Gram-negative bacteria are bacteria that do not retain crystal violet dye in the Gram staining protocol. In a Gram stain test, a counterstain (commonly safranin) is added after the crystal violet, coloring all Gram-negative bacteria with a red or pink color. The test itself is useful in classifying two distinct types of bacteria based on the structural differences of their cell walls. On the other hand, Gram-positive bacteria will retain the crystal violet dye when washed in a decolorizing solution.
The pathogenic capability of Gram-negative bacteria is often associated with certain components of Gram-negative cell walls, in particular, the lipopolysaccharide (also known as LPS or endotoxin) layer. In humans, LPS triggers an innate immune response characterized by cytokine production and immune system activation. Inflammation is a common result of cytokine (from the Greek cyto, cell and kinesis, movement) production, which can also produce host toxicity.
When treated as a clade, the term "negibacteria" is sometimes used.
The following characteristics are displayed by Gram-negative bacteria:
The proteobacteria are a major group of Gram-negative bacteria, including Escherichia coli, Salmonella, Shigella, and other Enterobacteriaceae, Pseudomonas, Moraxella, Helicobacter, Stenotrophomonas, Bdellovibrio, acetic acid bacteria, Legionella and alpha-proteobacteria as Wolbachia and numerous others. Other notable groups of Gram-negative bacteria include the cyanobacteria, spirochaetes, green sulfur and green non-sulfur bacteria.
Medically relevant Gram-negative cocci include three organisms, which cause a sexually transmitted disease (Neisseria gonorrhoeae), a meningitis (Neisseria meningitidis), and respiratory symptoms (Moraxella catarrhalis).
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