Biochemical tests are used to identify bacterial species by differentiating them on the basis of biochemical activities. The difference in protein and fat metabolism, carbohydrate metabolism, enzyme production, compound utilization ability, etc. are some factors that aid in bacterial identification.

Importance of Biochemical Tests

Biochemical tests help in identification of higher plants and animals on the basis of external and internal structural differences. These tests advance in structural observations which are not visible to naked eyes or even under the microscope. There are many bacterial species sharing similarities in size, shape, etc. and can only be differentiated with detection of certain properties that can be recognized using biochemical tests.

Some Common Biochemical Tests

All species of bacteria possess a unique set of metabolic activates which are controlled by bacterial enzymes. These bacterial enzymes determine the type of test suitable in recognition of structural differences and metabolic activities. 

Intracellular Enzymes

Intracellular enzymes function internally and are responsible for synthesis of new protoplasmic substances in bacteria. They permit simple substances through the cell membrane and produce cellular energy that helps in utilization of these cellular substances. Some of tests based on these enzymes include:

• Catalase Test

Many aerobic bacteria produce an enzyme known as catalase that detoxifies hydrogen peroxide. The positivity of test is determined by gas (O2) formation in form of bubbles.

• Oxidase Test

Bacteria which function on aerobic respiration often possess cytochrome C and a cytochrome C oxidase. The test uses oxidase reagent applied filter paper, timely color change of which indicates the positivity of result.

• Indole Test

Bacteria possessing an enzyme tryptophanse can hydrolyze with amino acid tryptophan to form indole, ammonia and pyruvic acid. The timely color change during the test determines the success of the test.   

Some other tests include Vogus-Proskeur test (VP test), methyl red test, triple sugar iron test, litmus milk test, etc.

Extracellular Enzymes

These enzymes are secreted by the bacterial cells and function outside the cells. They are synthesized inside the cell and assist in breakdown of complex macromolecules into smaller units after secretion. These smaller weight substances then can permeate the cell membrane and utilized it with the help of intracellular activities. Some extracellular enzyme test include:

• Coagulase test

Some bacteria produce enzymes known as coagulase that converts fibrinogen to fibrin, which means it can coagulate plasma. During the test if coagulate plasma and coagulate remain stable and undissolved, it indicates towards the positivity of the test.

• Dnase test

Many species of bacteria contain an enzyme known as Dnase, which can hydrolyze DNA. Presence of Dnase can be determined by cultivation on agar plate containing DNA.

Some other tests that study extracellular enzymes include gelatinase test, starch hydrolysis test and lipid hydrolysis test.

Biochemical tests are among the most important methods of microbial identification as they reduce time and cost ensuring the accuracy in identification of unknown samples, even in blood test labs.