Bacteria that Form a Resting Stage

Several genera that inhabit the soil can form a resting stage that enables them to survive the dry periods that are typical in many soils. Of these various types of dormant cells, endospores are by far the most resistant to environmental extremes.

Endospore-Formers

Bacillus and Clostridium species are common Gram-positive rod-shaped bacteria that form endospores; the position of the spore in the cell can be used as an aid in identification (figure 11.16). Clostridium species, which are obligate anaerobes, were discussed earlier. Bacillus species may be either obligate aerobes or facultative anaerobes, and many have notable characteristics. A suspension of endospores of B. stearothermophilus, a ther-mophilic species, is used as a biological indicator in quality control procedures to ensure that an autoclave is working properly. Bacillus anthracis causes the disease anthrax, which can be acquired from contacting its endospores in soil or in animal hides or wool. Unfortunately, the spores can also be used as an agent in bioterrorism. Once inhaled, the spores germinate and cause disease. Likewise, spores that enter through breaks in the skin can germinate and cause disease, although considerably less severe. ■ autoclave, p. 114 ■ anthrax, p. 503

The Genus Azotobacter

Azotobacter species are Gram-negative pleomorphic, rod-shaped bacteria that live in alkaline soil. They can form a type of resting cell called a cyst (figure 11.17). These have negligible metabolic activity and can withstand drying and ultraviolet radiation but are not highly resistant to heat. Their formation involves

Figure Clostridium Tetani

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Figure 11.16 Endospore-Formers (a) Endospores forming in the midportion of the cells of Bacillus anthracis. (b) Endospores forming at the ends of the cells in Clostridium tetani. Both of these species can cause fatal disease, but many other species of endospore-formers are harmless.

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Figure 11.16 Endospore-Formers (a) Endospores forming in the midportion of the cells of Bacillus anthracis. (b) Endospores forming at the ends of the cells in Clostridium tetani. Both of these species can cause fatal disease, but many other species of endospore-formers are harmless.

284 Chapter 11 The Diversity of Prokaryotic Organisms

284 Chapter 11 The Diversity of Prokaryotic Organisms

Cyst Forming Azospirillum

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Figure 11.17 Azotobacter (a) Vegetative cells; (b) cyst.

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Figure 11.17 Azotobacter (a) Vegetative cells; (b) cyst.

changes in the cell wall of a vegetative bacterium, causing the cell to shorten and the wall to thicken; the enzyme content of the cyst is similar to that of the cell from which it arises. Cysts differ from endospores in both their manner of formation and their lesser degree of resistance to harmful agents.

Azotobacter species are also notable for their ability to fix nitrogen in aerobic conditions; recall that the enzyme nitroge-nase is inactivated by O2. Apparently, the exceedingly high respiratory rate of Azotobacter species consumes O2 so rapidly that an anaerobic environment is maintained inside the cell.

Myxobacteria

The myxobacteria are a group of aerobic Gram-negative rods that have a unique developmental cycle as well as a resting stage. When conditions are favorable, cells secrete a slime layer that other cells then follow, creating a swarm of cells. But then, when nutrients are exhausted, the behavior of the group changes. The cells begin to move toward each other and congregate. They then pile up to form a complex structure called a fruiting body, which is often brightly colored (figure 11.18). In some species the fruiting body is quite elaborate, consisting of a mass of cells elevated and supported by a stalk made of a hardened slime. The cells within the fruiting body differentiate to become a spherical, dormant form called a microcyst. These are considerably more resistant to heat, drying, and radiation than are the vegetative cells of myxobacteria, but they are much less resistant than bacterial endospores.

Myxobacteria are important in nature as degraders of complex organic substances; they can digest living or dead bacteria and certain algae and fungi. Scientifically, even though the mechanisms the cells use for group communication are still poorly understood, these bacteria serve as an important model for developmental biology. Included in the myxobacteria are members of the genera Chondromyces, Myxococcus, and Stigmatella.

The Genus Streptomyces

The genus Streptomyces encompasses more than 500 species of Gram-positive bacteria that resemble fungi in their pattern of

(a)

Figure 11.18 Fruiting Bodies of Myxobacteria These are the elaborate fruiting bodies of a species of Chondromyces: (a) photograph; (b) scanning electron micrograph.

Figure 11.18 Fruiting Bodies of Myxobacteria These are the elaborate fruiting bodies of a species of Chondromyces: (a) photograph; (b) scanning electron micrograph.

growth. Like the fungi, they form a mycelium, which is a visible mass of branching filaments. The filaments are called hyphae. At the tips of the hyphae, chains form of characteristic spores called conidia (figure 11.19). These dormant spores are desiccation resistant and are readily dispersed in air currents to potentially more favorable locations. Note that while this pattern of growth resembles fungi, which are eukaryotes, Streptomyces species are much smaller and are prokaryotes.

Streptomyces species are obligate aerobes that produce a variety of extracellular enzymes, which enables them to degrade a variety of organic compounds. They are also responsible for the characteristic "earthy" odor of soil; like the cyanobacteria, they produce geosmin. One species of Streptomyces, S. somalien-sis, can cause an infection of subcutaneous tissue called an actin-omycetoma.

Streptomyces species naturally produce a wide array of medically useful antibiotics, including streptomycin, tetracy-cline, and erythromycin. The role that these antimicrobial compounds play in the life cycle of Streptomyces has not been proven,

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Figure 11.19 Streptomyces A photomicrograph showing the spherical conidia at the ends of the filamentous hyphae.

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Figure 11.19 Streptomyces A photomicrograph showing the spherical conidia at the ends of the filamentous hyphae.

but it is quite possible that they provide the organism a competitive advantage.

11.6 Thriving in Terrestrial Environments 285

plant a portion of a plasmid; in Agrobacterium tumefaciens that plasmid is called the Ti plasmid (for "tumor-inducing"). The transferred DNA encodes the ability to synthesize a specific plant growth hormone, causing uncontrolled growth ofthe plant tissue and resulting in a tumor. The transferred DNA also encodes enzymes that direct the synthesis of an opine, an unusual amino acid derivative; Agrobacterium can then use this compound as a nutrient source (see Perspective 8.2).

Scientists have now modified the Ti plasmid, turning it into a commercially valuable tool. By removing those genes that cause tumor formation, the plasmid can be used as a vector to introduce DNA into plant cells. ■ vector, pp. 221,232

The Rhizobia

Gram-negative rod-shaped bacteria that fix nitrogen and form intimate relationships with legumes, a group of plants that bear seeds in pods, make up a group called rhizobia. This group includes members of the genera Rhizobium, Sinorhizobium, Bradyrhizobium, Mesorhizobium, and Azorhizobium. The bacteria live within cells in nodules formed on the root of the plant (figure 11.21). The plant synthesizes the protein leghemoglobin,

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Bacterial Vaginosis Facts

Bacterial Vaginosis Facts

This fact sheet is designed to provide you with information on Bacterial Vaginosis. Bacterial vaginosis is an abnormal vaginal condition that is characterized by vaginal discharge and results from an overgrowth of atypical bacteria in the vagina.

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  • ASPHODEL TOOK-TOOK
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