Cell Walls

Prokaryotic cells are surrounded by a generally rigid cell wall, protecting the cell from osmotic lysis (Figs. 5.5 and 5.6). The cell wall has also been important in identification and classification of bacteria, providing a major division between gram-positive and gram-negative bacteria, defined on the basis of the Gram stain. The cell wall of gram-positive cells consists of a single thick layer of peptidoglycan, surrounding the cytoplasmic membrane. Peptidoglycan is a polymer consisting of a backbone of alternating N-acetylglucosamine and N-acetylmuramic acid residues connected to cross-linked peptide chains of four amino acids. Gram-positive cell walls also usually contain teichoic acids, polymers of glycerol or ribitol, linked by

• Flagellum (provides motility)

FIGURE 5.5 Illustration of the major characteristics of prokaryote cell structure. (See Fig. 5.6 for detailed structure of the bacterial cell wall.)

• Flagellum (provides motility)

FIGURE 5.5 Illustration of the major characteristics of prokaryote cell structure. (See Fig. 5.6 for detailed structure of the bacterial cell wall.)

Gram-negative cell wall

Lipopolysaccharide Porin F'tiossptidpd

Gram-positive cell wall

Lipoteichoic acid Teichoic acid / /

Outer membrane I |

Lipopolysaccharide Porin F'tiossptidpd

Lipoteichoic acid Teichoic acid / /

Outer membrane I |

Peptidoglycan Lipoprotein

Protein

FIGURE 5.6 Detailed structure of gram-negative and gram-positive cell walls.

membrane Phospholipid

Peptidoglycan Lipoprotein

Protein

FIGURE 5.6 Detailed structure of gram-negative and gram-positive cell walls.

phosphate groups and containing amino acids and sugars. In the more complex gram-negative cell wall, the peptidoglycan layer is much thinner and is surrounded by an outer membrane enclosing a periplasmic space, which contains enzymes involved in nutrient acquisition, electron transport, and protection from toxins. In contrast, archaeal cell walls have variable chemical structure, consisting of proteins, glycoproteins, or polysaccharides, but do not contain peptidoglycan (Fig. 5.6).

The cell walls of many bacteria are encased within extracellular material (Fig. 5.5), ranging from apparently rigid and distinct capsules of specific thickness to more diffuse (chemically and physically) extracellular polymeric substances. Many roles have been assigned to this material, including protection from predation, adhesion to solid surfaces, and biofilm formation. In the free-living N2-fixing bacterium Azoto-bacter, extracellular material is important in creating anaerobic regions required for N2 fixation. Biofilm formation is particularly important, with suggestions that the majority of the soil microbial community is attached to particulate matter (clay minerals, soil organic matter, plant roots, and animals). Particulate material provides a concentration of nutrients necessary for microbial growth, and surface attachment has been shown to increase survival of bacteria and to protect them from environmental stress, including low pH, starvation, and inhibition by antibiotics and heavy metals. An example is the production by nitrifying bacteria, in model soil systems, of copious amounts of extracellular material that effectively forms a blanket over colonies, such that individual cells are not visible. This occurs despite the fact that these autotrophic organisms gain barely sufficient energy from oxidation of ammonium or nitrite, use much of this energy to generate reducing equivalents, and require more reducing equivalents because of the requirement to fix CO2. However, once formed, biofilms of these organisms are protected from a wide range of factors to which suspended cells are susceptible. Attachment of cells to surfaces is also facilitated by short, hair-like fimbriae, while similar structures, sex pili, are involved in cell-cell contact associated with plasmid transfer.

Organic Gardeners Composting

Organic Gardeners Composting

Have you always wanted to grow your own vegetables but didn't know what to do? Here are the best tips on how to become a true and envied organic gardner.

Get My Free Ebook


Post a comment