Question

1. Explain the principle of nitrate reduction test?

   (b) Alpha hemolysis (c) Beta hemolysis (d) Gamma hemolysis.

Answer 1

Answer =

The principle of nitrate reduction test as follows:

Nitrate reduction test determines the production of an enzyme called nitrate reductase, which results in reduction of nitrate.

A heavy inoculum of test organism is incubated in a broth containing nitrate.The organisms capable of producing the nitrate reductase enzyme reduce the nitrate, present in broth, to nitrite which may then further reduced to nitric oxide , nitrous oxide ,or nitrogen.

This test is based on the detection of nitrite and its ability to form a red compound when it reacts with sulfanilic acid to form a complex that is nitrite-sulfanilic acid which then reacts with a alpha -naphthylamine to give a red precipitate ,which is water soluble azo dye.

only when nitrate is present in the medium,red color will be produced.

If there is no red color in the medium after addition of sulfanilic acid and alpha -naphthylamine means only that nitrite is not present in the medium.

(b) Alpha hemolysis - is a partial or green hemolysis associated with reduction of the red cell hemoglobin.Alpha hemolysis is caused by hydrogen peroxide produced by the bacterium, oxidizing hemoglobin to green methemoglobin.

(c) Beta hemolysis = is associated with complete lysis of red cells surrounding the colony.

is caused by two hemolysins O and S

O hemolysin is inactive in the presence of oxygen, thus stabbing of the plate increases the intensity of the hemolysis reaction

S is an oxygen -stable cytotoxin.

(d) Gamma hemolysis - colonies show neither typical alpha nor beta hemolysis.There may be slight discoloration in the medium.