Unicellular Growth Forms

Cell shape depends on internal turgor pressure acting against cell wall components, whose elasticity varies during cell growth. These interactions can be used to explain the differences between rod-shaped and coccoid cells and some of the more detailed aspects of cell shape and morphology. In addition, there is increasing evidence for the involvement of an actin-like skeleton controlling cell growth and shape in bacterial cells. In fact, unicellular bacteria exhibit a wide range of cell forms, including spiral cells, vibrios, pleomorphic cells, stalked cells, "bacteroids," and even square bacteria. Archaea also exhibit a range of unusual growth forms, although the mechanisms generating and ecological significance of these forms are unclear.

The shape, form, and size of prokaryotic cells are important characteristics when considering the ecology of bacteria in the soil. Nutrient uptake will be determined, to some extent, by the surface area:volume ratio for a cell. This factor has been shown to contribute to the ability of spiral-shaped bacteria (e.g., Spirillum) to outcompete rod-shaped pseudomonads under conditions of substrate limitation. Shape and size may also be important in susceptibility to predation, with evidence that protozoa and flagellates choose prey partly on the basis of these factors.

Bacterial shape and size are not fixed and, for Escherichia coli, cell volume can vary 27-fold depending on growth rate. In addition, many bacteria take on different forms depending on environmental conditions. A striking example is the N2-fixing rhizobia, which form nodules on the roots of leguminous crops (Chap. 14). Initial contact with the plant root is through attraction to the root of flagellated, motile, rod-shaped bacteria. After infection, flagella are shed and bacteria form swarmer cells. Rapid division of these cells leads to formation of infection threads and then nodules that contain masses of Rhizobium cells, the majority of which are misshapen bacteroids, with bulging cell walls and unusual morphologies. Another example is Arthrobacter, commonly isolated from the soil, which grows as rod-shaped blue cells at high growth rates and as coccoid purple cells under nutrient-limiting conditions.

These effects are accentuated in starving cells and the transition from growth to starvation is frequently associated with a significant decrease in cell size, changes in cell characteristics (Fig. 5.3), rapid turnover of cell material, a decrease in ribosome number, and expression of a suite of starvation genes (Kjelleberg, 1993). Many of these genes encode high-affinity nutrient uptake systems with broad substrate specificity. These enable cells to sequester a wider range of substrates at low concentrations, giving them an advantage over organisms that, under conditions of nutrient excess, have much greater maximum specific growth rates. Starved forms are resistant to environmental stress and, although not as resistant as bacterial endospores, starved vegetative cells can survive better than growing cells. Direct observation of prokary-otic cells in soil indicates that many are much smaller than typical laboratory-grown organisms, with significant proportions of cells passing through 0.4 ^m pore size filters. There is evidence that these cells are less able to grow in laboratory culture.

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