Intact skin is the first line of defense against many organisms. In addition to the physical barrier presented by skin, the fatty acids secreted by sebaceous glands in the skin have antibacterial and antifungal activity. The increased fatty acid production that occurs at puberty is thought to explain the increased resistance to ringworm fungal infections observed at this time. The low pH of the skin (between 3 and 5), which is due to these fatty acids, also has an antimicrobial effect. Although many organ isms live on or in the skin as members of the normal flora, they are harmless as long as they do not enter the body.
Another important defense is the mucous membrane of the respiratory tract, which is lined with cilia and covered with mucus. The coordinated beating of the cilia drives the mucus up to the nose and mouth, where the trapped bacteria can be expelled. This mucociliary apparatus, the ciliary elevator, can be damaged by alcohol, cigarette smoke, and viruses; the dam age predisposes the host to bacterial infections. Other protective mechanisms of the respiratory tract involve alveolar macro phages, lysozyme in tears and mucus, hairs in the nose, and the cough reflex, which prevents aspiration into the lungs.
Loss of the physical barrier provided by the skin and mucous membranes predisposes to infection. Table 8–1 describes the organisms that commonly cause infections associated with the loss of these protective barriers.
The nonspecific protection in the gastrointestinal tract includes hydrolytic enzymes in saliva, acid in the stomach, and various degradative enzymes and macrophages in the small intestine. The vagina is protected by the low pH generated by lactobacilli that are part of the normal flora.
Additional protection in the gastrointestinal tract and in the lower respiratory tract is provided by defensins, a large family of antimicrobial peptides expressed predominantly by leukocytes and epithelial cells. These small, positively charged (cationic) proteins have broad-spectrum microbicidal activity against both gram-positive and gram-negative bacteria. They kill bacteria using a number of different mechanisms.
Normal flora organisms of the skin, nasopharynx, colon, and vagina occupy these ecologic niches, preventing pathogens from multiplying in these sites, a process called colonization resistance. If the normal flora is disrupted after a course of antibiotics, certain organisms such as Clostridioides difficile and Candida albicans can flourish and cause diseases such as pseudomembranous colitis and vaginitis, respectively.
