Potassium Hydroxide Test explained

Many Bacillus and Clostridium species organisms that have lost some of the integrity of their cell wall, appear Gram negative on staining resulting in possible misidentification.

The potassium hydroxide test may aid in differentiation between Gram positive and Gram negative organisms and is a useful complement to the Gram stain and the antibiotic disc test. Like the Gram stain reaction, the test is based on differences in the chemistry of the bacterial cell wall.

In the presence of potassium hydroxide, Gram negative cell walls are broken down, releasing viscid chromosomal material which causes the bacterial suspension to become thick and stringy. Gram positive organisms remain unaffected. Hence the alternative name for this procedure, the “String Test”.

In relation to the potassium hydroxide test, Public Health England has issued a technical report, including safety information. The report can be accessed here.

Posted by Tim Sandle