Temperature and Disease

Wide variations exist in the temperature of various parts of the human body. Although the heart, brain, and gastrointestinal tract are near 37°C, the temperature of the extremities may be much lower. For these reasons, some microorganisms can cause disease in certain body parts but not in others. For example, Hansen's disease (leprosy) typically involves the coolest regions of the body (ears, hands, feet, and fingers) because the causative organism, Mycobacterium leprae, grows best at these lower temperatures. The same situation applies to syphilis, in which lesions appear on the genitalia and then on the lips, tongue, and throat. Indeed, for more than 30 years the major treatment of syphilis was to induce fever by deliberately introducing the agent that causes malaria, which induces very high fevers. Hot-bath spas were commonly recommended as a less drastic treatment. ■ Hansen's disease, p. 670 ■ syphilis, p. 648

Oxygen (O2) Requirements

The oxygen (O2) level in different environments varies greatly, providing many different habitats with respect to its availability.

Gaseous oxygen accounts for about 20% of the earth's atmosphere. Beneath the surface of soil and in swamps, however, very limited amounts, if any, may be available. The human body alone provides many different habitats. While the surface of the skin is exposed to the atmosphere, the stomach and intestines are anaerobic, or contain no O2.

Like humans, some bacteria have an absolute requirement for O2. Others thrive in anaerobic environments, and many of these are killed if O2 is present. The O2 requirements of some organisms can be determined by growing them in a shake tube. To prepare a shake tube, a tube of nutrient agar is boiled, which both melts the agar and drives off the O2. The agar is then allowed to cool to 50°C. Next, the test organism is added and dispersed by gentle shaking or swirling. The agar is allowed to harden and the tube is incubated at an appropriate temperature. Because the solidified agar impedes the diffusion of O2, the level of O2 in the tube is stratified—O2 levels are high at the top, whereas the bottom portion is anaerobic. The bacteria grow in the region that has the level of O2 that suits their requirements (table 4.2).

Based on their O2 requirements, prokaryotes can be separated into these groups:

■ Obligate aerobes have an absolute or obligate requirement for oxygen (O2). They use it to transform energy in the process of aerobic respiration. This and other ATP-generating pathways will be discussed in detail in chapter 6. Obligate aerobes include members of the genus Pseudomonas, a diverse group of Gramnegative rods that are common in the environment.

■ aerobic respiration, pp. 137,148 ■ ATP, p. 24

■ Obligate anaerobes cannot multiply if any O2 is present; in fact, they are often killed by traces of O2 because of its toxic derivatives, which will be discussed shortly. Obligate anaerobes may transform energy by fermentation or by anaerobic respiration; the details of these processes will be discussed in chapter 6. Obligate anaerobes include members of the genus Bacteroides, the major inhabitants of the large intestine. Another obligate anaerobe is Clostridium botulinum, the causative agent of botulism. It is estimated that one-half of all the cytoplasm on earth is in anaerobic bacteria! ■ fermentation, pp. 137,151 ■ anaerobic respiration, pp.137,149

■ Facultative anaerobes grow better if O2 is present, but can also grow without it. The term facultative means that the organism is flexible, in this case in its requirements for O2. Facultative anaerobes use aerobic respiration if oxygen is available, but use fermentation or anaerobic respiration in its absence. Growth is more rapid when oxygen is present because aerobic respiration yields the most ATP of all these processes. Examples of facultative anaerobes include E. coli, a common inhabitant of the large intestine, and the yeast Saccharomyces (a eukaryote), which is used to make bread and alcoholic beverages.

■ Microaerophiles require small amounts of O2 (2% to 10%) for aerobic respiration; higher concentrations are inhibitory. Examples include Spirillum volutans, which is common in aquatic habitats, and Helicobacter pylori, which causes gastric and duodenal ulcers.

■ Aerotolerant anaerobes are indifferent to O2. They can grow in its presence, but they do not use it to transform energy. Because they do not use aerobic or anaerobic respiration, they are also called obligate fermenters. They include Lactobacillus delbrueckii subspecies bulgaricus, which is used in yogurt-making, and Streptococcus pyogenes, which causes strep throat.

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