Normal Flora

The normal flora is important in protecting the body against invasion by pathogens. This section focuses mainly on flora of the oral cavity and intestine. The esophagus has a relatively sparse population, consisting mostly of bacteria from the mouth and upper respiratory tract. When empty of food, the normal stomach is devoid of microorganisms because they are killed by the action of acid and pepsin. ■ normal flora, pp. 375,461

The Mouth

Of all the species of bacteria introduced into the mouth from the time of birth onward, relatively few can colonize the oral cavity. Streptococcal species, Gram-positive, chain-forming cocci that produce lactic acid as a by-product of carbohydrate metabolism, are the most numerous. One species of Streptococcus preferentially colonizes the upper part of the tongue, another colonizes the teeth, while still another colonizes the mucosa of the cheek. Streptococci and other bacteria attach specifically to receptors on host tissues, allowing the microorganisms to resist the scrubbing action of food and the tongue and the flushing action of salivary flow. The fact that the cells lining different parts of the mouth have differing receptors accounts for the distribution of the various species of streptococci. The host limits the numbers of bacteria on its mucous membranes by constantly shedding the superficial layers of cells and replacing them with new ones; the rate of this shedding correlates with the number of microorganisms present on the surface.

Because teeth are a non-shedding surface, large collections of bacteria can build up on them in a biofilm. These masses of bacteria, called dental plaque (figure 24.3), form because the bacteria attach to specific receptors on each other or on the tooth, and they may be bound together by extracellular poly-saccharides. There can be up to 100 billion bacteria per gram of plaque. Metabolic by-products of one species are utilized by

2 mm

Figure 24.3 Scanning Electron Micrograph of Dental Plaque The many kinds of bacteria composing the plaque exhibit specific attachments to the tooth and to each other and may be bound together by extracellular polysaccharides.

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Figure 24.3 Scanning Electron Micrograph of Dental Plaque The many kinds of bacteria composing the plaque exhibit specific attachments to the tooth and to each other and may be bound together by extracellular polysaccharides.

another. Plaque organisms consume oxygen, thereby creating conditions that permit the growth of strict anaerobes. In fact, colonization of the mouth by strictly anaerobic flora requires the presence of teeth, because only teeth provide sufficiently anaerobic habitats for growth of these organisms. Dental plaque, gingival crevices, and fissures in the teeth are such habitats. ■ lactic acid production, p. 152 ■ biofilms, 104


Only small numbers of bacteria live in the upper small intestine because they are continually flushed away by the rapid passage of digestive juices. The predominant organisms are usually aerobic and facultatively anaerobic Gram-negative rods and some streptococci. Lactobacilli and yeasts such as Candida albicans are found in small numbers. The bacterial population increases as the intestinal contents move toward the large intestine.

In contrast to the relatively scanty numbers of organisms in the small intestine, the large intestine contains very high numbers of microorganisms, approximately 1011 bacteria per gram of feces. These large numbers occur because of the abundance of nutrients in undigested and indigestible food material. Bacteria make up about one-third of the fecal weight. The numbers of anaerobic bacteria, notably including members of the genus Bacteroides, generally exceed other organisms by about 100-fold. Of the fecal microorganisms able to grow in the presence of air, facultatively anaerobic, Gram-negative rods, particularly Escherichia coli and other enterobacteria, predominate. Fecal organisms are an important source of opportunistic pathogens, especially for the urinary tract. The enormous population of bacteria comprises species that can work together enzymatically to change numerous materials. The currently recommended high-fiber diets contain substances that are indigestible by the gastric and intestinal juices but are readily degraded by intestinal organisms, which often produce large amounts of carbon dioxide, hydrogen, and methane gas. Abdominal discomfort and discharge of intestinal gas, flatus, from the anus is an unfortunate result. Bacterial enzymes can also convert various substances in food to carcinogens and therefore may be involved, along with diet, in the production of intestinal cancer. ■ opportunistic pathogens, p. 462

24.3 Bacterial Diseases of the Upper Alimentary System 601

Intestinal bacteria synthesize a number of useful vitamins, including niacin, thiamine, riboflavin, pyridoxine, vitamin B12, folic acid, pantothenic acid, biotin, and vitamin K. These vitamins are important when a person's diet is inadequate.

The normal flora helps prevent colonization of the large intestine by pathogens. A sometimes life-threatening disease called antibiotic-associated or pseudomembranous colitis can follow antibiotic therapy. The disease is caused by the toxin-producing anaerobic bacterium, Clostridium difficile, which readily colonizes the intestine of people whose normal intestinal flora has been reduced by antimicrobial chemotherapy. The toxins of C. difficile are lethal to intestinal epithelium and cause small patches called pseudomembranes, composed of dead epithelium, inflammatory cells, and clotted blood, to form on the intestine. Fever, abdominal pain, and profuse diarrhea result. Suppression of intestinal flora with antibacterial medications can also increase susceptibility to other pathogens such as Salmonella enterica, to be discussed later.

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  • kalvin
    What are the causative agent of normal flora?
    3 years ago

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