Fig. 1.29. Representation of several forms of bacteria. (A) Coccus cell, (B) rod cell with flagella, (C) rod cell with an endospore, (D) rod cells in a filament.

Fig. 1.29. Representation of several forms of bacteria. (A) Coccus cell, (B) rod cell with flagella, (C) rod cell with an endospore, (D) rod cells in a filament.

of a hollow cylindrical tube, composed of polymerized flagellin monomers. The flagellin protein monomers pass into the hollow flagella and polymerize at the tip. The flagellum is anchored on to the cell by a hook and motor complex which consists of about 100 polypep-tides. Rotation of the flagellum is caused by proton pump proteins, which establish an electromotive gradient. Normally, the positions of the flagella are polar or medial in species that swim in free water. They are positioned laterally on those that swim or glide in thin water films on solid substrates. Motile forms can move towards nutrients or away from chemicals by chemotaxis. Similarly, filamentous species can grow towards or away from chemicals through the direction of elongation and branching. Many genera do not have the capacity to move on their own, and depend on Brownian movement and external disturbance to be carried. In soils, bacteria normally occur as more or less aggregated colonies, so that species patchiness is expected at the 10-2000 ^m scale. They are disaggregated during sampling, and during preparative steps for enumeration or culture. Bacterial abundances can be as high as 1010/g dry soil in litter and surface soils, representing dozens or hundreds of species. They have been reported at great depth, 200-400 m below the surface, at 104-106/g soil and were demonstrated to be active by 14C uptake (Lengeler et al., 1999, p. 784). However, it is most probable that they are carried by water flow-through rather than being normal residents of that habitat.

Bacteria consist of a cytoplasm bounded by a cell membrane and an outer cell wall. The cytoplasm of bacteria is not compartmentalized by membranes, and the single circular chromosome is free in the cytoplasm. The cytoplasm contains prokaryotic ribosomes for protein translation, and sometimes other inclusions depending on physiological conditions. There are no endomembranes, no membrane-bound vesicles or vacuoles for endo-/exocytosis, and no cytoskeleton. However, phototrophic genera such as the Cyanobacteria, have invaginations of the inner cell membrane to increase the surface area of bound photosynthesis enzyme complex. The cell membrane is chemically different from that of eukaryotes, and is bound by an external cell wall. This confers to bacteria physical and physiological properties different from those of eukaryotes.

The cell wall is the barrier between the habitat and the cytoplasm. Its composition and function are central to bacteriology. Bacterial systematics traditionally were based on cell wall chemistry and metabolic biochemistry. More recently, it has been complemented by comparative DNA sequence phylogenies. The cell wall is responsible for keeping the cytoplasm molecules in, selectively transporting nutrient molecules into the cytoplasm, maintaining a favourable physiological environment inside the cytoplasm and providing a chemical and mechanical barrier to the habitat (Fig. 1.30). The two principal

Archaea outer membrane periplasm cell membrane 8 nm

Gram positive Gram negative

Gram positive Gram negative outer membrane periplasm

porin proteins lipopolysaccharide slime layer = outer membrane — peptidoglycan murein wall cell membrane porin proteins lipopolysaccharide

Was this article helpful?

0 0

Post a comment