Gram negative

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 simialr 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.