Chemotaxis

Motile bacteria sense the presence of chemicals and respond by moving in a certain direction—a phenomenon called chemotaxis. If a compound is a nutrient, it may serve as an attrac-tant, enticing cells to move toward it. On the other hand, if the compound is toxic, it may act as a repellent, causing cells to move away.

The movement of a bacterium toward an attractant is anything but direct (figure 3.40). When E. coli travels, it progresses in a given direction for a short time, then stops and tumbles for a fraction of a second, and then moves again in a relatively straight line. But after rolling around, the cell is often oriented in a completely different direction. The seemingly odd pattern of movement is due to the rotation of the flagella. When the flagella rotate counterclockwise, the bacterium is propelled in a forward movement called a run. The flagella of E. coli and other peritri-chously flagellated bacteria rotate in a coordinated fashion, forming a tight propelling bundle. After a brief period, the direction of rotation of the flagella is reversed. This abrupt change causes the cell to stop and roll, called a tumble. Movement toward an attractant is due to runs of longer duration that occur when cells are going in the right direction; this occurs because cells tumble less frequently when they sense they are moving closer to an attractant. In contrast, they tumble more frequently when they sense they are moving closer to a repellent.

In addition to reacting to chemicals, some bacteria can respond to variations in light, phototaxis. Other bacteria can respond to the concentration of oxygen, aerotaxis. Organisms that require oxygen for growth will move toward it, whereas bac-

(a) No attractant or (b) Gradient of attractant concentration repellent

Figure 3.40 Chemotaxis (a) Cells move via a random series of short runs and tumbles when the attractant or repellent is uniformly distributed. (b) The cell tumbles less frequently resulting in longer runs when it senses that it is moving closer to the attractant.

(a) No attractant or (b) Gradient of attractant concentration repellent

Figure 3.40 Chemotaxis (a) Cells move via a random series of short runs and tumbles when the attractant or repellent is uniformly distributed. (b) The cell tumbles less frequently resulting in longer runs when it senses that it is moving closer to the attractant.

Magnetic particles

Figure 3.41 Magnetic Particles Within a Magnetotactic Bacterium The chain of particles of magnetite (Fe3O4) within the spirillum Magnetospirillum (Aquaspirillum) magnetotacticum serve to align the cell along geomagnetic lines (TEM).

Magnetic particles

Figure 3.41 Magnetic Particles Within a Magnetotactic Bacterium The chain of particles of magnetite (Fe3O4) within the spirillum Magnetospirillum (Aquaspirillum) magnetotacticum serve to align the cell along geomagnetic lines (TEM).

teria that grow only in its absence tend to be repelled by it. Certain motile bacteria can react to the earth's magnetic field by the process of magnetotaxis. They actually contain a row of magnetic particles that cause the cells to line up in a north-to-south direction much as a compass does (figure 3.41). The magnetic forces of the earth attract the organisms so that they move downward and into sediments where the concentration of oxygen is low, which is the environment best suited for their growth.

Pili

Pili are considerably shorter and thinner than flagella, but they have a similar structural theme to the filament of flagella—a string of protein subunits arranged helically to form a long cylindrical molecule with a hollow core (figure 3.42). The functions of pili, however, are distinctly different from those of flagella.

Many types of pili enable attachment of cells to specific surfaces; these pili are also called fimbriae. At the tip or along the length of the molecule is located another protein, an adhesin, that adheres by binding to a very specific molecule. For example, certain strains of E. coli that cause a severe watery diarrhea can attach to the cells that line the small intestine. They do this through specific interactions between adhesins on their pili and the intestinal cell surface. Without the ability to attach, these cells would simply be propelled through the small intestine along with the other intestinal contents. ■ enterotoxigenicE. coli, p. 615

Pili also appear to play a role in the movement of populations of cells on solid media. Twitching motility, characterized by short, jerking movements, and some types of gliding motility, characterized by smooth sliding motion, involve pili.

Another type of pilus is involved in conjugation, a mechanism of DNA transfer from one bacterial cell to another. A sex pilus is used to join those two cells. An example is the F pilus of E. coli. Typically, sex pili are somewhat longer than the types of pili that mediate other characteristics. ■ conjugation, p. 206

Nester-Anderson-Roberts: I I. Life and Death of I 3. Microscopy and Cell I I © The McGraw-Hill

Microbiology, A Human Microorganisms Structure Companies, 2003

Perspective, Fourth Edition

Chapter 3 Microscopy and Cell Structure

I— Flagellum

I— Flagellum

Epithelial cell

Bacterium with pili

Bacterium

Epithelial cell

Bacterium with pili

Figure 3.42 Pili (a) Pili on an Escherichia coli cell.The short pili (fimbriae) mediate adherence; the F pilus is involved in DNA transfer. (11,980x). (b) Escherichia coli attaching to epithelial cells in the small intestine of a pig.

Bacterium

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