Microbial Diseases of the Digestive System

Overview of the Digestive System

The digestive system is a complex series of organs and glands that processes food, extracts nutrients, and eliminates waste. It is a major site for microbial colonization and infection due to its exposure to ingested materials.

• Main organs: Oral cavity, pharynx, esophagus, stomach, small intestine, large intestine, rectum, anus

• Accessory organs: Salivary glands, liver, gallbladder, pancreas

Normal microbiota are abundant, especially in the mouth and large intestine, and play roles in digestion and immune defense.

Normal Microbiota and Host Defenses

Normal Microbiota

• Millions of bacteria per milliliter of saliva

• Large numbers in the large intestine (up to 100 billion bacteria per gram of feces)

Host Defenses

• Stomach: Acidic environment inhibits many pathogens

• Small intestine: Paneth cells produce α-defensins and other antimicrobial peptides

Bacterial Diseases of the Mouth

Structure of the Tooth

Teeth are susceptible to microbial colonization due to their non-shedding surfaces, allowing biofilm (dental plaque) to accumulate.

• Enamel: Hard, outermost layer

• Dentin: Underlies enamel, less mineralized

• Pulp cavity: Contains nerves and blood vessels

• Gingiva: Gum tissue surrounding teeth

Dental Caries (Tooth Decay)

Streptococcus mutans is the primary pathogen responsible for dental caries. It metabolizes carbohydrates, tolerates acidity, and synthesizes dextrans that help form biofilms.

• Biofilm formation: S. mutans adheres to tooth surfaces, especially in the presence of sucrose, producing sticky dextran polymers.

• Acid production: Fermentation of sugars produces acids that demineralize enamel.

Stages of Tooth Decay

Tooth decay progresses through several stages, beginning with plaque formation and advancing to pulp involvement if untreated.

1. Healthy tooth with plaque

2. Decay in enamel

3. Advanced decay

4. Decay in dentin

5. Decay in pulp

Periodontal Disease

Periodontal diseases affect the supporting structures of the teeth, including the gums and bone. Porphyromonas species are major pathogens.

• Gingivitis: Inflammation of the gums

• Periodontitis: Destruction of supporting bone and periodontal ligament, formation of periodontal pockets

Summary Table: Bacterial Diseases of the Mouth

Bacterial Diseases of the Lower Digestive System

General Concepts

• Infection: Growth of a pathogen in the GI tract (incubation: 12 hours to 2 weeks)

• Intoxication: Ingestion of preformed toxins (symptoms: 1 to 48 hours after ingestion)

• Gastroenteritis: Inflammation of the stomach and intestines, causing diarrhea and dysentery

• Treatment: Oral rehydration therapy is often essential

Staphylococcal Food Poisoning

• Pathogen: Staphylococcus aureus

• Symptoms: Nausea, vomiting, diarrhea

• Mechanism: Intoxication by enterotoxin (superantigen)

• Diagnosis: Phage typing

• Treatment: None (self-limiting)

Shigellosis (Bacillary Dysentery)

• Pathogen: Shigella spp.

• Symptoms: Tissue damage, dysentery

• Mechanism: Infection; produces endotoxin and Shiga exotoxin

• Diagnosis: Isolation of bacteria

• Treatment: Fluoroquinolones

Salmonellosis and Typhoid Fever

• Salmonellosis Pathogen: Salmonella enterica

• Typhoid Fever Pathogen: Salmonella typhi

• Symptoms: Nausea, diarrhea (salmonellosis); high fever, significant mortality (typhoid)

• Mechanism: Infection; endotoxin production

• Diagnosis: Isolation and serotyping

• Treatment: Oral rehydration (salmonellosis); quinolones/cephalosporins (typhoid)

Cholera and Other Vibrios

• Cholera Pathogen: Vibrio cholerae O:1 and O:139

• Symptoms: Profuse watery diarrhea ("rice-water stools")

• Mechanism: Cholera toxin (exotoxin) causes secretion of Cl-, HCO3-, and water

• Treatment: Rehydration, doxycycline

Escherichia coli Gastroenteritis

• EPEC: Enteropathogenic E. coli – forms pedestals on host cells

• EIEC: Enteroinvasive E. coli – invades intestinal mucosa

• EAEC: Enteroaggregative E. coli – produces enterotoxin, watery diarrhea

• EHEC: Enterohemorrhagic E. coli – produces Shiga toxin, can cause hemolytic uremic syndrome (HUS)

Helicobacter Peptic Ulcer Disease

• Pathogen: Helicobacter pylori

• Symptoms: Peptic ulcers

• Mechanism: Infection; urease production neutralizes stomach acid

• Diagnosis: Urea breath test, bacterial culture

• Treatment: Antimicrobial drugs

Viral Diseases of the Digestive System

Mumps

• Pathogen: Mumps virus

• Symptoms: Swollen parotid glands

• Incubation: 16–18 days

• Treatment: Preventive vaccine

Viral Hepatitis

Hepatitis is inflammation of the liver, caused by several viruses (A, B, C, D, E) with different transmission routes and outcomes.

Fungal Diseases of the Digestive System

Mycotoxin Intoxications

Protozoan Diseases of the Digestive System

Giardiasis

• Pathogen: Giardia lamblia

• Symptoms: Diarrhea due to protozoan adherence to intestinal wall

• Reservoir: Water or mammals

• Diagnosis: FA test

• Treatment: Metronidazole; quinacrine

Cryptosporidiosis

• Pathogen: Cryptosporidium hominis

• Symptoms: Self-limiting diarrhea; severe in immunosuppressed

• Reservoir: Cattle; water

• Diagnosis: Acid-fast stain; FA; ELISA

• Treatment: Oral rehydration

Amebic Dysentery

• Pathogen: Entamoeba histolytica

• Symptoms: Abscesses, significant mortality

• Reservoir: Humans

• Diagnosis: Microscopy; serology

• Treatment: Metronidazole

Helminthic Diseases of the Digestive System

Overview of Helminths

• Kingdom: Animalia

• Phyla: Platyhelminthes (flatworms: trematodes/flukes, cestodes/tapeworms), Nematoda (roundworms)

• Features: Chemoheterotrophic, multicellular, complex life cycles, reduced digestive/nervous systems

Life Cycle of Helminths

• Monoecious: Both male and female reproductive organs in one animal

• Dioecious: Separate male and female individuals

• Stages: Egg → larva(e) → adult

Clonorchis sinensis (Liver Fluke)

• Transmission: Ingestion of undercooked, infested fish

• Symptoms: Often asymptomatic; fever, abdominal pain, jaundice

• Diagnosis: Eggs in fecal matter

• Treatment: Praziquantel

Taenia spp. (Tapeworms)

• T. saginata: Beef tapeworm

• T. solium: Pork tapeworm

• Transmission: Ingestion of undercooked meat containing larvae

• Definitive host: Humans

• Intermediate host: Cattle (T. saginata), pigs (T. solium)

Neurocysticercosis

Neurocysticercosis is a severe complication of T. solium infection, where larvae invade the central nervous system, causing neurological symptoms.

Additional info: For all diseases, prevention includes proper food handling, sanitation, and, where available, vaccination. Oral rehydration is a cornerstone of treatment for diarrheal diseases.