The Cell Wall Structure of Bacteria

The Cell Wall

The study of the bacterial cell wall dates back to more than five decades when Salton and Horne (1951) described the structure of cell wall for the first time; this was later confirmed by electron microscopic studies. Cell wall is a dense layer surrounding the plasmamembrane and functions to give shape and rigidity to the cell. Concentration of dissolved solutes inside a bacterial cell like that of E. coli develops turgor pressure estimated at 2 atmospheres which is roughly the same as the pressure in an automobile tier. This amount of pressure is counterbalanced by the cell-wall

Peptidoglycan

Peptidoglycan, the main constituent or back-bone bacterial cell wall, consists of two parts: a glycan or sugar portion and a peptide portion. The glycan portion is made up of alternating units of N-acetylglucosamine and N-acetylmuramic acid bonded each other by β-1, 4-linkages. The peptide portion is a short-chain composed of four amino acids (L-alanine, D-glutamine, either L-lysine or diaminopimelic acid, and D-alanine) connected with each other by peptide-linkages and hence is called tetrapeptide chain. The two adjacent of different tetrapeptide chains are interlinked by a cross-linkage (peptide interbridge). The type and extent of cross-linkages may vary among different species. In some species, the cross-linkage forms between the carboxyl group (-CO-) of an amino acid in one tetrapeptide chain and amino group (-NH-) of an amino acid in other tetrapeptide chain (Fig.3.10). In others, a pentaglycine chain is used to link two tetrapeptide side chains.

Structural organization of peptidoglycan showing β-1, 4-linkage (peptide interbridege and peptide linkage.



It is these cross-linkages that provide rigidity to the peptidoglycan which helps protecting the cell against osmotic socks exerted on it.

Structural Organization of peptidoglycan Showing Cross-linkages (peptide interbridge) by a Pentaglycine chain


However, the amino acid diaminopimelic acid (DAP) does not occur in the peptidoglycan of all bacteria; only all gram-negative bacteria and some gram-positive bacteria possess it. Most of the gram-positive bacteria have amino acid lysine instead of DAP. Another unusual feature of the peptidoglycan (i.e. the bacterial cell wall) is the presence of two amino acids that have the D-configuration, D-alanine and D-glutamine. It is because in proteins amino acids are always of L-configuration.

Gram-positive Cell Walls

The cell wall of Gram-positive bacteria (Bacillus, Streptococcus, etc.) appears as a thick homogenous layer, and mainly consists of peptidoglycan (up to 90%). The remainder being made up of proteins, polysaccharides and teichoic acid. Teichoic acids are acidic polysaccharides, which lie on the outer surface of the peptidoglycan, and are covalently bonded with it. Their functions are not known with certainty; they are considered to affect the passage of ions, thereby help maintain the cell wall at a relatively low pH so that self-produced enzymes (autolysins) do not degrade the cell wall. Other functions are also attributed to teichoic acid such as binding metals and acting as receptor sites for some viruses

Gram-negative Cell Walls

The wall of gram-negative bacteria (Rhizobium, Escherichia, Salmonella, etc.) is biochemically far more complex than of gram-positive bacteria and appears usually trilayered (Fig. 3.14B). The innermost layer is the plasma membrane of the cell made up of phospholipid bilayer; the middle layer is the peptidoglycan (10% or less), and the outer most layer represents the outer membrane. The region between the inner plasma membrane and the outer membrane is called periplasmic space. The inner half of the outer membrane is similar to the plasma membrane, but the outer half contains lipoplysaccharides (fat-carbohydrates) in place of phospholipids

An overall structure of peptidoglycan. M=N-acetylmuramic acid, G = N-acetyglucosamine

Cell wall composition of gram-positive bacteria A. and gram-negative bacteria	Cell wall composition of gram-positive bacteria B. Shown alongwith plasma-membrane

The outer membrane is present outside the thin peptidoglycan layer (Fig. 3.15) Brauns lipoprotein is the most abundant protein occurring in the outermembrane. It is a small lipoprotein covalently joined to the underlying peptidoglycan and embedded in the outer membrane by its hydrophobic end. Brauns lipoprotein joins the outermembrane and peptidoglycan so firmly that both can be isolated as a single unit. There are, however, a special type of porin proteins present in the outer membrane. Three porin molecules cluster together and span the outer membrane to form a narrow channel through which molecules smaller than about 600-700 daltons can pass.

The most unusual constituents of the outer membrane are its lipopolysaccharides (LPSs). The latter are large, complex molecules consisting of three parts: lipid A, the core polysaccharide, and the O side chain or O antigen. Lipid A is burried in the outer membrane while the remaining core polysaccharide and 0 side chain project from the surface. Lipid A is a major constituent of lipopolysaccharide and helps stabilize the outer membrane. Lipid A often is toxic and functions as an endotoxin.

The core polysaccharide usually contains charged sugars and phosphate and contributes to the negative charge on the bacterial surface. O side chain or O antigen is a polysaccharide consisting of several peculiar sugars and varies in composition between bacterial strains. 0 side chains rapidly change their nature to avoid detection and thus help bacteria to thwart host defences.

The chemistry of lipid A and the polysaccharide components varies among species of gram-negative Bacteria, but the major components (lipid A–KDO–core–O-specific)  are typically the same. The O-specific polysaccharide varies greatly among species. KDO, ketodeoxyoctonate; Hep, heptose; Glu, glucose; Gal, galactose; GluNac, N-acetylglucosamine; GlcN, glucosamine; P, phosphate. Glucosamine and the lipid A fatty acids are linked through the amine groups. The lipid A portion of LPS can be toxic to animals and comprises the endotoxin complex.

Molecular level digrammatic representation of the cell (alongwith plasma membrane) of a gram negative bacteium

1.	Outer Membrane	6.	Lipoprotein	11.	Phospholipid
2.	Periplasmic Space	7.	Porin Protein	12.	Braun's Protein
3.	Plasma Membrane	8.	'O' Side Chain	13.	Peptidoglycan
4.	Lipopoly Saccharide	9.	Core Polysaccharide	14.	Intergral Proteins
5.	Other Om Protein	10.	Lipid A	15.	Integral Proteins

Outer membrane serves as protective barrier. Despite its permeability to small molecules due to porin proteins, the outer membrane prevents or slows the entry of bile salts, antibiotics, lysozymes and other toxic substances which might kill or injure the bacterium. As a result, infections with gram­-negative bacteria are often more difficult to treat. Since teichoic acid is absent and the peptidoglycan is less in amount, the wall of gram-negative bacterium is less rigid as compared to that of gram-­positive one.

Difference in Cell Walls of Gram-positive and Gram-negative Bacteria

Gram   -positive	Gram-negative
1. Cell wall appears thick and homogenous.	1. Cell wall appears thin and usually tri-layered.
2. Peptidoglycan comprises upto 90% of the cell wall hence more rigid.	2. Peptidoglycan comprises only 10% or less of the cell wall hence less rigid.
3. Besides peptidoglycan, there are teichoic acids, other polysaccharides and proteins in the cell wall.	3. Besides peptidoglycan, there are phospholipids, proteins and lipopolysaccharides in the cell wall. Teichoic acids are absent.
4.Teichioc acids are the main surface antigens	4. Lipopolysacchrides are the main surface antigens.
5. More sensitive to wall attacking antibiotics like penicillin.	5. Less sensitive to wall attacking antibiotics like penicillin.