24.1 Anatomy and Normal Microbiota of the Digestive System
- The digestive tract, consisting of the oral cavity, pharynx, esophagus, stomach, small intestine, and large intestine, has a normal microbiota that is important for health.
- The constant movement of materials through the gastrointestinal canal, the protective layer of mucus, the normal microbiota, and the harsh chemical environment in the stomach and small intestine help to prevent colonization by pathogens.
- Infections or microbial toxins in the oral cavity can cause tooth decay, periodontal disease, and various types of ulcers.
- Infections and intoxications of the gastrointestinal tract can cause general symptoms such as nausea, vomiting, diarrhea, and fever. Localized inflammation of the GI tract can result in gastritis, enteritis, gastroenteritis, hepatitis, or colitis, and damage to epithelial cells of the colon can lead to dysentery.
- Foodborne illness refers to infections or intoxications that originate with pathogens or toxins ingested in contaminated food or water.
24.2 Microbial Diseases of the Mouth and Oral Cavity
- Dental caries, tartar, and gingivitis are caused by overgrowth of oral bacteria, usually Streptococcus and Actinomyces species, as a result of insufficient dental hygiene.
- Gingivitis can worsen, allowing Porphyromonas, Streptococcus, and Actinomyces species to spread and cause periodontitis. When Prevotella intermedia, Fusobacterium species, and Treponema vicentii are involved, it can lead to acute necrotizing ulcerative gingivitis.
- The herpes simplex virus type 1 can cause lesions of the mouth and throat called herpetic gingivostomatitis.
- Other infections of the mouth include oral thrush, a fungal infection caused by overgrowth of Candida yeast, and mumps, a viral infection of the salivary glands caused by the mumps virus, a paramyxovirus.
24.3 Bacterial Infections of the Gastrointestinal Tract
- Major causes of gastrointestinal illness include Salmonella spp., Staphylococcus spp., Helicobacter pylori, Clostridium perfringens, Clostridioides difficile, Bacillus cereus, and Yersinia bacteria.
- C. difficile is an important cause of hospital acquired infection.
- Vibrio cholerae causes cholera, which can be a severe diarrheal illness.
- Different strains of E. coli, including ETEC, EPEC, EIEC, and EHEC, cause different illnesses with varying degrees of severity.
- H. pylori is associated with peptic ulcers.
- Salmonella enterica serotypes can cause typhoid fever, a more severe illness than salmonellosis.
- Rehydration and other supportive therapies are often used as general treatments.
- Careful antibiotic use is required to reduce the risk of causing C. difficile infections and when treating antibiotic-resistant infections.
24.4 Viral Infections of the Gastrointestinal Tract
- Common viral causes of gastroenteritis include rotaviruses, noroviruses, and astroviruses.
- Hepatitis may be caused by several unrelated viruses: hepatitis viruses A, B, C, D, and E.
- The hepatitis viruses differ in their modes of transmission, treatment, and potential for chronic infection.
24.5 Protozoan Infections of the Gastrointestinal Tract
- Giardiasis, cryptosporidiosis, amoebiasis, and cyclosporiasis are intestinal infections caused by protozoans.
- Protozoan intestinal infections are commonly transmitted through contaminated food and water.
- Treatment varies depending on the causative agent, so proper diagnosis is important.
- Microscopic examination of stool or biopsy specimens is often used in diagnosis, in combination with other approaches.
24.6 Helminthic Infections of the Gastrointestinal Tract
- Helminths often cause intestinal infections after transmission to humans through exposure to contaminated soil, water, or food. Signs and symptoms are often mild, but severe complications may develop in some cases.
- Ascaris lumbricoides eggs are transmitted through contaminated food or water and hatch in the intestine. Juvenile larvae travel to the lungs and then to the pharynx, where they are swallowed and returned to the intestines to mature. These nematode roundworms cause ascariasis.
- Necator americanus and Ancylostoma doudenale cause hookworm infection when larvae penetrate the skin from soil contaminated by dog or cat feces. They travel to the lungs and are then swallowed to mature in the intestines.
- Strongyloides stercoralis are transmitted from soil through the skin to the lungs and then to the intestine where they cause strongyloidiasis.
- Enterobius vermicularis are nematode pinworms transmitted by the fecal-oral route. After ingestion, they travel to the colon where they cause enterobiasis.
- Trichuris trichiura can be transmitted through soil or fecal contamination and cause trichuriasis. After ingestion, the eggs travel to the intestine where the larvae emerge and mature, attaching to the walls of the colon and cecum.
- Trichinella spp. is transmitted through undercooked meat. Larvae in the meat emerge from cysts and mature in the large intestine. They can migrate to the muscles and form new cysts, causing trichinosis.
- Taenia spp. and Diphyllobothrium latum are tapeworms transmitted through undercooked food or the fecal-oral route. Taenia infections cause taeniasis. Tapeworms use their scolex to attach to the intestinal wall. Larvae may also move to muscle or brain tissue.
- Echinococcus granulosus is a cestode transmitted through eggs in the feces of infected animals, especially dogs. After ingestion, eggs hatch in the small intestine, and the larvae invade the intestinal wall and travel through the circulatory system to form dangerous cysts in internal organs, causing hydatid disease.
- Flukes are transmitted through aquatic plants or fish. Liver flukes cause disease by interfering with the bile duct. Intestinal flukes develop in the intestines, where they attach to the intestinal epithelium.