The importance of the prokaryotic cell wall

 

Almost all prokaryotes have a cell wall outside their cytoplasmic membrane.  The cell wall is the major structure of the cell that determines the cell's shape and helps resist the pressure inside the cell generated by the high osmolarity of the cytoplasm. It is a very important structure for the cell because if there is a flaw in its integrity then the cell will burst and the cell will die.  

In bacteria the cell wall consists of a polymer called peptidoglycan which consists of glycan strands cross-linked by short peptides. The genes that encode the enzymes responsible for synthesizing peptidoglycan are conserved throughout the bacterial domain and are present in almost all bacteria with only a few exceptions, the most notable of which are the mycoplasma, which as intracellular parasites live in an an environment with an osmolarity similar to that of their cytoplasm so have no need for a rigid envelope. 

The archaea do not possess peptidoglycan, instead most have a proteinaceous S-layer but others have alternative cell wall components, for example the cell wall of the methanobacteria is made of a polymer called pseudomurein.  

Because the cell wall synthesis is essential for the growth of the bacerial cell the cell wall is a good target for antibiotics.  For example the β-lactams (like penicillin) and the so called 'antibiotic of last resort' vancomycin target different parts of the pathway that synthesizes peptidoglycan.   

The bacterial cytoskeleton

 

Bacteria have a cytoskeleton, in fact the domain has examples of all three major cytoskeletal proteins: actin; tubulin; and intermediate filaments. The best studied are: the actin homologs MreB, FtsA and ParM; and the tubulin homolog FtsZ. Together MreB and FtsZ control cell wall synthesis in such a way that the shape of the cell is maintained as it grows and divides. In Most rod shaped bacteria MreB controls cell wall synthesis as the rod elongates and FtsZ localises in a ring in the middle of the cell (called the z-ring) and assembles the rest of the cell wall synthetic machine to that point during division.

Bacteria that do not have a cell wall: L-forms

 

L-forms are derivatives of common strains of bacteria that lack the cell wall that is found in the strains from which they are derived. L-forms were first observed by Emmy Klieneberger in the 1930s, who named them after the Lister institute where she worked. 

In cultures of the bacteria Streptobacillus grown in a special media, Emmy Klieneberger observed round cells growing along side the normal rod-shaped bacteria.  She assumed that the round cells were a symbiote, a different species living along side the rod-shaped cells in a beneficial arrangement. It was not until conditions were found to propagate pure cultures of the round cells (or L-forms as they became known as) that it was recognised that the L-form was derived from the rod-shaped cells. The growth of pure cultures of L-forms was aided greatly by the advent of the antibiotic penicillin. Penicillin killed the rod-shaped bacteria, which had a cell wall and were susceptible to this antibiotic, while the L-from was not killed because it did not have a cell wall was resistant to the antibiotic. 

After this early work, L-forms of many common bacterial strain were observed and characterised. Typically, L-form strains were made by passaging cells many times on plates supplemented with an antibiotic like penicillin that target the cell wall. Emmy Klieneberger's special media, which contained high concentrations of the osmoprotectant sucrose, was also needed. In these conditions cell wall synthesis was inhibited but cells did not burst because the sucrose in the media meant the osmolarity in the media was the same as that of the cytoplasm. Over time a strain treated this way would accumulate mutations in genes for cell wall synthesis and become unable to synthesize peptidoglycan. Typically, to make an L-from strain required multiple passages and the whole process could take months. 

I and my colleagues at Newcastle University have developed a new technique for reproducibly generating L-forms of B. subtilis in a few generations [PubMed]. This allowed us to investigate their cell biology using the array of techniques available today. We have shown that L-forms divide by an unusual mechanism and that division does not require FtsZ. This is unusual because in almost all bacteria FtsZ is required for cell division to take place. Normally bacteria divide by growing then separating the cell into two equal parts. We showed that L-forms divide by a different mechanism, that often involved the round cells blebbing or extruding part of their cytoplasm which then breaks up to form several smaller cells. Because this mechanism of division is so different from that of normal bacteria, and it did not require to very important structures in the normal cell, the cell wall and Z-ring, we think that it may represent a mode of division that was used in ancient bacteria that existed before the evolution of the cell wall and the cytoskeleton.

Bacillus subtilis
L-form