some can also cause respiratory and urinary infections. Experiment 1: EMB agar EMB (Eosin methylene blue) agar is a selective, differential media that can be used in the isolationa detection of enteric bacteria from other types. It is a standard media to use for water quality testing and suspected sewage contamination. EMB agar contains eosin and crystal violet dyes which allow gram negativ e bacteria to grow, while inhibiting the growth of gram positive bacteria. EMB also differentiates the gram negative bacilli into those that can ferment lactose, the coliform bacteria, fre those that can't ferment lactose, the non-coliforms. Again, this metabolic ability is used for identification purposes. Lactose is added to the agar. Lactose fermenters grow on EMB as dark pur colonies, and when they grow rapidly can create a metallic green sheen as well. Non-coliforms, or non-lactose fermenters will appear as a pale lavender or colorless colonies. Materials

Ques part 1)

Answer 1

1. The ingredient eosin and crystal violet dye make the EMB agar selective for gram-negative bacteria by inhibiting the growth of gram-positive bacteria. These ingredients also contain lactose which differentiates gram-negative bacteria from gram-positive bacteria.

2. Gram-positive bacteria are inhibited from growing on EMB agar.

3. Lactose ingredient makes the EMB agar differential.

4. Lactose fermentation test is another test used to confirm the gram-negative bacilli.

5. positive presumptive test indicate that the water sample may be significant growth of bacteria especially E.coli. So the bacteria will get grown on an agar plate and the positive test for coliforms confirms the presence of bacteria in the water sample.