Starch hydrolysis: This test is used to detect the enzyme amylase, which breaks down starch. After incubation the plate is treated with Gram’s iodine. If starch has been hydrolyzed (broken down) then there is a reddish color or a clear zone around the bacterial growth; if it has not been hydrolyzed then there is a black/blue area indicating the presence of starch. Simply use inoculating loop to spread bacteria onto plate surface. After the bacteria have grown, you add a few drops of Gram’s iodine to the plate and look for the color immediately after adding the iodine.
Carbohydrate utilization: Bacteria produce acidic products when they ferment certain carbohydrates. The carbohydrate utilization tests are designed to detect the change in pH which would occur if fermentation of the given carbohydrate occurred. Acids lower the pH of the medium which will cause the pH indicator (phenol red) to turn yellow. If the bacteria do not ferment the carbohydrate then the media remains red. If gas is produced as a by product of fermentation, then the Durham tube will have a bubble in it.
Citrate utilization: Tests for the ability of bacteria to convert citrate (an intermediate of the Kreb’s cycle) into oxaloacetate (another intermediate of the Kreb’s cycle). In this media, citrate is the only carbon source available to the bacteria. If it can not use citrate then it will not grow. If it can use citrate, then the bacteria will grow and the media will turn a bright blue as a result of an increase in the pH of the media. To inoculate this slant, use the transfer loop.
MRVP (methyl red-Vogues Proskauer): this test is used to determine two things. The MR portion (methyl red) is used to determine if glucose can be converted to acidic products like lactate, acetate, and formate. The VP portion is used to determine if glucose can be converted to acetoin.