Question

1. Why are biochemical tests performed on microbes?

2. What are 2 functions of pH indicators found in media?

3. Define the citrate and malonate tests. How do they work?

4. How are differential tests based on enzymes performed in a micro lab?

5. What is combinatorial media?

Answer 1

Ans-1) Biochemical tests are performed on microbes to identify them. Identification of different species of microbes is based on the differences in the biochemical activities of different microbes.

Ans-2) The two functions of pH indicators are-:

a) It is used to determine the end point of titrations through color change.

b) These indicators give characteristic colours in different media according to different pH.

Ans-3) Citrate test is used to detect the ability of an organism to use citrate as the only source of energy and carbon. In this test, bacteria are inoculated on a medium that contains sodium citrate, a pH indicator known as bromothymol blue and an inorganic ammonium salt that is used as a source of nitrogen. The enzyme citrase present in the microbe breaks down citrate to oxaloacetate and acetate. The oxaloacetate is further broken down to pyruvate and carbon dioxide. Formation of sodium bicarbonate and ammonia by using sodium citrate and ammonium salts results in alkaline pH. This changes the colour of the medium from green to blue.

Malonate test is a colorimetric test used to detect the ability of bacteria to use malonate as a source of carbon. This test leads to production of alkaline metabolites that induces a color change. An organism that can utilize both sodium malonate as its carbon source and ammonium sulphate as its nitrogen source forms an alkalinity due to the production of sodium hydroxide. This leads to alkaline reaction which changes the colour of the medium containing malonate from its original green colour to prussian blue or light blue. Organisms that are unable to utilize malonate and ammonium sulfate and do not ferment dextrose produce no colour change. Organisms that are malonate-negative but ferment dextrose result in the formation ofyellow color due to the acidity of the medium.