DEFECTS IN ANATOMICAL OR PHYSIOLOGICAL BARRIERS TO INFECTION
One of the commonest predisposing causes of infection is a defect in an anatomical or physiological barrier to infection. Intact epithelial membranes, especially strati-fied squamous epithelial surfaces such as the skin, constitute an extremely effective barrier to infection. Thus, integumentary damage caused by burns, eczema, and trauma (including surgery), predisposes to infection. Skull fractures, particularly damage of the cribriform plate, may result in recurrent episodes of pyogenic menin-gitis. The existence of sinus tracts between deeper tissues and the skin surface or alternatively, the presence of foreign bod-ies or avascular areas (e.g., within bone) predisposes to infection. Obstruction to the drainage of hollow tubes and viscera also predisposes to infection, for example, obstruction of the biliary tract, urinary tract, or bronchi. Impaired vascular perfusion of the tissues due to edema and angiopathy (including microvascular changes follow-ing diabetes mellitus) also predisposes to infection.
Alteration of the normal com-mensal flora by broad-spectrum antibiotic therapy predisposes to colonization by antibiotic-resistant potential pathogens, which may cause infectious or toxin-induced complications, for example, pseu-domembranous colitis caused by Clos-tridium difficile toxin, multidrug-resistant Staphylococcus aureus infection. Surgical instruments, perfusion lines, and cath-eters may promote microbial invasion past the anatomical or physiological barriers.
Finally, damaged tissues, for example, damaged cardiac valves, provide a nidus for the establishment of infection.
Infections that recur in the same ana-tomical site are often due to defective anatomical or physiological barriers and hence should induce a diligent search for such factors. Microorganisms that cause infection in patients with this category of defects comprise pyogenic bacteria such as staphylococci and commensal organ-isms from the skin or intestinal tract. Fungi, especially Candida, may be another pathogen under these circumstances.