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Today, anaerobic habitats still abound. Mud and tightly packed soil limit the diffusion of gases, and any O2 that does penetrate is rapidly consumed by aerobically respiring chemotrophs. This creates anaerobic conditions just below the surface. Aquatic environments may also become anaerobic if they contain high levels of nutrients, permitting the rapid growth of O2-consuming microbes. This is evident in polluted lakes, where fish may die because of a lack of dissolved O2. The bodies of humans and other animals also provide numerous anaerobic environments. It is estimated that 99% of the bacteria that inhabit the intestinal tract are obligate anaerobes. Even the skin and the oral cavity, which are routinely exposed to O2, have anaerobic microenvironments. These are created via the localized depletion of O2 by aerobes. â–  obligate anaerobes, p. 88

Anaerobic Chemolithotrophs

Chemolithotrophs oxidize reduced inorganic chemicals such as hydrogen gas (H2) to obtain energy. Those that grow anaer-obically obviously cannot use O2 as a terminal electron acceptor and instead must use an alternative such as carbon dioxide or sulfur. Relatively few anaerobic chemolithotrophs have been discovered, and most are members of the Domain Archaea. Some that inhabit aquatic environments will be discussed later.

The Methanogens

The methanogens are a group of archaea that generate ATP by oxidizing hydrogen gas, using CO2 as a terminal electron acceptor. This process generates methane (CH4), a colorless, odorless, flammable gas:

Many methanogens can also use alternative energy sources such as formate; some can use methanol or acetate as well. A wide variety of morphological varieties of methanogens have been described, including rods, cocci, and spirals (figure 11.2). Representative genera of methanogens include Methanococcus and Methanospirillum.

5 mm

5 mm

Figure 11.2 Methanogens (a) Phase-contrast micrograph of a Methanospirillum species. (b) Scanning electron micrograph of a Methanosarcina species.

5 mm

5 mm

Figure 11.2 Methanogens (a) Phase-contrast micrograph of a Methanospirillum species. (b) Scanning electron micrograph of a Methanosarcina species.

272 Chapter 11 The Diversity of Prokaryotic Organisms

Table 11.2 Medically Important Chemoorganotrophs

Organism

Medical Significance

Page Number

Disease Description Page Number

Gram-Negative Rods

Bacteroides

Enterobacteriaceae Enterobacter species Escherichia coli

Klebsiella pneumoniae Proteus species Salmonella Enteritidis

Salmonella Typhi Shigella species Yersinia pestis

Haemophilus influenzae Haemophilus ducreyi Legionella pneumophila

Pseudomonas aeruginosa

Gram-Negative Rods—Obligate

Chlamydia pneumoniae Chlamydia psittaci Chlamydia trachomatis

Coxiella burnetii Ehrlichia chaffeenis Orientia tsutsugamushi Rickettsia prowazekii Rickettsia rickettsii Gram-Negative Curved Rods Campylobacter jejuni

Helicobacter pylori

Vibrio cholerae

Vibrio parahaemolyticus

Obligate anaerobes that commonly inhabit the mouth, intestinal tract, and genital tract. 291 708,719

Causes abscesses and bloodstream infections.

Normal flora of the intestinal tract. 282

Normal flora of the intestinal tract. Some strains cause urinary tract infections; some strains 282 614,636

cause specific types of intestinal disease. Causes meningitis in newborns.

Normal flora of the intestinal tract. Causes pneumonia. 282 577

Normal flora of the intestinal tract. Causes urinary tract infections. 282 636

Causes gastroenteritis. Grows in the intestinal tract of infected animals; acquired 282 616

by consuming contaminated food.

Causes typhoid fever. Grows in the intestinal tract of infected humans; transmitted in feces. 282 616

Causes dysentery. Grows in the intestinal tract of infected humans; transmitted in feces. 282 613

Causes bubonic plague, which is transmitted by fleas, and pneumonic plague, which 282 723

is transmitted in respiratory droplets of infected individuals.

Causes ear infections, respiratory infections, and meningitis in children. 291 571, 666

Causes chancroid, a sexually transmitted disease 291 644

Causes Legionnaires' disease, a lung infection. Grows within protozoa; acquired by inhaling 289 584

contaminated water droplets.

Causes burns, urinary tract, and bloodstream infections. Ubiquitous in the environment. Grows in 281 697

nutrient-poor aqueous solutions and is resistant to many disinfectants and antimicrobial medications.

Intracellular Parasites

Causes atypical pneumonia, or "walking pneumonia." Acquired from an infected person. 292

Causes psittacosis, a form of pneumonia.Transmitted by birds. 292

Causes a sexually transmitted disease that mimics the symptoms of gonorrhea. Also causes 292 646

trachoma, a serious eye infection, and conjunctivitis in newborns.

Causes Q fever. Acquired by inhaling organisms shed by infected animals. 292

Causes human ehrlichiosis.Transmitted by ticks. 292

Causes scrub typhus.Transmitted by mites. 292

Causes epidemic typhus.Transmitted by lice. 292 533

Causes Rocky Mountain spotted fever.Transmitted by ticks. 292 541

Causes gastroenteritis. Grows in the intestinal tract of infected animals; acquired 291 617

by consuming contaminated food.

Causes stomach and duodenal ulcers. Neutralizes stomach acid by producing urease, 291 605

resulting in the breakdown of urea to form ammonia.

Causes cholera, a severe diarrheal disease. Grows in the intestinal tract of infected humans; 289 611

acquired by drinking contaminated water.

Causes gastroenteritis. Acquired by consuming contaminated seafood. 289

(Continued)

Table 11.2 (Continued)

Organism

Medical Significance

Page Number

Disease Description Page Number

Gram-Negative Cocci

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