Microbial Diseases of the Digestive System: Structure, Function, and Pathogenesis

Chapter 24: Microbial Diseases of the Digestive System

A Glimpse of History

This section provides historical context for the study of digestive system diseases, focusing on cholera as a model for understanding transmission and control of microbial diseases.

• Cholera's Origin and Spread: Ancient Sanskrit writings suggest cholera originated in the Far East. Increased global mobility in the 19th century facilitated its spread to Europe and North America, causing major epidemics.

• John Snow's Discovery (1854): Demonstrated that cholera is waterborne by tracing an outbreak to a contaminated well in London. His intervention (removal of the pump handle) reduced cases, even before the germ theory was established.

• Robert Koch (1883): Isolated Vibrio cholerae, the causative agent of cholera.

• Modern Outbreak Example: The 2008 Zimbabwe cholera outbreak highlighted the importance of public health infrastructure, water sanitation, and rapid response in controlling epidemics.

24.1. Anatomy, Physiology, and Ecology of the Digestive System

Digestive Tract Overview

The digestive tract is a major interface with the environment and a complex ecosystem for microbes.

• Structure: Hollow tube from mouth to anus; the gastrointestinal tract refers specifically to the stomach and intestines.

• Microbial Ecology: The tract provides nutrients for microbes, most of which live in harmony with the host. The mucous membrane (one cell layer thick) separates microbes from underlying tissue; damage can allow pathogen invasion.

• Accessory Organs: Salivary glands, liver, and pancreas produce enzymes and substances to aid digestion.

The Upper Digestive System

• Components: Mouth, salivary glands, esophagus, stomach.

• Mouth and Salivary Glands:

  • Chewing and saliva (containing amylase) begin starch digestion.

  • Teeth are protected by enamel; a protein film (pellicle) forms from saliva, allowing bacterial attachment and biofilm (dental plaque) formation.

  • Mineral salts can deposit, forming dental calculus (tartar).

  • Damage to enamel allows entry of microbes, leading to dental caries (tooth decay).

  • Gingival crevice can harbor microbes, leading to gingivitis (gum inflammation).

  • Saliva (about 1,500 mL/day) contains secretory IgA, lysozyme, and lactoferrin; over 600 bacterial species inhabit the mouth, with Streptococcus species being most common.

• Esophagus: Connects mouth to stomach; peristalsis moves food. Microbial population is sparse due to secretory IgA in mucus and saliva.

• Stomach: Elastic, muscular organ that stores and digests food. Highly acidic gastric juices denature proteins and activate pepsinogen to pepsin. Most bacteria cannot survive the acidity.

The Lower Digestive Tract

• Small Intestine:

  • Receives alkaline digestive fluids from pancreas and liver, neutralizing stomach acid.

  • Bile emulsifies fats and contains bile salts that kill many bacteria.

  • Villi and microvilli increase surface area (~250 m2), enhancing nutrient absorption.

  • Dendritic cells, M cells, and Peyer's patches monitor microbial populations.

• Large Intestine:

  • Main function is water and vitamin absorption.

  • Bacteria constitute about one-third of fecal weight (up to 1011 cells/gram), with anaerobic Bacteroides dominating.

  • Microbiota degrade indigestible substances, synthesize vitamins (e.g., niacin, vitamin K), and prevent pathogen colonization.

• Pancreas: Produces hormones and digestive enzymes; secretes ~2 L/day into the small intestine.

• Liver: Produces bile (stored in gallbladder), inactivates toxins, and can develop jaundice if bile ducts are obstructed.

24.2. Bacterial Diseases of the Upper Digestive System

Dental Caries (Tooth Decay)

• Signs and Symptoms: Often advanced before symptoms appear; discoloration, roughness, and tooth breakage; severe pain is a common first sign.

• Causative Agent: Streptococcus mutans and related species; thrive in acidic environments, produce lactic acid from fermentation, and synthesize glucans from sucrose.

• Pathogenesis: Bacteria adhere to pellicle, form plaque, and ferment sugars, lowering pH and promoting caries formation.

• Epidemiology: Most common infectious disease; influenced by sucrose intake, dental care, and genetics. Incidence peaks in teens.

• Treatment/Prevention: Drilling and filling, reducing sucrose intake, fluoridation, brushing, and flossing.

Periodontal Disease

• Gingivitis: Swelling and redness of gums; may progress to chronic periodontitis, damaging tooth-supporting structures.

• Causative Agents: Dental plaque at gum line; mainly Gram-negative anaerobes such as Porphyromonas gingivalis, Treponema denticola, and Tannerella forsythia.

• Pathogenesis: Plaque and tartar extend into gingival crevice, inciting inflammation and tissue destruction.

• Epidemiology: Affects nearly 90% of people by age 65; risk factors include smoking and impaired immunity.

• Treatment/Prevention: Cleaning, minor surgery, brushing, and flossing.

Acute Necrotizing Ulcerative Gingivitis (ANUG)

• Also known as: Trench mouth; associated with methamphetamine use ('meth mouth').

• Signs and Symptoms: Bleeding, painful gums, abscessed and broken teeth, foul breath.

• Causative Agent: Treponema spirochete (often with other anaerobes).

• Pathogenesis: Poorly understood; plaque with spirochetes invades tissue. Methamphetamine use reduces saliva, increasing risk.

• Epidemiology: Associated with poor hygiene, nutrition, high sugar diet, stress, and immunodeficiency.

• Treatment/Prevention: Hydrogen peroxide, plaque/tartar removal, daily oral hygiene, professional cleaning.

Table: Important Infections of the Teeth and Gums

Feature	Dental Caries	Periodontal Diseases	Acute Necrotizing Ulcerative Gingivitis
Signs/Symptoms	Discoloration, roughness, pain, tooth breakage	Gum swelling, bleeding, bad breath, tooth loosening	Bleeding, painful gums, abscessed teeth, foul breath
Causative Agent	Streptococcus mutans	Gram-negative anaerobes (e.g., Porphyromonas, Treponema)	Treponema spirochete (with other anaerobes)
Pathogenesis	Plaque formation, acid production, enamel damage	Plaque/tartar in gingival crevice, inflammation, tissue destruction	Plaque invasion, tissue destruction, reduced saliva
Epidemiology	Worldwide, peaks in teens	Common in elderly, smokers, immunocompromised	Poor hygiene, nutrition, drug use
Treatment/Prevention	Fillings, hygiene, fluoridation	Cleaning, surgery, hygiene	Hydrogen peroxide, hygiene, professional cleaning

Helicobacter pylori Gastritis

• Discovery: Barry Marshall demonstrated H. pylori causes stomach ulcers.

• Signs and Symptoms: Often asymptomatic; can cause belching, vomiting, peptic ulcers, and stomach cancer.

• Causative Agent: Helicobacter pylori: short, curved, Gram-negative, microaerophilic, with multiple flagella.

• Pathogenesis:

  • Survives stomach acid by producing urease (converts urea to ammonia, creating an alkaline microenvironment).

  • Uses flagella to burrow into mucus layer; produces CagA protein (alters host cells) and VacA toxin (damages epithelial cells).

  • Results in decreased mucus production, persistent infection, and variable outcomes (ulcers, cancer).

Additional info: The notes continue with further details on viral and protozoan diseases of the digestive system, as well as more bacterial pathogens, which are covered in subsequent slides and sections.