The total number of organisms in the oral cavity is

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and in sites such as tonsillar crypts, where anaerobic conditions can develop readily. The total number of organisms in the oral cavity is very high, and it varies from site to site. Saliva usually contains a mixed flora of about 10 8 organisms per milliliter, derived mostly from the various epithelial colonization sites. The stomach contains few, if any, resident organisms in health because of the lethal action of gastric hydrochloric acid and peptic enzymes on bacteria. The small intestine has a scanty resident flora, except in the lower ileum, where it begins to resemble that of the colon. The colon carries the most prolific flora in the body (Figure 1–4). In the adult, feces are 25% or more bacteria by weight (about 10 10 organisms per gram). More than 90% are anaerobes, predominantly members of the genera Bacteroides, Fusobacterium, Eubacterium, and Clostridium . The remainder of the flora is composed of facultative organisms such as Escherichia coli, enterococci, yeasts, and numerous other species. There are considerable differences in adult flora depending on the diet of the host. Those whose diets include substantial amounts of meat have more Bacteroides and other anaerobic Gram- negative rods in their stools than those on a predominantly vegetable or fish diet. Respiratory Tract The external 1 cm of the anterior nares is lined with squamous epithelium. The nares have a flora similar to that of the skin except that it is the primary site of carriage of a pathogen, Staphylococcus aureus . About 25% to 30% of healthy people carry this organism as either resident or transient flora at any given time. The nasopharynx has a flora similar to that of the mouth; however, it is often the site of carriage of
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potentially pathogenic organisms such as pneumococci, meningococci, and Haemophilus species. The respiratory tract below the level of the larynx is protected in health by the action of the epithelial cilia and by the movement of the mucociliary blanket; thus, only transient inhaled organisms are encountered in the trachea and larger bronchi. The accessory sinuses are normally sterile and are protected in a similar fashion, as is the middle ear by the epithelium of the eustachian tubes. Genitourinary Tract The urinary tract is sterile in health above the distal 1 cm of the urethra, which has a scanty flora derived from the perineum. Thus, in health the urine in the bladder, ureters, and renal pelvis is sterile. The vagina has a flora that varies according to hormonal influences at different ages. Before puberty and after menopause, it is mixed, nonspecific, and relatively scanty, and it contains organisms derived from the flora of the skin and colon. During the childbearing years, it is composed predominantly of anaerobic and microaerophilic members of the genus Lactobacillus , with smaller numbers of anaerobic Gram-negative rods, Gram-positive cocci, and yeasts that can survive under the acidic conditions produced by the lactobacilli. These conditions develop because glycogen is deposited in vaginal epithelial cells under the influence of estrogenic hormones and metabolized to lactic acid by lactobacilli. This process results in a
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