Anaerobic Chemoorganotrophs Fermentation

Numerous types of anaerobic bacteria obtain energy using the process of fermentation, producing ATP only by substrate-level phosphorylation. The end products of fermentation include a variety of acids and gases that are generally characteristic for a given species. Consequently, they can often be used as identifying markers.

The Genus Clostridium

Members of the genus Clostridium are Gram-positive rods that can form endospores (see figure 3.46). They are common inhabitants of soil, where the vegetative cells live in the anaerobic microenvironments created when aerobic organisms consume available O2. Their endospores, a dormant form, are indifferent to O2 and can survive for long periods by withstanding measures of heat, desiccation, chemicals, and irradiation that would kill all vegetative bacteria. When the appropriate conditions are renewed, these endospores germinate, and the resulting vegetative bacteria may once again multiply. Vegetative cells that arise from soil-borne endospores are responsible for a variety of diseases, including tetanus (caused by C. tetani), gas gangrene (caused by C. perfringens), and botulism (caused by C. botulinum). Some species of Clostridium are normal inhabitants of the intestinal tract of humans and other animals. ■ endospores, p. 67 ■ tetanus, p. 698 ■ gas gangrene, p. 701 ■ botulism, p. 672

As a group, Clostridium species ferment a wide variety of compounds, including sugars, cellulose, and ethanol. Some of the end products are commercially valuable; for example, C. acetobutylicum produces acetone and butanol. Some species can ferment amino acids by an unusual process that oxidizes one amino acid, using another as a terminal electron acceptor. This generates a variety of foul-smelling end products associated with putrefaction.

The Lactic Acid Bacteria

Gram-positive bacteria that produce lactic acid as a major end product of their fermentative metabolism make up a group called the lactic acid bacteria. They include members of the genera Streptococcus, Enterococcus, Lactococcus, Lactobacillus, and Leuconostoc. Most can grow in aerobic environments, but all are obligate fermenters and thus derive no benefit from

11.1 Anaerobic Chemotrophs 275

O2. They can be readily distinguished from other bacteria that grow in the presence of O2 because they lack the enzyme cata-lase. This enzyme converts hydrogen peroxide, a toxic derivative of oxygen, into water and O2. ■ obligate fermenters, pp. 89, 151

Streptococcus species are cocci that typically grow in chains of varying lengths (figure 11.3). They inhabit the oral cavity, generally as normal flora. Some, however, are notable for their adverse effect on human health. One of the most important is S. pyogenes (Group A strep), which causes pharyngitis (strep throat) and other diseases. Unlike the streptococci that typically inhabit the throat, S. pyogenes is ^-hemolytic, an important characteristic used in its identification (see figure 4.6).

■ Streptococcus pyogenes, p. 565 ■ hemolysis, p. 94

Species of Lactococcus and Enterococcus were at one time included in the genus Streptococcus. The genus Lactococcus now includes those species used by the dairy industry to produce fermented milk products such as cheese and yogurt. Species of the genus Enterococcus typically inhabit the intestinal tract of humans and other animals. ■ fermentation of dairy products, p. 804

Members of the genus Lactobacillus are rod-shaped organisms that grow as single cells or loosely associated chains. They are common among the microflora of the mouth and the healthy human vagina during child-bearing years. In the vagina, they metabolize glycogen, which has been deposited in the vaginal lining in response to the female sex hormone, estrogen. The low pH that results helps the vagina resist infection. Lactobacilli are also often present in decomposing plant material, milk, and other dairy products. Like the lactococci, they are important in the production of fermented foods (figure 11.4).

The Genus Propionibacterium

Propionibacterium species are Gram-positive pleomorphic (irregular shaped) rods that produce propionic acid as their primary fermentation end product. Significantly, they can also ferment lactic acid. Thus, Propionibacterium species can extract residual energy from a waste product of another group of bacteria.

Propionibacterium species are important to the dairy industry, because their fermentation end products play an indispensable role in the production of Swiss cheese. The propionic

Figure 11.3 Streptococcus Species (a) Gram stain. (b) Scanning electron micrograph.

Figure 11.3 Streptococcus Species (a) Gram stain. (b) Scanning electron micrograph.

10 mm

10 mm

5 mm

Figure 11.4 Lactobacillus Species from Yogurt

5 mm

Figure 11.4 Lactobacillus Species from Yogurt acid is responsible for the typical nutty flavor of Swiss cheese. The CO2, also a product of the fermentation, creates the signature holes in the cheese. Propionibacterium species are also found growing in the intestinal tract and in anaerobic microenvironments on the skin. ■ Swiss cheese production, p. 805

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Responses

  • Vilho Luostarinen
    What are anaerobic chemoorganotrophs?
    3 years ago
  • ritva
    What are anaerobic chemotrophs?
    2 years ago
  • aurelio lucchesi
    What are aerobic chemoorgnotrophs?
    6 months ago

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