Laboratory 8: Biochemical Characteristics of
Gram Negative Enteric Bacteria

MacConkey Agar, Indole, Methyl RedVogues Proskauer, & Citrate

MacConkey Agar, a Selective and Differential Medium

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.  The media also contains the pH indicator, neutral red.  Therefore Gram negative bacteria that ferment lactose turn pink.  Those that do not remain the color of the medium or turn tan. Upper right: Growth negative, Gram negative, lactose negative
Upper left: Growth positive, Gram negative, lactose positive
Lower center: Growth negative, Gram positive, lactose unknown

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The IMViC Series

Indole Production

 The indole test examines the ability of a bacteria to breakdown the amino acid trytophan.  The presence of   indole  is confirmed by the addition of a few drops of Kovac's Indole reagent.  The red layer at the top indicates a positive test.

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The Methyl Red Test

The Methyl Red Test examines a bacteria ability to ferment glucose to produce "many acids".  Such bacteria are known as mixed acid fermentors because they lower the pH of the medium to below 4.8, the point at which the indicator methyl red is truly red.

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The Voges Proskauer Test

In the butylene-glycol fermentation pathway, glucose is converted to acetylmethylcarbinol (AMC) and finally to butylene glycol.  The addition of alph napthol and potassium hydroxide tests for the presence of AMC.  The red tube on the right is positive.

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Citrate Utilization

Bacteria that can utilize citrate incorporate it aerobically producing alkaline end products.  The pH indicator contained in Simmons Citrate Agar is bromthymol blue.  A blue slant indicates a positive reaction.

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