The Gram stain is a differential staining technique which divides bacteria into two major groups, gram-positive and gram-negative. The Gram stain uses four different reagents:

i. Crystal Violet - stains all cells purple-blue in color

ii. Grams Iodine - acts as a mordant (a substance that formsan insoluble complex by bonding to the crystal violet)

iii. Ethyl Alcohol - a decolorizing agent that leaches the
gram-negative bacteria colorless yet can not remove the
Grams iodine complex in gram-positive bacteria.

iv. Safranin - serves as a counterstain which colors the gram-negative cells pink.

INTERPRETING TEST RESULTS:
GRAM + = purple stained bacteria
GRAM - = pink stained bacteria