Facultative Anaerobes

Facultative anaerobes preferentially use aerobic respiration if O2 is available. As an alternative, however, they can use fermentative metabolism.

The Genus Corynebacterium

Members of the genus Corynebacterium commonly inhabit soil, water, and the surface of plants. They are Gram-positive pleomorphic rods that are often club-shaped and arranged to form V shapes or palisades (koryne is Greek for "club") (figure 11.14). Bacteria that exhibit this characteristic microscopic morphology are sometimes referred to coryneforms or diph-

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theroids. Corynehacterium species are generally facultative anaerobes, although some are strict aerobes. They form volutin granules, which are storage forms of phosphate. These are sometimes called metachromatic granules to reflect their characteristic staining with the dye methylene blue. Many species of Corynebacterium reside harmlessly in the throat, but toxin-producing strains of C. diphtheriae can cause the disease diphtheria. ■ volutin, p. 67 ■ metachromatic granules, p. 67 ■ diphtheria, p. 568

The Family Enterobacteriaceae

Members of the family Enterobacteriaceae, frequently referred to as enterics or enterobacteria, are Gram-negative rods. Their name reflects the fact that most reside in the intestinal tract of humans and other animals (Greek enteron means "intestine"), although some thrive in rich soil. Enterics that are part of the normal flora of the intestine include Enterobacter, Klebsiella, and Proteus species as well as most strains of E. coli. Those that cause diarrheal disease include Shigella species, Salmonella Enteritidis, and some strains of E. coli. Life-threatening systemic diseases include typhoid fever, caused by Salmonella Typhi, and both the bubonic and pneumonic forms of plague, caused by Yersinia pestis. ■ diarrheal disease, p. 610 ■ typhoid fever, p. 616 ■ plague, p. 723

Members of the family Enterobacteriaceae are facultative anaerobes that ferment glucose and, if motile, have peritrichous flagella. The family includes about 40 recognized genera that can be distinguished using biochemical tests. A battery of such tests is often used to simultaneously determine the genus and species. Within a given species, many different strains have been described. These are often distinguished using serological tests that detect differences in their cell walls, flagella, and capsules (figure 11.15). ■ peritrichous flagella, p. 64

Enterics that ferment lactose are included in a group called coliforms. This is an informal grouping of certain common intestinal inhabitants such as E. coli that are easy to detect in food and water; they are used by regulatory agencies as an indicator of fecal pollution. Their presence indicates a potential health risk because fecal-borne pathogens might also be present. Typically, coliforms are enumerated using membrane filtration or the most probable number (MPN) method. ■ coliform, p. 793 ■ membrane filtration, p. 99 ■ MPN, p. 99

Flagella (H antigen)

10 mm

Flagella (H antigen)

Capsule (K antigen)

Cell wall (O antigen)

Figure 11.15 Schematic Drawing of a Member of the Family

Enterobacteriaceae The cell structures used to distinguish different strains are shown.

Capsule (K antigen)

Cell wall (O antigen)

Figure 11.14 Corynebacterium The Gram-positive pleomorphic rods are often arranged to form V shapes or palisades.

Figure 11.15 Schematic Drawing of a Member of the Family

Enterobacteriaceae The cell structures used to distinguish different strains are shown.

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