Question

Please describe two differential plates, How do they work and what information do they give you about the bacteria?

Answer 1

Answer: Differential media plates are the bacteriological media plates that is used to differentiate closely related or group of organisms by growing on the same media. Owing to presence of specific compounds or ingredients in the media, group of microorganisms that differ in some biochemical characteristics shows specific visible changes in the colony appearance or surrounding media that is used to differentiate the microorganisms.

MacConkey agar media and Mannitol salt agar media are the two differential agar plates.

MacConkey agar media: It is differential media used to differentiate gram-negative enteric bacteria from other bacteria based on their ability to ferment lactose. This media contains bile salts and crystal violet that selectively allow the growth of gram-negative bacteria and inhibit the growth of gram-positive bacteria. The neutral red pH indicator in the media differentiate the gram-negative bacteria that can ferment lactose or not. The gram-negative bacteria that can ferment lactose produce acid (pH 6.8) that lower the pH of the agar in the plates, leading to pink color colonies. Further, the bile salts precipitate and forms hazy appearance around those colony. In contrast, gram-negative bacteria that cannot ferment lactose will form normal color colonies. Hence, MacConkey agar media provide information about the ability of bacteria that can ferment lactose or not.

Mannitol salt agar media: This media is used to differentiate microorganisms based on their ability to ferment mannitol or not. This media contains mannitol and phenol red. Bacteria that ferment mannitol produces yellow colonies with yellow zones around them. This is because fermentation of mannitol produces acid in the medium, leading to change in the color of phenol red from red to yellow. The bacteria that cannot ferment mannitol produces pink or red color colonies. So, mannitol salt agar media gives information about the ability of the bacteria that can ferment mannitol or not.