The cells of the mucous membranes, or mucosa, are constantly bathed with mucus and other secretions that help wash microbes from the surfaces. Some mucous membranes have mechanisms that propel microorganisms and viruses, directing them toward areas where they can be eliminated more easily. For example, peristalsis, the rhythmic contractions of the intestinal tract that propels food and liquid, also helps expel microbes. The respiratory tract is lined with ciliated cells; the hairlike cilia constantly beat in an upward motion, propelling material including microbes away from the lungs to the throat where they can then be swallowed. The flow of urine regularly flushes organisms from the urinary tract.■ cilia, p. 74
15.2 First-Line Defenses 375
Both the skin and mucous membranes have a variety of antimicrobial substances that inhibit or kill microorganisms. Sweat, for example, is high in salt; as it evaporates it leaves a salty residue, inhibiting many organisms that might otherwise proliferate on the skin.
Lysozyme, the enzyme that degrades peptido-glycan, is found in tears, saliva, and in mucus that bathes mucous membranes. It is also found within the body, in phagocytic cells, blood, and the fluid that bathes tissues. Lysozyme is primarily effective against Gram-positive bacteria, whose peptidoglycan is more likely to be exposed and therefore accessible to the enzyme; recall that in Gram-negative bacteria, the peptidoglycan layer is sandwiched between the cytoplasmic and outer membranes (see figures 3.33 and 3.34). ■ lysozyme, p. 60
Peroxidase enzymes are found in saliva and milk; they are also found within body tissues and inside phagocytes. These enzymes break down hydrogen peroxide and, in the process, produce potent oxidizing compounds. For example, the interaction of peroxidase, hydrogen peroxide, and chlorine produces hypochlorite, the active ingredient in bleach. Bacteria that produce the enzyme catalase, however, may avoid the damaging products associated with peroxidase activity; catalase breaks down hydrogen peroxide to produce water and oxygen, potentially destroying the compound before it can interact with peroxidase. Catalase-negative organisms are more sensitive to peroxidase killing. ■ catalase, p. 89
Lactoferrin is an iron-binding protein found in saliva, mucus, and milk; it is also found in some types of phagocytic cells. A similar compound, transferrin is found in blood and tissue fluids. Iron, an important part of some enzymes, is one of the major elements required for growth (see table 4.3). By sequestering iron, the lactoferrin and transferrin effectively withhold the essential element from most microbes. Some bacteria, however, make compounds that capture iron in body fluids and secretions, thus circumventing this defense.
Defensins are short antimicrobial peptides found on mucous membranes and within phagocytic cells. They are thought to function by inserting into bacterial membranes, forming pores that disrupt the integrity of this essential barrier.
The population of microorganisms routinely found growing on the body surfaces of healthy individuals is called the normal flora. Although these organisms are not technically part of the immune system, the protection they provide is considerable.
One protective effect of the normal flora is competitive exclusion of pathogens. For example, the normal flora prevents adherence of invading organisms to the host by covering binding sites that might otherwise be used for attachment. The population also consumes available nutrients that could otherwise
376 Chapter 15 The Innate Immune Response be used by less desirable organisms. Members of the normal flora also produce compounds that are toxic to other bacteria. For example, in the hair follicles of the skin, Propionibacterium species degrade the lipids found in body secretions, releasing fatty acids that inhibit the growth of many potential disease-producers. In the gastrointestinal tract, some strains of E. coli synthesize colicins, proteins that are toxic to some strains of bacteria. Lactobacillus species growing in the vagina produce lactic acid as a fermentation end product, resulting in an acidic pH that inhibits the growth of many potential disease-causing organisms. Disruption of the normal flora, which occurs when antibiotics are used, can predispose a person to various infections. Examples include antibiotic-associated colitis, caused by the growth of toxin-producing strains of Clostridium difficile in the intestine, and vulvovaginitis, caused by excessive growth of Candida albicans in the vagina. ■ antibiotic-associated colitis, p. 601 ■ vulvovaginitis, p. 640 The normal flora also stimulates the host defenses, effectively providing a moderate amount of "exercise" to the system, thereby enhancing its function. Other aspects of the normal flora will be discussed in chapter 19.
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