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Gram Reaction – 3% Potassium Hydroxide

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I.            Alternative test for Gram Reaction – 3% Potassium Hydroxide

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A.    Add a heavy inoculum of pure culture of bacteria grown on a solid medium to a drop of 3% potassium hydroxide (KOH) solution (3 grams KOH per 100 mL distilled water) on a clean glass slide.

B.     Stir for about one minute, occasionally lifting the loop to look for thickening and “stringing” of the slurry.


1.Gram negative        2 . Gram positive
Result:


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1.          Gram positive: bacteria will not appear to change the viscosity of the KOH solution.

2.          Gram negative: bacteria will cause the KOH solution to become stringy or mucoid in appearance and consistency.

D.    QUALITY CONTROL:
1.          Gram Positive: Staphylococcus sp. 

2.          Gram Negative: Escherichia coli

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