Cetrimide Agar Test- Principle, Procedure and Result Interpretation

Objective of Cetrimide Agar Test

This test is primarily used to isolate and purify Pseudomonas aeruginosa from contaminated specimens.

Principle of Cetrimide Agar Test

The test is used to determine the ability of an organism to grow in the presence of cetrimide, a toxic substance that inhibits the growth of many bacteria by causing the release of nitrogen and phosphorous, which slows or kills the organism. P. aeruginosa is resistant to cetrimide.


Enzymatic digest of gelatin (20 g), MgCl2 (1.4 g), K2SO4 (10 g), cetrimide (cetyltrimethylammonium bromide) (0.3 g), agar (13.6), pH 7.2.

Procedure of Cetrimide Agar Test

1. Inoculate a cetrimide agar slant with 1 drop of an 18- to 24-hour brain-heart infusion broth culture.
2. Incubate at 35°-37°C for up to 7 days.
3. Examine the slant for bacterial growth.

Result Interpretation of Cetrimide Agar Test

Positive: Growth, variation in color of colonies.
Negative: No growth.

Limitations of Cetrimide Agar Test

Some enteric organisms will grow and exhibit a weak yellow color in the media. This color change is distinguishable from the production of fluorescein. Additional testing is necessary to confirm a diagnosis of P. aeruginosa.

Quality Control

Positive: Pseudomonas aeruginosa (ATCC27853)—growth and color change; yellow-green to blue-green colonies
Negative: Escherichia coli (ATCC25922)—no growth and no color change