Tuesday, June 12, 2007
Bacteria Have Cell Walls
Most species of bacteria have a cell wall. The rigid cell wall prevents the bacterial cell from expanding in solutions where the salt concentrations are lower than the salt concentration inside the cell. If it wasn't for the cell wall, bacteria wouldn't be able to live in fresh water or sea water.
Gram positive bacteria have a thick cell wall on the exterior that picks up the purple Gram stain (named after Hans Christian Gram). Gram negative bacteria, such as the E. coli cell shown in the figure, do not stain with the dye because the thinner cell wall lies between the inner and outer membranes.
The cell wall is made up of peptidoglycan, which, as the name implies, is a combination of polysaccharide (glycan) and peptides. The polysaccharide consists of alternating N-acetylglucosamine (GlcNac) and N-acetylmuramic (MurNAc) resides [see Glycoproteins].
During cell wall synthesis a short peptide of five amino acid residues is attached to the polysaccharide. The sequence of this peptide varies slightly from species to species. In some gram negative bacteria the sequence is L-alanine- D-isoglutamate- L-lysine- D-alanine- D-alanine. These short chains are linked to each other by another peptide consisting of five glycine residues. When the cross-links are formed, the terminal D-alanine residue is cleaved off so the final structure has only a single D-alanine at the end. (The significance of this cleavage will become apparent in subsequent postings.)
The completed peptidoglycan cell wall is extremely rigid because of the peptide crosslinks between the polysaccharide chains. In the figure above, the original peptide chain is colored blue and the second pentaglycine peptide is colored red. The right end of the red pentaglycine is covalently attached to the blue D-alanine residue at the bottom of an adjacent polysaccharide chain as shown in the cartoon in the upper right corner of the figure.