Gram-negative bacteria
The gram staining characteristics of bacteria are denoted as negative or positive, dependent on if the bacteria take up and retain the crystal violet stain or not. Gram-negative bacteria lose the crystal violet stain in Gram's technique of staining. This is characteristic of bacteria that have a cell wall made up of a thin layer of a particular substance (called peptidoglycan). The thin peptidoglycan layer in their cell wall is sandwiched between an inner cytoplasmic cell membrane and also a bacterial outer membrane. Just after staining with crystal violet, an alcohol wash is utilized which decolorizes the bacteria showing that their peptidoglycan layer is too thin to retain the stain and enabling identification.
Cell envelope:
The Gram-negative cell envelope contains a different outer membrane composed by phospholipids and lipopolysaccharides which face the external environment. The highly charged nature of lipopolysaccharides confers a total negative charge towards Gram-negative cell wall. The chemical structure of the outer membrane lipopolysaccharides is normally unique to specific bacterial strains and is responsible for most of the antigenic properties of these strains. Many species of Gram-negative bacteria are pathogenic. This pathogenicity is usually associated with the lipopolysaccharide (LPS) layer of the Gram-negative cell envelope.
Gram-negative bacteria include:
- Actinobacillus
- Brucella
- Campylobacter
- Cyanobacteria
- Enterobacter
- Erwinia
- Escherichia coli
- Franciscella
- Helicobacter
- Hemophilus
- Klebsiella
- Legionella
- Moraxella
- Neisseria
- Pasteurella
- Proteus
- Pseudomonas
- Salmonella
- Serratia
- Shigella
- Treponema
- Vibrio
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