General Microbiology question. If I gave you a broth mix of three bacteria; Bact

Question

General Microbiology question.

If I gave you a broth mix of three bacteria; Bacterium 1 is a gram-positive bacterium, Bacterium 2 is a lactose-fermentation-positive negative bacterum and Bacterium 3 is a lactose-fermentation-negative gram-negative bacterium, what two or three plates would you use to isolate them from one another and distinguish them? Also, thoroughly describe what you would expect each plate to contain/grow and what each growth might look like.

I'm not really sure if this has anything to do with the question above, but during lab we used PEA (Phenyl Ethyl Alcohol), MSA (Mannitol Salt Agar), EMB (Eosin Methylene Blue) Agar, MacConkey Agar, and Nutrient Agar.

Explanation / Answer

I use PEA agar to isolate gram-positive bacteria, EMB agar to differentiate lactose positive and lactose negative gram-negative bacteria.

Eosin Methylene Blue is a selective stain for Gram-negative bacteria. EMB contains dyes that are toxic for Gram positive bacteria and bile salt which is toxic for Gram negative bacteria other than coliforms.

This medium can distinguish lactose fermenters from non-lactose fermenters.

Rapid lactose fermentation produces acids, which lower the pH. And the colonies appears purple.

Lactose non-fermenters may increase the pH by deamination of proteins. This ensures that the dye is not absorbed. The colonies will be colorless.

Phenylethyl Alcohol Agar is a selective medium used for the isolation of gram- positive organisms like Staphylococci and Streptococci. Since there was a little growth on this medium, this means bacteria are gram-positive.

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