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Many bacteria have adhesin mediated affinity for

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antibacterial substances such as bile and lysozyme. Many bacteria have adhesin-mediated affinity for receptors on specific types of epithelial cells; this facilitates colonization and multiplication and prevents removal by the flushing effects of surface fluids and peristalsis. Various microbial interactions also determine their relative prevalence in the flora. These interactions include competition for nutrients, inhibition by the metabolic products of other organisms. Normal Flora at Different Sites
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The total normal flora of the body probably contains more than 1000 distinct species of microorganisms. The major members known to be important in preventing or causing disease, as well as those that may be confused with etiologic agents of local infections, are summarized in Table 1–3 and are described in greater detail in subsequent chapters. Blood, Body Fluids, and Tissues In health, the blood, body fluids, and tissues are sterile. Occasional organisms may be displaced across epithelial barriers as a result of trauma or during childbirth; they may be briefly recoverable from the bloodstream before they are filtered out in the pulmonary capillaries or removed by cells of the reticuloendothelial system. Such transient bacteremia may be the source of infection when structures such as damaged heart valves and foreign bodies (prostheses) are in the bloodstream. Skin The skin plays host to an abundant flora that varies somewhat according to the number and activity of sebaceous and sweat glands. The flora is most abundant on moist skin areas (axillae, perineum, and between toes). Staphylococci and members of the Propionibacterium genus occur all over the skin, and facultative diphtheroids (corynebacteria) are found in moist areas. Propionibacteria are slim, anaerobic, or microaerophilic Gram-positive rods that grow in subsurface sebum and break down skin lipids to fatty acids. Thus, they are most numerous in the ducts of hair follicles and of the sebaceous glands that drain into them. Even with antiseptic scrubbing, it is difficult to eliminate bacteria from skin sites, particularly those bearing pilosebaceous units. Organisms of the skin flora are resistant to the bactericidal effects of skin lipids and fatty acids, which inhibit or kill many extraneous bacteria. The conjunctivae have a very scanty flora derived from the skin flora. The low bacterial count is maintained by the high lysozyme content of lachrymal secretions and by the flushing effect of tears. Intestinal Tract The mouth and pharynx contain large numbers of facultative and strict anaerobes. Different species of streptococci predominate on the buccal and tongue mucosa because of different specific adherence characteristics. Gram-negative diplococci of the genera Neisseria and Moraxella (Branhamella) make up the balance of the most commonly isolated facultative organisms. Strict anaerobes and microaerophilic organisms of the oral cavity have their niches in the depths of the gingival crevices surrounding the teeth and in sites such as tonsillar crypts, where anaerobic conditions can develop readily.
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