25.1 Structure and Function of the Digestive System

• The digestive system is essentially a tubelike structure, the gastrointestinal (GI) tract, or alimentary canal—mainly the mouth, pharynx (throat), esophagus (food tube leading to the stomach), stomach, and the small and large intestines

25.2 Normal Microbiota of the Digestive System

• Bacteria heavily populate most of the digestive system. In the mouth, each milliliter of saliva can contain millions of bacteria.

• The small intestine also contains important antimicrobial defenses, most significantly, millions of specialized, granules filled cells called Paneth cells

25.3 Bacterial Diseases of the Mouth

• These accumulations, called dental plaque, are a type of biofilm and are intimately involved in the formation of dental caries, or tooth decay.

• Localized acid production within deposits of dental plaque results in a gradual softening of the external enamel

  • Even people who avoid tooth decay might, in later years, lose their teeth to periodontal disease, a term for a number of conditions characterized by inflammation and degeneration of structures that support the teeth

• This resulting inflammation, called gingivitis, is characterized by bleeding of the gums while the teeth are being brushed

• Gingivitis can progress to a chronic condition called periodontitis, an insidious condition that generally causes little discomfort.

  • Acute necrotizing ulcerative gingivitis, also termed Vincent’s disease or trench mouth, is one of the more common serious mouth infections.

25.4 Bacterial Diseases of the Lower Digestive System

• An infection occurs when a pathogen enters the GI tract and multiplies.

  • M (microfold) cells translocate antigens and microorganisms to the other side of the epithelium where they can contact lymphoid tissues (Peyer’s patches) to initiate an immune response

• An intoxication is caused by the ingestion of such a preformed toxin.

• Severe diarrhea accompanied by blood or mucus is called dysentery.

• The general term gastroenteritis is applied to diseases causing inflammation of the stomach and intestinal mucosa.

• A leading cause of gastroenteritis is staphylococcal food poisoning, an intoxication caused by ingesting an enterotoxin produced by S. aureus

  • All salmonellae are considered pathogenic to some degree, causing salmonellosis, or Salmonella gastroenteritis.

• The causative agent of cholera, one of the most serious gastrointestinal diseases, is Vibrio cholerae, a slightly curved, gramnegative rod with a single polar flagellum

• Enteropathogenic E. coli (EPEC) is a major cause of diarrhea in developing countries and is potentially fatal in infants.

  • Enteroaggregative E. coli (EAEC) is a group of coliforms found only in humans.

• It has long been observed that travel broadens the mind and loosens the bowels, leading to the common name of traveler’s diarrhea

• Clostridium difficile–associated diarrhea is a disease condition that has appeared in recent decades and has been described as being responsible for more deaths than all other intestinal infections combined.

25.5 Viral Diseases of the Digestive System

• Mumps typically begins with painful swelling of one or both parotid glands 16 to 18 days after exposure to the virus

• Hepatitis is an inflammation of the liver. At least five different viruses cause hepatitis, and probably more remain to be discovered or become better known

  • The hepatitis A virus (HAV) is the causative agent of hepatitis A

  • Hepatitis B is caused by the hepatitis B virus (HBV).

• HBV and HAV are completely different viruses: HBV is larger, its genome is double-stranded DNA, and it is enveloped.

• Hepatitis D can occur as either acute (coinfection form) or chronic (superinfection form) hepatitis.

• Hepatitis E (HEV) is spread by fecal–oral transmission, much like hepatitis A, which it clinically and structurally resembles

25.6 Fungal Diseases of the Digestive System

• Aflatoxin poisoning can cause serious damage to livestock when their feed is contaminated with A. flavus.

25.7 Protozoan Diseases of the Digestive System

• G. intestinalis is the cause of giardiasis, a prolonged diarrheal disease.

• Cryptosporidiosis is caused by the protozoan Cryptosporidium.

• Amebic dysentery, or amebiasis, is spread mostly by food or water contaminated by cysts of the protozoan ameba Entamoeba histolytica

25.8 Helminthic Disease of the Digestive System

• Taeniasis develops when the adult tapeworm infects the human intestine

  • The most serious, and much more common, disease is neurocysticercosis, which arises when the larvae develop in areas of the central nervous system, such as the brain.

• Hookworm infections were once a very common parasitic disease in the southeastern states.

• Whipworm infestations, known as trichuriasis, are widespread in tropical areas of the world, especially Asia.