Bacteria cell shape

Bacteria also contain filamentous proteins that resemble F-actin and which may be utilized for cell-shape determination 301c Actin-based motility is used by some bacteria and other pathogens during invasion of host cells (Box 19-C). It is employed by sperm cells of Ascaris and of C. elegans, which crawl by an ameboid movement that utilizes treadmilling of filaments formed from a motile sperm protein, which does not... 

More recently, a closer prokaryotic homolog of actin was characterized. This protein, called MreB, plays an important role in determining cell shape in rod-shaped, fdamentous, and helical bacteria. The internal structures formed by MreB are suggestive of the actin cytoskeleton of eukaryotic cells, although they are far less extensive. Even though this protein is only approximately 15% identical in sequence with actin, MreB folds into a very similar three-dimensional structure. 

The localization of different PBPs and peptidoglycan hydrolases in several bacterial species has been determined using immunofluorescence and, more recently, using GFP-fusions [106, 107,108,109,110]. From these studies, it appears that in coccoid bacteria cell wall synthesis occurs mainly at the septum, while in rod-shaped bacteria different PBPs are localized to sites of cell division and peripheral cell wall synthesis. Furthermore, the localization of the enzymes seems to be dependent on both interactions with other enzymes and with substrate [98]. These visualization techniques have also helped explain the apparent functional redundancy of the PBPs. For example, in S. pneumoniae, different PBPs are speciflcally localized to sites of... 

The regulated polymerization of actin can generate forces that move certain bacteria and viruses or cause changes in cell shape. 

Although inhibition of the transpeptidase is demonstrably important, there are additional targets for the actions of penicillins and cephalosporins these collectively are termed penicillinbinding proteins (PBPs). The transpeptidase responsible for peptidoglycan synthesis is one of these PBPs. Other PBPs are necessary for cell division and cell shape and other essential processes thus, the lethality of penicillin for bacteria involves both lytic and nonlytic mechanisms. 

In the toxic action of tensides, besides the toxicity which follows from the chemical structure of tensides, its physico-chemical effect is important, since it leads to the hydration and swelling of cells. The cell swelling is increased on decreasing the water surface tension (with increasing concentration of tensides), and thus, metabolic processes in cells are suppressed. With a long-term burden or excess swelling the cells are destroyed. Bacteria, moulds and yeast cells, which have firm cellular walls are more sensitive to tenside effects than, for example, protozoa, which have elastic cell walls, which allow limited changes of the cell shape [38]. 

If the aggregates of rod-shaped bacteria cells are assumed to coalesce into spheres, then the foregoing equations predict the variation of scattering eflBciency with the size-refractive index parameter p. Experimentally,... 

When bacteria are exposed to 3-lactams a small amount of the antibiotic becomes covalently linked to the cells. The amount bound varies with the nature of the organism and the antibiotic but it is always very low, of the order of 200-4000 molecules per cell. These covalently tound molecules are responsible for damaging and, ultimately, killing the cell. They are bound to transpeptidases and carboxypeptidases and effectively inhibit the enzymes. Since no other metabolic or biosynthetic activities are affected the cells continue to grow but are unable to produce cross-linked peptidoglycan. Abnormal cell shapes develop and the cells eventually stop growing. The precise fate of the cells depends upon the nature and the concentration of the 3"lactam involved and upon the organism itself. Lysis is a common phenomenon but filaments and cells with other unusual shapes can persist without lysis. 

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