Two indicators media: Introduction and Uses

Two indicators media

Outlines of the topic two indicators media are introduction and use. Medium carrying two indicators are the most common question asking in the examination as viva, therefore today we concern with those media as shown below.

Two indicators medium are:

1. Hekton enteric agar

It contains two indicators

a)acid fuchsin and

b) bromothymol blue.

Introduction:

Hektoen enteric agar(HEA) is named after the Hektoen Institute in Chicago, where researchers developed the agar. It is a selective and differential agar primarily used to recover Salmonella and Shigella from patient specimens. It contains indicators of lactose fermentation and hydrogen sulfide production; as well as inhibitors to prevent the growth of Gram-positive bacteria

E. coli-Yellow to pink

Shigella-Greenish,translucent

Salmonella-greenish, black translucent

2. Moller’s formula based decarboxylase medium

a) Bromocresol purple

b) Cresol red

This medium is useful for the decarboxylase test of Gram-negative bacteria. Decarboxylases are a group of substrate-specific enzymes that are capable of reacting with the carboxyl (COOH) portion of amino acids, forming alkaline-reacting amines and byproduct carbon dioxide. Increased pH of the medium is detected by the color change of the pH indicators bromcresol purple and cresol red. Decarboxylation shows Purple color- test Positive and no decarboxylation Yellow color- test negative. As quality control following organisms are used: Positive Control for lysine: Klebsiella pneumoniae, ornithine: Enterobacter cloacae, and arginine: Enterobacter cloacae. Whereas for negative control lysine: Enterobacter cloacae, ornithine: Klebsiella pneumoniae and arginine: Klebsiella pneumoniae.

Decarboxylase test of Pseudomonas

Lysine: Result variable

Ornithine: Result variable

Arginine: Result variable

Decarboxylase test of Aeromonas

Lysine: Negative

Ornithine: Negative

Arginine: Positive

Decarboxylase test of Vibrio

Lysine: Positive

Ornithine: Positive

Arginine: Negative

Decarboxylase test of Plesiomonas

Lysine: Positive

Ornithine: Positive

Arginine: Positive

3. Thiosulphate Citrate bile salt Sucrose (TCBS) agar

a) Bromothymol blue and

b)Thymol blue

Introduction of TCBS agar: It is a selective and differential medium for Vibrio cholerae. Vibrio cholerae shows a green colony on this medium.

Note: Nearly all indicators give their name in alkaline reaction except methyl red and neutral red. For example bromothymol blue gives its name blue in alkaline reaction (pH ≥7.6), similarly phenol red gives its name red in alkaline reaction (pH >7.0).

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