Mucous membranes of the respiratory tract, genitourinary tract, and respiratory tract provide a habitat for numerous kinds of bacteria, many of which have already been discussed. For example, Streptococcus and Corynebacterium species reside in the respiratory tract, Lactobacillus species inhabit the vagina, and Clostridium species and members of the family Enter-obacteriaceae thrive in the intestinal tract. Some of the other genera are discussed next.
Members of the genus Bacteroides are small, strictly anaerobic Gram-negative rods and coccobacilli. They inhabit the mouth, intestinal tract, and genital tract of humans and other animals. Bacteroides fragilis and related species constitute about 30% of the bacteria in human feces and are often responsible for abscesses and bloodstream infections that follow appendicitis and abdominal surgery. Since many are killed even by brief exposure to O2, they are difficult to study.
Bifidobacterium species are irregular Gram-positive rod-shaped anaerobes that reside primarily in the intestinal tract of humans and other animals. They are the predominant members of the intestinal flora of breast-fed infants and are thought to provide a protective function by excluding disease-causing bacteria. Formula-fed infants are also colonized with members of this genus, but generally the concentrations are lower. Bifidobacterium species ferment sugars to produce lactic acid and acetic acid.
Members of the genera Campylobacter and Helicobacter are genetically related curved Gram-negative rods that include species of major medical importance. As microaerophiles, they require specific atmospheric conditions to be successfully cultivated. Campylobacter jejuni causes diarrheal disease in humans. It typically resides in the intestinal tract of domestic animals, particularly poultry. Helicobacter pylori inhabits the stomach, where it can cause stomach and duodenal ulcers. It has also been implicated in the development of stomach cancer. An important factor in its ability to survive in the stomach is its production of the enzyme urease. This enzyme breaks down urea to produce ammonia, which neutralizes the acid in the immediate surroundings. ■ stomach ulcers, p. 605
Members of the genus Haemophilus are tiny Gram-negative coccobacilli that, as their name reflects, are "blood loving." They require one or more compounds found in blood, such as hematin and NAD. Many species are common flora of the respiratory tract, but H. influenzae can also cause ear infections, respiratory infections, and meningitis, primarily in children. Haemophilus ducreyi causes the sexually transmitted disease chancroid. ■ ear infections, p. 571 ■ meningitis, p. 664 ■ chancroid, p. 651
Neisseria species are Gram-negative kidney-bean-shaped cocci that typically occur in pairs. They are common inhabitants of animals including humans, growing in the oral cavity and other
mucous membranes. Neisseria species are obligate aerobes, and many are nutritionally fastidious. Those noted for their medical significance include N. gonorrhoeae, which causes the sexually transmitted disease gonorrhea, and N. meningitidis, which causes meningitis. ■ fastidious, p. 91 ■ gonorrhea, p. 644 ■ meningitis, p. 667
Members of the genus Mycoplasma are part of the taxonomic class characterized by the absence of a cell wall. Because these bacteria lack cell walls, they are pliable and can pass through the pores of filters that retain other bacteria. Most have sterols in their membrane, which provides added strength and rigidity, protecting the cells from osmotic lysis. They are among the smallest forms of life, and their genomes are thought to be the minimum size for encoding the essential functions for a free-living organism; the genome of Mycoplasma genitalium is only 5.8 x 105 base pairs, which is approximately one-eighth the size of the E. coli genome.
Medically, the most significant member of this group is M. pneumoniae, which, as its name implies, causes a form of pneumonia. This type of pneumonia cannot be treated with penicillin and other antimicrobial drugs that interfere with peptidoglycan synthesis, because these organisms lack a cell wall. Colonies of Mycoplasma species growing on solid media produce a characteristic "fried egg" appearance (figure 11.28).
Members of the genera Treponema and Borrelia are spirochetes that typically inhabit body fluids and mucous membranes of humans and other animals. Recall that spirochetes are characterized by their corkscrew shape and axial filaments. Although they have a Gram-negative cell wall, they are often too thin to
292 Chapter 11 The Diversity of Prokaryotic Organisms be viewed by conventional microscopy. Treponema species are obligate anaerobes or microaerophiles that often inhabit the mouth and genital tract. Study of the species that causes syphilis, T. pallidum, is difficult because it has never been grown in culture. Its genome has been sequenced, however, providing evidence that it is a microaerophile with a metabolism that is highly dependent on its host. It lacks critical enzymes of the TCA cycle and a variety of other pathways.
Three species of Borrelia are medically significant microaerophiles that are transmitted by arthropods such as ticks and lice. B. recurrentis and B. hermsii both cause relapsing fever; Borrelia burgdorferi causes Lyme disease. A striking feature of Borrelia species is their genome, which is composed of a linear chromosome and many linear and circular plasmids.
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