Bacteria are categorised into two main subgroups 'Gram negative' and 'Gram positive' bacteria (with the exception of Mycobacterium Tuberculosis which falls into neither of the two groups.) The Gram method of classification is dependent upon cell wall structure. In this article I am going to focus on the 'Gram negative' wall structure.
A Gram negative bacteria has a base lipid bi-layer similar to an eukaryotic plasma membrane, surrounded by the periplasmic space. A thin peptidoglycan layer, much thinner in comparison to that of the gram positive bacteria then arises seperating the two periplasmic compartments. Peptidoglycan is a polymer made up of two repeating units of N-acetylmumaric acid and N-acetylglucosamine which form linear chains due to cross linkages formed by the tetrapeptide side chains of the monomers. The outermost external barrier of a gram negative bacteria is a lipid like bi-layer, but this is highly disimilar from that of an eukaryotic plasma membrane. The inner leaflet of this outer membrane is studded with lipoproteins which associate to the cytoskelton and peptidoglycan layer. The outer leaflet is made up of LPS lipidpolysaccharide; composed of Lipid A a fucntional endotoxin when released and an O polysacharide tail.
Due to the lipid characteristics of the outermost membrane of the cell wall the gram negative bacteria are stained pink when gram stained.
Gram-negative refers to a classification of bacteria based upon their cell wall structure. Gram-negative bacteria appear red as a result of Gram testing, whereas Gram-positive stain purple.
The structure of the Gram-negative bacterial cell wall is what distinguishes it from Gram-positive bacteria. Gram-negative bacteria contain a much thinner layer of Peptidoglycan in comparison to a Gram-positive bacterial cell wall. The Gram-negative cell wall consists of a unique outer membrane, containing lipopolysaccharides, murein lipoproteins and porin channels. There is also a periplasmic space between the peptidoglycan cell wall, and the cell membrane.
Gram-negative bacteria exhibit stronger resistance to Antibiotics such as Lysozyme and pennicillin G, as well as greater resistance to dyes and detergents. The lipopolysaccharide consists of a core polysaccharide, Lipid A and O-antigen. This lipolysaccharide layer is important in excluding large hydrophobic susbstances from interacting with the cell. Lipid A attaches to the outer membrane ensuring that the lipopolysaccharide remains attached to the cell .