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Chapter: Biology of Disease: Infectious Diseases and Treatments

Infections of the Gastrointestinal Tract

All regions of the gastrointestinal tract (GIT) are subject to infection. Saliva traps and removes many pathogens and these can also be killed by stomach acid . Unfortunately new ones are constantly introduced through breathing and eating.


All regions of the gastrointestinal tract (GIT) are subject to infection. Saliva traps and removes many pathogens and these can also be killed by stomach acid . Unfortunately new ones are constantly introduced through breathing and eating.

Infections of the oral cavity (Figure 3.6) differ in type and symptoms to those of the stomach and intestines. Inflammation of oral tissues caused by fungal infection, actinomycosis, often occurs following injuries, such as an accidental bite to the lining of the mouth or fracture of the jaw. Immunosuppression resulting from viral infections, AIDS, cancer treatment or treatment with broad spectrum antibiotics can all allow the yeast Candida albicans (Figure3.10) to invade and colonize the mucous membrane, eventually producing athick layer of yeast cells called candidiasis or thrush.

Some bacteria can resist removal by saliva and become immobilized by binding to surface receptors of cells in the mouth, eventually forming biofilms and microcolonies. Oral streptococci, such as Streptococcus sanguis and Streptococcus mutans (Figure 3.11), secrete glycosyltransferases that mediatetheir adhesion to extracellular carbohydrates on tooth surfaces leading to the formation of dental plaque, which is a complex mixture of bacteria and extracellular materials. These bacteria, together with Actinomyces species, can cause caries by forming plaque on the tooth enamel, where they catabolize sugars to produce acid that demineralizes enamel and allows the dentine to be eroded. Abscesses of the roots of teeth can also be caused by mixed bacterial infections.

Periodontal (gum) diseases are inflammatory conditions that attack the gums, bone, and other supporting structures of the teeth. The extent of the inflammatory response depends upon the types of pathogens involved and the effectiveness of the immune response. However, they are major causes of tooth loss in adults. Gingivitis is the earliest form of periodontal disease and occurs when plaque accumulates on the teeth near the gums, which become inflamed and bleed easily. If detected and treated early, gingival tissues will return to normal without long-lasting damage. Untreated gingivitis progresses to periodontitis, which is also known as pyorrhea. Plaque hardens and extends from the gum line to the tooth root causing the gums to detach from the teeth and form pockets. Periodontal pockets create room for greater bacterial activity, particularly of facultative and obligate anaerobic bacteria leading to a progressive cycle of tissue damage until eventually the bone supporting the teeth is destroyed resulting in their loss.

Stomach and intestinal infections are caused by viruses, bacteria, protozoa and worms, all of which may be transmitted in food, contaminated drinking water or by fecal–oral contact. The need for strict personal hygiene is emphasized because these are the most frequent infections of children under five years of age. Approximately 40% of cases of diarrhea in children are caused by rotaviruses (Figure 3.12). In the very young this is potentially lethal and the WHO has estimated that out of the nearly two billion annual diarrhea diseases worldwide, three million end fatally.

Figure 3.13 indicates a number of pathogens that can infect the GIT. Theacidic environment and proteolytic enzymes of the stomach kill most

ingested microorganisms. However, the motile bacterium, Helicobacterpylori has specific receptors enabling it to bind to the gastricepithelium. It secretes urease, which catalyzes the hydrolysis of urea releasing ammonia that neutralizes stomach acid, and cytotoxins that damage the cells. This causes chronic inflammation of the gastric mucosal membrane and can lead to stomach and duodenal ulcers . The partially digested food (chyme) in the stomach is made alkaline in the small intestine by secretions of the gut and pancreas and by bile salts . In the ileum and jejunum, nonenveloped viruses, such as rotaviruses and adenoviruses, may infect enterocytes and damage the intestinal mucous membrane with disruption of water and electrolyte resorption. This can result in intestinal cramps, vomiting, watery diarrhea and a raised temperature. Enteropathogens, such as Vibrio cholerae and forms of Escherichia coli that are enterotoxic (ETEC) or enteropathogenic (EPEC) all have similar effects. The protozoan parasites Giardia lamblia and Cryptosporidium parvum are water-borne parasites that can infect the GIT. Giardia lamblia (Figure 3.14) frequently causes chronic disease, with watery diarrhea and, in some cases, a subfebrile temperature leading to malnutrition in children as a result of malabsorption. Cryptosporidium parvum can adhere to the epithelium of the small intestine and cause a shortening of the villi, which may be the cause of the diarrhea.

The lower portion of the ileum has areas of lymphoid tissue called Peyer’s patches (Figure 3.13) composed of so-called M (microfold) cells, rather than the usual enterocytes and goblet cells . These cells are able to translocate materials directly to the lymph follicles found beneath the mucosal surface. Invasive bacteria such as Campylobacter jejuni (Figure 3.15), Salmonellae and Yersiniae can use M cells to enter the submucosal area. Here they can multiply and destroy the adjacent epithelium, form abscesses and spread through the lymph and blood systems into the mesenteric lymph nodes, spleen and liver. The infection can also spread into the colon, causing inflammation of the colon or colitis. The ileum and colon can also be attacked by the bacteria Yersinia enterocolitica, Salmonella enterica and Campylobacterjejuni resulting in abdominal cramps, vomiting, watery, occasionally bloody,diarrhea and fever. Shigella dysenteriae and Escherichia coli pathotypes, EHEC (enterohemorrhagic) and EIEC (enteroinvasive) can cause a hemorrhagic colitis with bloody stools and subfebrile to febrile temperatures. The pathogenic protozoan Entamoeba histolytica is thought to infect 50 million people and kill about 100 000 per year worldwide due to amebic liver abscesses. Lastly, Clostridium difficile, a normal inhabitant of the gut, is an opportunistic pathogen. It is especially common in older people in hospitals and nursing homes and has been implicated in iatrogenic infections following medical interventions, such as antibiotic therapy. Infection with Clostridium difficile is now recognized as the major causative agent of colitis and diarrhea, which may occur following antibiotic intake and can be fatal in older patients.


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