Gram Negative Bacteria

Definition

A gram-negative bacteria is a prokaryotic cell with a cell wall containing small amounts of peptidoglycan and an outer membrane containing lipopolysaccharides, proteins, and other complex macromolecules. The distinction between gram-positive and gram-negativebacteria is based on a differential stain, the Gram stain, which depends on the structure of the cell wall (see cell wall). After Gram staining, gram-positive bacteria appear purple and gram-negative bacteria appear red. This difference in reaction to the Gram stain arises because of differences in the cell wall structure which promote the retention of the purple dye, crystal violet, in the gram-positive bacteria after decoloring with ethanol, while in the gram-negative, the stain is not retained. The Gram stain is one of the most useful staining procedures in microbiology. In addition to the peptidoglycan, gram-negative bacteria contain an additional wall layer, the outer membrane. This layer is effectively a second...