Laboratory 8: Biochemical
Characteristics of
Gram Negative Enteric Bacteria
MacConkey Agar, Indole,
Methyl
Red, Vogues 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 |
return to top
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.
return to top
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.
return to top
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.
return to top
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.
return to top
