Cetrimide Agar Test

Many of the biochemical tests are identified and performed in the laboratories and in experiments to differentiate a characteristic of a microbe and they are differentiated from other organisms.

One such biochemical test is cetrimide agar test. Cetrimide agar test is performed in order to identify the Pseudomonas aeruginosa and it also helps in differentiating it from other organisms.

Cetrimide Agar

Cetrimide agar is one of the selective and differential mediums which is used for isolating the Pseudomonas aeruginosa from other clinical and non-clinical specimens.

Cetrimide is considered as one of the selective agents which inhibits the detergent activity of most of the bacteria.

Cetrimide agar is also known as Pseudomonas cetrimide Agar or Pseudosol agar.

What is Cetrimide Agar Test?

Cetrimide agar test works on the principle based on the ability of the organisms to grow in the presence of cetrimide.

Cetrimide is one of the toxic quarterly ammonium detergents. This ammonium detergent is toxic to most of the bacteria except few organisms such as Pseudomonas aeruginosa.

P. aeruginosa plays active role in causing infections in the urinary tract and other infections like bacteremia, which occurs especially in humans.

This Pseudomonas aeruginosa is usually identified on their basic characteristics such as production of pyocyanin, and its blue colour its ability to soluble in water, presence of a phenazine Pigment, morphology of their colony and its characteristic grape like odour.

Pseudomonas aeruginosa is the only species of Pseudomonas or its gram-negative rod which has the capability to excrete pyocyanin, and this capability is usually detected by performing this cetrimide agar test.

Cetrimide Agar Test Objective

 Aim of this test is to identify the species, Pseudomonas aeruginosa and to differentiate it from other micro-organisms.

 To detect the ability of various organisms to grown on the cetrimide agar.

Cetrimide Agar Test Principle

As mentioned above, cetrimide is a quaternary ammonium salt, and it also acts as a cationic detergent in most of the bacterial cells.

This cationic detergent is considered as one of the toxic substances. Cetrimide reduces the surface tension of the medium at the time of point of contact which results in complex precipitants which denatured the effect of proteins that were present on the bacterial membranes.

Where as the inhibition of growth in various micro-organisms is observed in this cetrimide agar medium including Pseudomonas species except P. aeruginosa.

When cetrimide is Brought in contact with the bacteria there will be release of nitrogen and phosphorus in the bacterial cell.

In such case, the organisms like P. Aeruginosa and other species of pseudomonas cannot withstand this germicidal activity.

This media also contains gelatine peptone which provides necessary nutrients for Pseudomonas aeruginosa.

Sodium chloride helps in maintaining the osmotic equilibrium in the cetrimide medium and it also prevents the change of pH during the growth in the medium.

Micro Organisms Tested

Cetrimide agar is usually used for testing the isolated colonies of non-glucose fermentation, gram negative rods which are suggestive for Pseudomonas aeruginosa.

Cetrimide Agar Test Reagent

Cetrimide agar can be purchase commercially in the format of dehydrated powder or it can be prepared in the laboratory using the necessary chemicals and materials available.

Supplies:

 Sterile inoculating loops

 Sterilized sticks

 Uv light

Cetrimide Agar Test Procedure

Media Preparation

About 46.7grams of dehydrated powder is added in a beaker containing 1000 millilitres of distilled water. Instead of dehydrated powder, media prepared in lab can also be used.

The prepared medium is now heated till it boils, such that the medium dissolves completely.

The boiled medium is then transferred into the tubes and it is sterilized in an autoclave at a temperature of 121ºC or at 15 lbs pressure for about 15 minutes.

After autoclaving the tubes are taken out and placed in a slanted position and it is allowed to cool till the temperature reaches about 40 to 45ºC.

The slanted position is maintained until it develops a butt in a depth of 1.5 to 2.0 cm.

Cetrimide Agar Test

A well isolated colony is collected from an 18 to 24-hour culture using sterile inoculating loops or needles.

The cetrimide agar tubes are then inoculated by streaking the surface of the slants along the medium, in the direction of froth and back and it is also ensured for proper inoculation with the inoculating stick.

The caps of the tubes should not be closed tightly so that the medium gets an adequate aeration.

Further the tubes are incubated aerobically at a temperature of about 35 to 37ºC for about 7 days.

The test tubes are checked daily, and they must be kept at least for 7 days without discarding in order to get the proper and appropriate results.

Cetrimide Agar Test Result

Positive Result: A positive result is usually determined by observing the growth in the medium, and sometimes along with a growth there will also be a change in colour from yellow green to dark blue green. Positive results are usually detected in P. aeruginosa.

Negative Result: Negative result is usually determined when there is no change in colour or no growth is seen. Negative results are usually detected in Escherichia coli.