Chapter 25: Microbial Diseases of the Digestive System 

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

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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

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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.

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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.

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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

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25.6 Fungal Diseases of the Digestive System

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

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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

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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.

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