Bacteria are bounded by a cell wall. The cell wall defines the shape of the microorganism, exerts some control as to what enters and exits the bacterium, and, in the case of infectious microorganisms, can participate in the disease process.

Many bacteria can be classified as either Gram-positive or Gram-negative. The Gram stain is a method that differentiates bacteria based on the structure of their cell wall. Gram-positive bacteria retain the crystal violet stain that is applied to the bacteria, and appear purple. In contrast, gram-negative bacteria do not retain this stain, but are "counterstained" red by the safranin stain that is applied later. The basis of these different staining behaviors lies in the composition of the cell walls of each Gram type.

Gram-positive bacteria have a cell wall that consists of a single membrane and a thick layer of...