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Potassium hydroxide test

Potassium hydroxide test

Potassium hydroxide test.

A. Application of potassium hydroxide solution onto a microscopic slide.
B. Bacteria are transferred and mixed with the KOH solution.
C. Result with gramnegative bacteria where the solution will be viscous and form a mucoid string.
D. Result with grampositive bacteria where the solution will not be viscous.
Click on the image to enlarge it.


The purpose of the potassium hydroxide test (KOH test) is to identify gram negative bacteria. KOH dissolves the thin layer of peptidoglycan of the cell walls of gram negative bacteria, but does not affect gram positive cell walls. Disintergration of gram negative cell walls lyses the cell and release its contents, including the DNA. The DNA will make the solution very viscous and the solution will stick to the plastic loop when touched. Gram positive bacteria will not be affected by KOH, because they have thicker peptidoglycan layer in the cell wall. Thus, the cells will not be lysed, the DNA not released and no viscosity will be observed.


  • Microscopic slide
  • Plastic loop
  • 3% KOH


  • Apply one drop of 3% KOH on a microscopic slide.
  • Use o loop to transfer a generous amount of bacteria (cultivated for 24-48 h) to the drop of KOH
  • Stir carefully
  • The solution of gram negative bacteria will be viscous and form a mucoid string within 30 sec
  • Use positive and negative controls

Positive results: The solution with the bacteria (gram negative) will be viscous

Negative results: The solution with the bacteria (gram positive) will not be viscous


The purpose of the KOH test is to quickly distinguish between gram negative and gram positive bacteria as a complement to Gram staining. The test is not useful for anaerobic bacteria.




Updated: 2017-12-20.

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Swedish University of Agricultural Sciences