The Gram stain reaction is not always truly indicative of the organisms true cell wall structure. Poorly
controlled decolorizing can obviously be a source of misleading results, but even with adequate
technique some organisms are problematic. For example, some strains of Bacillus or Clostridium
consistently stain Gram negative even though they have a Gram positive type cell wall.
The KOH String Test relies on the differential resistance to 3% potassium hydroxide between gram
positive and negative cells, where a portion of a colony is mixed with a small volume of 3% KOH on a
glass slide for no more than 60 seconds. If the cells lyses, the liberated cellular DNA makes the mixture
viscous or “stringy.” The positive string test indicates a gram negative organism