| A staining method used to classify different species of bacteria. Gram staining is a microbiological procedure that categorizes bacteria based on the physical and chemical structure of their outer surface. This procedure is commonly used for detection and identification of bacteria both in research and clinical laboratories. Bacteria are loosely categorized into two types, gram-positive and gram-negative. • Gram-positive bacteria have a thick layer made up of polymers of protein-sugar molecules called peptidoglycan. Gram staining of the peptidoglycan layer (a.k.a. the cell wall) with a chemical called crystal violet results in purple coloration of the gram-positive bacteria. Addition of acetone or alcohol dehydrates the bacteria, causing it to retain the purple color. • Gram-negative bacteria have an additional outer membrane whose properties resist acetone/alcohol-dehydration and result in loss of the crystal violet stain. A counterstain, made up of fuchsin, stains these bacteria red or pink. |