Entry of Organisms and Evasion of Local Defenses
Skin, mucus, ciliated epithelium, and secretions containing antibacterial substances (e.g., lysozyme) are the natural barriers of the human and animal hosts that prevent microbial entry. However, these barriers are sometimes broken (e.g., a break in the skin, an ulcer in the intestine, or a tumor, etc.), thereby allowing the entry of microbes into the host (Table 10-5). On entry, the microbes spread through blood circulation to other sites in the body (Fig. 10-2).
Skin: The stratified squamous epithelium of the skin with itssuperficial cornified anucleate layers is a simple and efficient mechanical barrier to prevent microbial invasion. Organisms gain access to the underlying tissues only by breaks or by way of hair follicles, sebaceous glands, and sweat glands that traverse the stratified layers.
The surface of the skin continuously desquamates and thereby tends to shed contaminating organisms. The skin also inhibits the growth of most extraneous microorganisms due to its low moisture, low pH, and the presence of substances with an antibacterial activity.
Mucus: Viscous mucus secreted by goblet cells protects theepithelium lining the respiratory and gastrointestinal tracts and urogenital system. Microorganisms become trapped in the mucus layer and may be swept away before they reach the epithelial cell surface. Secretory IgA, secreted into the mucus, and other secreted antimicrobials (such as lysozyme and lacto-ferrin) facilitate this cleansing process.
Ciliated epithelial cells: These cells constantly move themucus away from the epithelial surfaces. For example, mucus in the respiratory tract—particles larger than 5 mm are washed and trapped in the mucus. Similarly, the multilayered transi-tional epithelium of the urinary tract uses the flushing effect of urine, and its relatively low pH acts as an additional defense mechanism to limit microbial entry and growth.
Secretions: The high level of hydrochloric acid and gastricenzymes in the normal stomach kills many ingested bacteria. Others are susceptible to pancreatic digestive enzymes or to the detergent effect of bile salts.