Question

why are carbohydrate and proteins tests used to identify organisms and how do you perform the tests?

Answer 1

Carbohydrate and protein tests are used to identify organisms as most of the organic compounds found in living organisms are carbohydrates and proteins, nucleic acids and lipids, these are called macromolecules. Therefore to identify the presence of microorganisms, biochemical tests are performed to identify the presence of carbohydrates and proteins. Most of the bacteria found are identified and also classifies based on their reaction in a biochemical test.

Tests performed to identify the organism

1. Starch hydrolysis
2. Protein hydrolysis
3. Fermentation of carbohydrates
4. Indole and hydrogen sulphide production
5. Catalase activity

Starch hydrolysis

• In this test, the bacteria to be tested are grown on the agar plates containing starch.
• If that specific bacteria has the potential to digest the starch in that medium and after the incubation period the iodine is added to the plate then as the starch has already been digested by the bacteria there will be no change of color.
• On the other hand, if the starch is not hydrolyzed then when iodine is added to the starch then it forms a dark blue or deep purple line.

Protein Hydrolysis

• This test is also sometimes referred to as the casein hydrolysis test
• The casein protein is a large polymer of amino acid; it is also too large in size to enter through the cell membrane.
• Some microorganisms have the ability to degrade the microorganisms; they do so by producing proteolytic exoenzymes, called proteinase.
• After the inoculation and the incubation period, the agar plates will exhibit an area of proteolysis with a transparent opaque free area ( shows colony growth)
• After the inoculation and the incubation period, the agar plates may also sometimes exhibit an area of opacity, which tells that no proteolysis has taken place( shows no colony growth)

Fermentation of carbohydrates

• Through this test, we can identify the specific carbohydrate that has been fermented by the bacteria by looking at the end products of fermentation.
• When the carbohydrates are fermented by the bacteria, two main types of end products are produced
  • Acid end products
  • Acid and gas end product

But the end products are never gas alone

• A Durham tube is provided in the tubed broth to collect the gas produced, the indicator in the media changes color from purple to yellow if the carbohydrate is fermented by the bacteria
• If the carbohydrate is not fermented by the bacteria, the color does not change

Indole and hydrogen sulphide production

• In this test iron and sulphide compounds are added to the medium to identify the production of hydrogen sulphide gas
• Hydrogen sulphide gas is produced if the bacteria reduce the sulphur compound from the medium.
• If the hydrogen sulphide gas is produced it interacts with the iron compound to form the black precipitate. ( indicates that hydrogen sulphide gas is produced and identifies a particular organism)

Catalase activity

• Catalase is an enzyme found in most of the bacterias, which breaks down hydrogen peroxide into water and free oxygen.
• If the bacteria present in the medium has the enzyme catalase, then the hydrogen peroxide added to the culture is degraded into water and oxygen. ( positive catalase test)
• If the bacteria present in the medium does not have the enzyme catalase, then the hydrogen peroxide is not disintegrated. (negative catalase test)