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.

6 comments:

  1. I've wondered from time to time just what "Gram negative" or "Gram positive" means. Now I know. Thanks.

    I learn something interesting almost every time I come to your blog, Larry.

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  2. The pentaglycine bridge is the structure in S. aureus. E. coli uses a different structure, diaminopimelic acid, to cross link the PG. There is a fair amount of variability. BTW, on the topic of the post above, the diagrams of Penicillin and D-ala-d-ala are terrific, I'm going to use them in my next micro class.

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  3. "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."

    Ah, so that is the difference! One less mystery of life - still ℵ0 mysteries to go.

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  4. Thanks Larry but could you answer a quick question. Is it true (or commonly accepted) that it is this rigid bacterial cell wall that has prevented the bacteria from evolving multi-cellular forms and diversifying as the eukaryotes have?

    This is my understanding but does the cellulaose cell wall of plants not pose a similar problem?

    I don't expect you to 'do requests' but I would greatly appreciate an article (or even a comment or a good reference) on how the biological species concept is applied to bacteria as the definitions that I have read refer to populations of inter-breeding organisms but don't discuss how this can be applied to organisms that don't reproduce sexually.

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  5. Neil asks,

    Thanks Larry but could you answer a quick question. Is it true (or commonly accepted) that it is this rigid bacterial cell wall that has prevented the bacteria from evolving multi-cellular forms and diversifying as the eukaryotes have?

    No. There are several multicellular species of bacteria. Perhaps the most obvious are the myxobacteria. They do not have rigid cell walls and neither do lots of other species. This shows that the presence of rigid cell walls in bacteria is not a barrier to developing multicellular organisms.

    Another common example is cyanobacteria. Many species form multicellular colonies with several different cell types such as spore-forming cells, and heterocysts for nitrogen fixation.

    I don't expect you to 'do requests' but I would greatly appreciate an article (or even a comment or a good reference) on how the biological species concept is applied to bacteria as the definitions that I have read refer to populations of inter-breeding organisms but don't discuss how this can be applied to organisms that don't reproduce sexually.

    This is a complicated problem. John Wilkins is one of the world's experts on species. In fact, he has just posted an article on the topic ["Species" in the Stanford Encyclopedia updated] and he has just had a paper accepted on Microbial Species. I suggest you read John's postings on the topic.

    If you're still confused—very likely because John is a philosopher—then don't be upset. Nobody has any better ideas than John.

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  6. Thanks Larry, I followed your links to John Wilkins's site and am now much the wiser concerning microbial species.

    The reason I asked the question was that I recently read an evolution denier assert that speciation had never been observed in bacteria and that the emergence of drug resistant bacteria was not due to or evidence for 'Darwinism' (sic).

    The more I learn about evolution the more transparent and pathetic the evolution deniers arguments appear.

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