The Gram-stain technique is used to classify bacteria as either Gram-positive or Gram-negative depending on their colour following a specific staining procedure originally developed by Hans Christian Gram. As the word "Gram" is derived from a name it is always written with an upper case "G".

Following the Gram stain procedure, and on visualization with a microscope Gram-positive bacteria appear dark blue or violet due to the crystal violet stain; Gram-negative bacteria, which cannot retain the crystal violet stain, appear red or pink due to the counterstain. Gram-positive bacteria retain the crystal violet due to a difference in structure of their cell wall, specifically the amount of peptidoglycan.

Gram-negative bacteria do not retain the crystal violet dye in the Gram stain protocol. Gram-negative bacteria will thus appear red or pink following the Gram stain procedure due to the effects of the counterstain (for example safranin).

The cell envelope is defined as the cell membrane and cell wall plus an outer membrane, if one is present. The cell envelope of Gram-negative bacteria contains an outer membrane composed by phospholipids and lipopolysaccharides which face the external environment. The lipopolysaccharides confer an overall negative charge to the Gram-negative cell wall. The chemical structure of the outer membrane lipopolysaccharides is often unique to specific bacterial strains. Many species of Gram-negative bacteria are pathogenic. This pathogenicity is often associated with the lipopolysaccharide layer of the Gram-negative cell envelope.

Gram-negative bacteria have a characteristic cell envelope structure very different from Gram-positive bacteria. Gram-negative bacteria have a cytoplasmic membrane, a thin peptidoglycan layer, and an outer membrane containing lipopolysaccharide. There is a space between the cytoplasmic membrane and the outer membrane called the periplasmic space or periplasm. The periplasmic space contains the peptidoglycan.

Genera of Gram-negative bacteria include:

• Acinetobacter
• Actinobacillus
• Bordetella
• Brucella
• Campylobacter
• Cyanobacteria
• Enterobacter
• Erwinia
• Escherichia coli
• Franciscella
• Helicobacter
• Hemophilus
• Klebsiella
• Legionella
• Moraxella
• Neisseria
• Pasteurella
• Proteus
• Pseudomonas
• Salmonella
• Serratia
• Shigella
• Treponema
• Vibrio
• Yersinia

Labels: bacteria, bacteriology, bacterium, gram negative bacteria, gram stain

Lipopolysaccharide (LPS) is the major component of the outer leaflet of the outer membrane of Gram-negative bacteria. The LPS molecule is composed of two biosynthetic entities: the lipid A - core and the O-polysaccharide (O-antigen). Most biological effects of LPS are due to the lipid A part, however, there is an increasing body of evidence indicating that O-antigen (O-ag) plays an important role in effective colonization of host tissues, resistance to complement-mediated killing and in the resistance to cationic antimicrobial peptides that are key elements of the innate immune system.

Recently, data has started to accumulate on the intricacies in the genetic regulation of the structural components of this molecule and this is highly relevant to the biological function of the molecule.

from Skurnik M and Bengoechea JA in Bacterial Polysaccharides: Current Innovations and Future Trends (2009) Ullrich M (Ed) Published by Caister Academic Press

Further reading: Bacterial Polysaccharides

Labels: gram negative bacteria, lipopolysaccharide, LPS, pathogenesis, polysaccharides, regulation