Question

1). What are some potentially pathogenic bacteria that are lactose fermenters that will grow on MacConkey agar?”

2). What are some potentially pathogenic bacteria that do not ferment lactose that will grow on MacConkey agar?”

Answer 1

Ans) 1) MacConkey Agar (MAC) is a selective and differential medium designed to isolate and differentiate enterics based on their ability to ferment lactose.

- Bile salts and crystal violet inhibit the growth of Gram positive organisms.

- Lactose provides a source of fermentable carbohydrate, allowing for differentiation.

- MacConkey Agar is selective for Gram negative bacteria due to the presence of bile salts and crystal violet.

- The only fermentable source of carbohydrate is lactose.

- Therefore Gram negative bacteria that ferment lactose turn pink.

2) Crystal violet penetrates Gram positive bacteria's cell wall, and is toxic for them, so it inhibits their growth, which makes MacConkey agar also a selective media.

- If gram positiva bacteria grow in it, make sure that: Crystal violet is included in agar's formula. The bacteria is indeed gram positive.