Please answer 4 and 5 4. (1 point), Compare and contrast the Methyl Red test and

Question

Please answer 4 and 5

4. (1 point), Compare and contrast the Methyl Red test and the Voges-Proskauer test 5. (1 point). Match the growth medium on the left to its description on the right A. Selective agar that permits the growth of halophiles Mannitol Salt Agar B. Selective and differential agar used to isolate halophiles and differentiate the mannitol-fermenters from the non fermenters MacConkey agar C. Selective agar used to isolate Gram positive bacteria Phenylethyl alcohol agar D. Selective and differential agar used to isolate Gram negative bacteria and differentiate lactose fermenters (appear red or will turn media pink if strong fermenter) from non-fermenters (appear tan). Eosin-methylene blue agar E. Selective and differential agar used to isolate Gram negative bacteria and differentiate lactose fermenters (appear thick pink or blue black with a metallic green sheen if Escherichia coll) from non- fermenters (appear colorless). Is particularly useful for identification of Escherichia coli due to metallic green appearance when grown on this medium. -7.5% sodium chloride agar

Explanation / Answer

4) Methyl red test : It is performed to detect the ability of an organism to produce stable acids end products (Mixed-acid fermentation) from supplied glucose. Some bacteria can metabolise glucose to pyruvic acid, which is further metabolized through the ‘mixed acid pathway to produce the stable acid. The acid thus produced decreases the pH to 4.5 or below and is indicated by a change in the colour of methyl red from yellow to red. In this test, the bacteria is grown in a broth containing glucose and if the bacteria has the ability to produce stable acidic end products, the colour of the methyl red changes from yellow to red, when added into the broth culture.

Voges Proskauer Test : It determines the ability of some organisms to produce non acidic or neutral end products, such as acetyl methyl carbinol, from the organic acids that result from glucose metabolism. The reagent used is Barritt's reagent which consists of a mixture of alcoholic a-naphthol and 40% potassium hydroxide solution. Acetyl methyl carbinol is oxidized to a diacety compound in the presence of potassium hydroxide. Diacetyl reacts with a-naphthol (catalyst) producing a red color and presents a positive result.

Thus both the tests give red color but, the methyl red test is used to detect the stable acidic end products while the Voges Proskauer is used to determine the non- acidic or neutral end products.

5) Mannitol salt agar : Option B Selective and differential agar used to isolate halophiles and differentiate the mannitol fermenters from the non-fermenters.

MacConkey agar : Option D selection and differentiation of gram negative bacteria and also differentiates between lactose fermenters and non-fermenters.

Phenylethyl alcohol agar : Option C Selective agar used to isolate Gram positive bacteria

Eosin-methylene blue agar : Option E differentiates between enteric lactose fermenters and nonfermenters; identifies e. coli

7.5% sodium chloride agar : Option A Selective agar that permits the growth of halophiles

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