Gram determination
Gram Stain
KOH method
1. Place approximately 50 ul of 3% (w/v) KOH on a clean glass slide.
2. Aseptically transfer bacterial cells from an agar plate to the drop of KOH with a sterile toothpick.
3. Agitate the cells with the toothpick. If the viscosity of the drop increases and a string of "goo" can be easily picked up with the tip of the toothpick, the cells are gram-negative. If the viscosity does not greatly increase, the cells are gram-positive.
From Suslow, T. V., M. N. Schroth, and M. Isaka. 1982. Application of a rapid method for gram differentiation of plant pathogenic and saprophytic bacteria without staining. Phytopathol. 72:917-918.