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 stratified 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 meningitis. The existence of sinus tracts between deeper tissues and the skin surface or alternatively, the presence of foreign bodies or avascular areas (eg 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 following diabetes mellitus) also predisposes to infection. Alteration of the normal commensal flora by broad-spectrum antibiotic therapy predisposes to colonization by antibiotic-resistant potential pathogens, which may cause infectious or toxin-induced complications, for example, pseudomembranous colitis caused by Clostridium difficile toxin, multi-drug resistant Staphylococcus aureus infection. Surgical instruments, perfusions lines, and catheters may promote microbial invasion past the anatomical or physiological barriers. Finally, damaged tissues, for example, damage cardiac valves, provide a nidus for the establishment of infection.