Question

Why do Staphylococcus and Micrococcus fail to grow on the MAC plate? Explain your answer based on cell wall composition and the composition of the media

Answer 1

Staphylococcus and Micrococcus are the Gram - Positive Bacteria and are inhibited on MacConkey Agar plate.

Composition of MacConkey:

Agar, Distilled water, Peptone, Sodium Chloride, Bile salts, Crystal Violet, Proteose Peptone, Lactose Monohydrate and Neutral Red.

Bile salts and Crystal Violet inhibit the growth of Gram Positive Bacteria (Staphylococcus and Micrococcus).

Bile salts in bacteria unfolds the proteins by breaking disulphide bonds. This disulphide stress causes homeostasis in the bacterial cells and thus it causes cell death.

Crystal Violet is a basic dye which binds to the peptidoglycan layer of Gram-positive Bacteria and gives Violet appearance.

The basic nature if the dye inhibits the growth of the bacteria.

Where as in case of Gram - Negative Bacteria peptidoglycan layer is very thin and the binding capacity is low and it has an outer membrane which minimise the action of dye.

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