A diverse group of Gram-negative bacteria possess projections called prosthecae, which are extensions of the cytoplasm and cell wall. These extensions are thought to provide increased surface area to facilitate absorption of nutrients. Some prosthecae enable the organisms to attach to a solid surface.
Because of their remarkable life cycle, Caulobacter species have served as a model for the research of cellular differentiation. Entirely different events occur in an orderly fashion at opposite ends of the cell. Caulobacter cells have a single polar prostheca, commonly called a stalk (figure 11.23). At the tip of the stalk is an adhesive holdfast, which provides a mechanism for attachment. To multiply, the cell elongates and divides by binary fission at the end opposite its stalk, producing a motile swarmer cell. This swarmer cell has a single polar flagellum, located at the pole opposite the site of division. This motile cell detaches and moves to a new location, where it adheres via a holdfast near the base of its flagellum. It then loses its flagellum, replacing it with a stalk. Only then can the daughter cell replicate its DNA and repeat the process. In favorable conditions, a single cell can divide and produce daughter cells many times. With each division a ring remains at the site of division, enabling a researcher to count the number of progeny.
Hyphomicrobium species are in many ways similar to Caulobacter species, except they have a distinct method of reproduction. The single polar prostheca of the parent cell enlarges at the tip to form a bud (figure 11.24). This continues enlarging and develops a flagellum, eventually giving rise to a motile daughter cell. The daughter cell then detaches and moves to a new location, eventually losing its flagellum and forming a polar prostheca at the opposite end to repeat the cycle. As with Caulobacter species, a single cell can sequentially produce multiple daughter cells.
11.7 Thriving in Aquatic Environments 287
Figure 11.23 Caulobacter (a) Photomicrograph; (b) life cycle.
Hypha lengthens more and produces another bud
Figure 11.24 Hyphomicrobium (a) Photomicrograph. Note the bud forming at the tip of the polar prostheca. (b) Life cycle.
288 Chapter 11 The Diversity of Prokaryotic Organisms
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