Gastrointestinal pathogens are transmitted by food or drink contaminated with fecal material. When hygiene fails, as may occur with natural disasters such as floods and earthquakes, diarrheal disease becomes rampant. Acidic gastric secretions are important defenses and are lethal for many gastrointestinal pathogens. Healthy volunteers do not become infected by Vibrio cholerae unless they are fed 1011 organisms, but neutralizing the stomach acid reduces the infectious dose by 10,000-fold. By contrast, some
Basis for Failure of Local Pathogen/Disease Site Major Local Defense(s) Defense (Examples) http://ebooksmedicine.net ingested agents, such as Shigella and Giardia cysts, are relatively resistant to gastric acid, so fewer than 100 organisms can cause illness.
Other normal defenses within the gastrointestinal tract include (1) the layer of viscous mucus covering the intestinal epithelium, (2) lytic pancreatic enzymes and bile detergents, (3) mucosal anti-microbial peptides called defensins, (4) normal flora, and (5) secreted IgA antibodies. IgA antibodies are made by plasma cells located in mucosa-associated lymphoid tissue (MALT). These lymphoid aggregates are covered by a single layer of specialized epithelial cells called M cells, which are important for transport of antigens to MALT. Numerous gut pathogens use M cells to enter the host from the intestinal lumen, including poliovirus, enteropathic E. coli, V. cholerae, Salmonella enterica serotype Typhi, and Shigella flexneri.
Infection via the gastrointestinal tract occurs when local defenses are weakened or the organisms develop strategies to overcome these defenses. Host defenses are weakened by low gastric acidity, by antibiotics that alter the normal bacterial flora (e.g., in pseudomembranous colitis due to C. difficile), or when there is stalled peristalsis or mechanical obstruction. Viruses that can enter the body through the intestinal tract (e.g., hepatitis A, rotavirus) are those that lack envelopes, because enveloped viruses are inactivated by bile and digestive enzymes.
Enteropathogenic bacteria cause gastrointestinal disease in several ways:
Toxin production in food. S. aureus and Bacillus cereus can contaminate and grow in food, where they release powerful enterotoxins that, when ingested, cause food poisoning without any bacterial growth in the gut.
Adhesion, local proliferation, and toxin production in the host. V. cholerae and enterotoxigenic E. coli bind to the intestinal epithelium and multiply in the overlying mucous layer, where they release exotoxins that cause epithelial cells to secrete large volumes of fluid, resulting in watery diarrhea.
Invasion. Shigella, Salmonella, and Campylobacter invade locally and damage the intestinal mucosa and lamina propria, causing ulceration, inflammation, and hemorrhage—changes manifested clinically as bloody diarrhea (dysentery). S. enterica serotype Typhi passes from the damaged mucosa through Peyer patches and mesenteric lymph nodes and into the bloodstream, resulting in a systemic infection.
Fungal infections of the gastrointestinal tract occur in immunologically compromised individuals. Candida, part of the normal gastrointestinal flora, shows a predilection for stratified squamous epithelium, causing oral thrush or membranous esophagitis, but also may spread to the stomach, lower gastrointestinal tract, and other organs.