Cell Wall

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Cell Wall

Cell Wall

The details of cell wall structure differ basically between Gram positive and Gram negative bacteria. Primarily, these two groups can be differentiated by Gram strain procedure (the name bearing the name of the microbiologist who devised the method). The cell wall of both Gram +ve and Gram -ve bacteria contain peptidoglycan and murein. Gram +ve bacteria have a thick (15 to 80 nm) peptidoglycan layer composed of chains of alternating subunits of N-acetylglucosamine and N-acetylmuramic acid.

These chains are cross linked (bridged) via tetrapeptide units extending from the N-acetylmuramic acid which imparts rigidity to the cell wall. Species specific cell wall antigens are defined by the protein chains and bridges. Many Gram -ve and some Gram +ve bacteria have diaminopimelic acid (a precursor of L-Iysine) in some tetrapeptide chains. All Gram +ve cell walls possess techoic acids bound to the peptidoglycan layer.

In Gram -ve bacteria, the peptidoglycan layer is thinner (1-2 nm) than Gram +ve bacteria. Unlike Gram +ve bacteria, the negative ones have a phospholipid outer membrane outside the peptidoglycan layer. There is a periplasmic space between the outer membrane and cytoplasmic membrane.

The outer membrane is protective in function against exposure to toxic substances, with porins (membrane proteins) serving to regulate the transport of materials through membrane pores. This regulatory mechanism is partly responsible for antibiotic resistance in Gram -ve bacteria.

Comparision between cell wall of Gram positive (left side of the figure) and Gram negative (right side of the figure) bacteria

A.	surrounded by a polysaccharide capsule
B.	whereas Gram negative bacteria have a thin peptidoglycan layer (A) sandwiched between an outer membrane
C.	and an inner cytoplasmic membrane
D.	the space between the two membranes is known as the periplasmic space
E.	The pilus
F.	and the flagellum
G.	are common to both types and are well embedded in the cell wall.

The presence of lipoprotein layer in the Gram -ve cell wall stabilises the outer membrane and cross links it to diaminopimelic acids in the peptidoglycan layer. Within the outside layer of the outer membrane, lipopolysaccharide (LPS) is embedded in Gram -ve bacteria which has a lipid component (endotoxin, lipid A) responsible for the toxic properties of this group of bacteria.

The polysaccharide component has a common core to all Gram -ve bacteria but the variable terminal segment exposed on the outer surface serves as a major surface antigen. This variability of the terminal segment has been the key to differentiate bacterial isolates based on serological methods.

Many bacteria are enclosed within a polysaccharide slimy layer known as capsule which prevents destruction from outside agents and also helps in adhesion to substrates. There are also pili (fimbriae) which help in adherence by its hair like structure.

Flagella made up of proteins help in the locomotion of bacteria. The number of flagella on a cell is of taxonomic value―monotrichous (single), lopotrichous (at both ends) and peritrichous (all over the surface). Some bacteria (Bacillus and Clostridum) often form spores to overcome dessication.

Gram Stain

Gram stain is commonly used to differentiate between Gram +ve and Gram -ve bacteria by exploiting the characteristic differences in their cell wall nature. Gram +ve bacteria have techoic acid in their walls whereas Gram -ve ones contain lipopolysaccharides.

In 1884, Christian Gram found that some bacteria were coloured deep blue while others remained colourless but absorbed a counter-stain. This finding helped in delineating major structural and taxonomic groups of bacteria. The initial Gram staining method has now undergone several modifications.

Gram stain uses four different reagents. Firstly, crystal violet renders all cells deep blue. A solution of potassium iodide (iodine solution) is then added when iodide replaces the chloride in the crystal violet molecule to form a water insoluble complex. Upto this reaction both Gram +ve and-ve cells react the same way but when a decolourising agent (ethanol, acetone) is added the deep blue stain is removed only from Gram -ve cells.

The mechanism underlying this reaction has been interpreted as differential binding to magnesium or nucleic acids or due to differential permeability.

However, evidence point out that the decolourising agent alcohol damages the outer membrane of Gram -ve bacteria thus allowing the crystal violet iodine complex to leak out while the undamaged Gram +ve bacterial cells retain the stain. A second stain (counter-stain) safranin is taken up by the colourless Gram -ve cells and makes them visible.

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