Bacterial cells are placed in 3% (w/v) potassium hydroxide (KOH) solution. This alkaline solution lyses (breaks down) the cell walls of Gram-negative bacteria, releasing the contents of the cells. Agitation of this solution with a toothpick creates a viscous liquid. The increase in viscosity is due to the release of the DNA, which can then be drawn up in a string on the toothpick. Gram-positive bacteria will not lyse in 3% KOH, so this demonstration can be used to determine Gram-stain reaction for bacterial identification.
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