Potassium Hydroxide Test: Result, Principle, Procedure, and Reagents

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Potassium Hydroxide Test Introduction

Many of the biochemical tests are used to detect the ability of the micro-organisms to act against certain chemicals and enzymes.

Potassium hydroxide test is used to identify the gram-negative bacteria.

Potassium hydroxide is one of the inorganic compounds which has its chemical formula KOH, and it is most commonly referred to as caustic potash.

It can be used in many chemical and biochemical tests.

Potassium hydroxide is also known as Potassium hydroxide dissolves with the peptidoglycan which is present as a thin membrane in the cell walls of the gram-negative bacteria.

But this does not affect the gram-positive cell walls.

Disintegration of these gram-negative cell walls lyses the cell thus releasing the contents from the cell including those of DNA.

The DNA makes the solution viscous and makes the solution to stick to the plastics loop when touched.

However, Gram positive bacteria does not affect the Potassium hydroxide, as it has thicker peptidoglycan layer present in the cell wall.

Hence the cells will not be lysed, so that there will no release of DNA and there is no viscosity observed.

What is Potassium Hydroxide Test?

Potassium hydroxide test is also known as KOH string test.

Potassium hydroxide test mostly relies on the differential resistance of about 3% in the potassium hydroxide, between the gram positive and the gram-negative cells.

Here a small portion of a colony is mixed up with a small volume of 3% KOH.

If the cell lyses, the cellular DNA will be liberated making the mixture viscous or stringy.

The positive string test indicates a gram-negative organism.

Hence the potassium hydroxide can also be known as String test.

Potassium hydroxide test also helps in differentiating the Gram positive and the Gram-negative organisms and this test is also helpful in complementing the gram stain and the antibiotic disc test.

Potassium Hydroxide Test Objective

The main aim of the test is to differentiate between gram negative and the gram-positive organisms.

Potassium Hydroxide Test Principle

Similar to the Gram stain reaction, the potassium hydroxide test is basically used to identify the differences in the chemical composition of the bacterial cell wall.

During the presence of this Potassium hydroxide, Gram negative cells will be lysed, this makes the Potassium hydroxide solution to dissolve easily with the thin layers of the cells, known as peptidoglycan.

However, Disintegration of Gram-negative cell wall will lyse the cells making it to release all of it contents, including DNA.

As a result, a viscid chromosomal material is released along with the contents and it is caused by the suspension of the bacteria to become thick and stringy.

The viscous and the solution sticks to the loops.

On the other hand, Gram positive bacteria are not affected by the Potassium hydroxide as it has thicker peptidoglycan layer along the cell so there are no chances of cell lyses and no contents will be spilled out of the cell so no viscosity can be detected.

Potassium Hydroxide Test Procedure

About one drop of 3% of potassium hydroxide solution is placed in a clean microscopic slide.

Few colonies of a suspect organism are emulsifying along with the drop of potassium hydroxide placed in the slide and makes it as a dense suspension.

Then the mixture is continuously mixed for about 60 seconds and they are gently pulled using a loop such that to pull it against the suspension.

Then the medium is detected for any changes.

Potassium Hydroxide Test Result

 For positive results, the organisms become thick, stringy and it forms a long strand within few seconds. This is seen only in the species of the Gram-negative bacteria.

 In case of negative results, the organisms leave the suspension without any changes in the absence of the stringing. These results are mostly seen in Gram positive bacteria.

Potassium Hydroxide Test Uses

 Potassium hydroxide tests are generally used in laboratories where the cultures are processed in large numbers, the above test is used is also used along with the gram stain in preliminary differentiation.

 This test is also useful in reacting as a complement in the Gram stain and in antibiotic tests.

Potassium Hydroxide Test Limitation

 Though, this test is useful, the test cannot be used to determine the results, as the negative tests does not prove any conclusive if an organism is Gram-positive.

 In old cultures which are being cultures before 48 hours, has the chances of turning the negative results into positive after 30 seconds of mixing the bacteria along with the potassium hydroxide solution, thus gives unreliable results. This type of results are most common in species like Achromonacter genera such as Brucells melitenis, Pseudomonas paucimobilis, Moraxella species, etc.

 False results mostly occur due to a heavy inoculum, where the solution appears in the form of gel, without a string form. In some cases, there is also chances of forming the inoculation with the mucoid colonies.  On the other hand, False results occur when there is a very little or light inoculum or if there is too much of potassium hydroxide than normal.

Potassium Hydroxide Test Citations


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