A Gram stain of mixed Staphylococcus aureus gram-positive cocci (in purple) and Escherichia coli gram-negative bacilli (in red)

Gram staining (or Gram's method) is a way of classifying bacteria into two large groups: gram-positive and gram-negative. The name comes from its inventor, Hans Christian Gram.

Gram's method stains bacteria according to the chemical and physical properties of their cell walls. First, a violet dye is put on the bacteria. This dye stains peptidoglycan, a thick layer that is only found in gram-positive bacteria. After the first stain, another stain (usually safranin or fuchsin) gives all gram-negative bacteria a red or pink colour.