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More Cell Wall
The Cell Wall
©2002 Timothy Paustian, University of Wisconsin-Madison
This discussion will restrict itself to the eubacterial cell wall. We are going to spend a fair amount of time talking about the cell wall, why is it so important?
The cell wall is a critical structure in bacterial cells. Most bacteria can not live without them. Inside of the bacterial cell there is a high solute concentration and a great pressure on the membrane (75 lb/in2). Outside of the cell there is a low solute concentrate. A fundamental law of physics is that water will tend to flow into a cell to equilibrate the amount of water inside and outside of the cell. Remember that membranes prevent most other molecules from crossing them, but water can. Without something supporting the membrane the cell would swell and burst. A cell wall protects bacteria from osmotic lysis
The cell wall also determines the shape of the cell. Any cell that has lost its cell wall, either artificially or naturally, becomes amorphic, without a defined shape.
Cell wall structure and synthesis is unique to procaryotes. (Plants also make cell walls, but they are completely different structures.) Many compounds found in the bacterial cell wall are found no where else in nature. There are numerous antibacterial agents that target the cell wall because mammals do not synthesize walls and therefore are immune to the toxic effects of these agents. You even synthesize an anti-cell wall enzyme. Lysozyme is an enzyme found in tears and saliva, that breaks down a component of cell walls and it is a critical part of the mammalian defense against bacterial invasion.
Before we begin this discussion of cell walls let me remind you that there are two basic types of bacterial cell wall structures that have been studied in detail. Gram positive (G+) and Gram negative (G-). Bacterial cells look very different following staining with the Gram stain. G+ cells are Purple and G- cells are red.
Figure 1 - A Gram stain of Gram + Staphylococcus cells.
Figure 2 - Gram stain of Gram - E. coli cells
The basis for this differential reaction relates to the cell wall. Look at the Electronmicrographs of a typical Gram + and a typical Gram - cell in the figures below.
Figure 3 - Electron micrograph of a G+ cell wall.
Figure 4 - Electron micrograph of a G- cell wall.
Gram + and - cells do share one thing in common that is unique to bacteria - peptidoglycan. We will talk about the structure of this and then move on to the arrangement of the cell walls.
peptidoglycan is a thick rigid layer that is found in both G+ and G- cells. It composed of a overlapping lattice of 2 sugars that are crosslinked by amino acid bridges. The exact molecular makeup of these layers is species specific.
The two sugars are N-acetyl glucosamine (NAG) and N-acetyl muramic acid (NAM). NAM is only found in the cell walls of bacteria and no where else. Attached to NAM is a side chain generally of four amino acids. Many bacterial cell walls have been looked at and the crossbridge is most commonly composed of...
The chemical structure of peptidoglycan
Note that the D-amino acids are different than the L-amino acids found in proteins. D-amino acids have the identical structure and composition as L-amino acids except that they are mirror images of the L amino acids (See figure below). Most biological systems have evolved to commonly handle only the L form of compounds. Bacteria however use the D-aminoacids in their cell walls and have enzymes called racemases to convert between D and L forms.
Figure 5 - A comparison of L and D amino acids. Note that while the structure is identical, it is impossible to superimpose them.
The NAM, NAG and amino acid side chain form a single peptidoglycan unit that can link with other units via covalent bonds to form a repeating polymer. The polymer is further strengthened by cross links between amino acid 3 (D-glutamic acid above) of one unit and amino acid 4 (DPA) of the next glycan tetrapeptide . In some G+ microbes there is often a peptide composed of glycine, serine and threonine in between the crossbridges. The chapter on metabolism has more information on cell wall synthesis.
The degree of cross-linking determines the degree of rigidity. In G+ cells the peptidoglycan is a heavily cross-linked woven structure that wraps around the cell. It is very thick with peptidoglycan accounting for 50% of weight of cell and 90% of the weight of the cell wall. Electron micrographs show the peptidoglycan to be 20-80 nm thick.
In G- bacteria the peptidoglycan is much thinner with only 15-20% of the cell wall being made up of peptidoglycan and this is only intermittently cross-linked. In both cases the peptidoglycan can be thought of as a strong, woven mesh the holds the cell shape. It is not a barrier to solutes, the openings in the mesh are large and all types of molecules can pass through them.
Figure 6 - A cartoon of the peptidoglycan mesh.
The cell wall is the site of action of many important antibiotics and antibacterial agents. Penicillin inhibits cells wall synthesis. Lysozyme an enzyme found in tears and saliva-attacks peptidoglycan. It hydrolyzes the NAG - NAM linkage.
The Gram + cell wall
A thick peptidoglycan layer constitutes most of the G+ cell wall. As a result, the G+ cell wall is very sensitive to the action of lysozyme and penicillin, or its derivatives. Penicillin is often the antibiotic of choice for infections caused by G+ organisms. An example being Streptococcus pyogenes which causes strep throat. This is almost always treated with some type of penicillin
Figure 7 - The Gram positive cell wall
Another structure in the G+ cell wall is teichoic acid. It is a polymer of glycerol or ribitol joined by phosphate groups. Amino acids, such as D-alanine are attached. Teichoic acid is covalently linked to muramic acid and links various layers of the peptidoglycan mesh together.
Figure 8 - The structure of teichoic acid
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