Microbial Diseases of the Digestive System – Key Vocabulary

Structure & Function of the Digestive System

• Gastrointestinal (GI) / Alimentary Canal Components
  • Mouth
  • Pharynx (throat)
  • Esophagus
  • Stomach
  • Small intestine
  • Large intestine
• Accessory Organs
  • Teeth & tongue
  • Salivary glands
  • Liver
  • Gallbladder
  • Pancreas
• Physiology
  • Digestion = mechanical & enzymatic reduction of macromolecules into absorbable units
  • Absorption = uptake of digestion products from intestinal lumen → blood & lymph

Immune Defenses Along the GI Tract

• 70\% of the entire immune system is positioned in the intestinal tract (Gut-Associated Lymphoid Tissue, GALT)
  • Includes lymph nodes & Peyer’s patches

Normal Microbiota

• Saliva: millions of bacteria per mL
• Stomach: very few microbes (low pH from HCl)
• Large intestine:
  • Dense population (≈ 10^{11} bacteria g⁻¹ feces)
  • Predominantly anaerobes & facultative anaerobes
  • Assist in polysaccharide digestion & vitamin synthesis

Dental & Periodontal Diseases

• Dental Caries (Tooth Decay)
  • Dental plaque = biofilm on tooth surface
  • Key pathogen: Streptococcus mutans
  – Converts sucrose → lactic acid (enamel demineralization)
  – Produces dextran (sticky polysaccharide) → plaque matrix
  • Dextran + bacterial fimbriae = strong adherence to enamel
  • High-sucrose diets ↑ risk (more substrate for acid & dextran production)
• Periodontal Spectrum
  • Gingivitis: gum inflammation/infection (streptococci, actinomycetes, anaerobic G– rods)
  • Periodontitis: destruction of bone & supporting tissues; chief culprit Porphyromonas
  • Progressive stages: healthy gingiva → gingivitis → periodontal pockets → periodontitis

Definitions for Lower GI Bacterial Illnesses

• Infection = pathogen enters GI tract & multiplies (longer incubation)
• Intoxication = ingestion of pre-formed exotoxin (rapid onset; fever uncommon)
• Diarrhea = watery stools; dysentery = diarrhea + blood & mucus
• Gastroenteritis = inflammation of stomach & intestinal mucosa
• Oral Rehydration Therapy (ORT) replaces water + electrolytes; lifesaving for children in regions lacking safe water

Major Bacterial Diseases of Lower Digestive System

Shigellosis (Bacillary Dysentery)

• Etiology: Shigella spp. (facultative anaerobic, G– rods); strictly human-to-human
• Produces Shiga toxin (inhibits protein synthesis)
• Clinical: diarrhea, abdo cramps, fever; often severe dysentery

Salmonellosis (Salmonella Gastroenteritis)

• Pathogen: Salmonella enterica (facultative anaerobic G– rods)
• Invades intestinal M cells → lymphatics → bloodstream; multiplies in macrophages
• Incubation \approx 12\text{–}36\,h; sx: fever, nausea, pain, diarrhea

Typhoid & Paratyphoid Fevers

• Pathogens: Salmonella Typhi & S. Paratyphi
• Produce Typhoid (AB) toxin
• Human reservoir only; phagocyte-borne systemic spread
• Sx: sustained high fever, headache, ulceration; diarrhea late (week 2–3)
• Rx: ceftriaxone or azithromycin; carriers require prolonged therapy

Cholera

• Agent: Vibrio cholerae
• Grows in small intestine; secretes cholera toxin
  • Toxin → epithelial cells secrete \text{Cl}^-, \text{HCO}_3^- & water
  • “Rice-water stools”; fluid loss \le 20\,L\,day^{-1}
  • Shock → organ failure → death if untreated
• Therapy: rapid IV or oral rehydration

Escherichia coli Gastroenteritis (EHEC)

• Serotype O157:H7 predominates
• Virulence:
  • Forms actin pedestals beneath adherent bacteria
  • Releases Shiga-like toxin (SLT) upon lysis (antibiotics can ↑ toxin)
• Disease spectrum: hemorrhagic colitis & HUS (hemolytic uremic syndrome)
• Lab: sorbitol-nonfermenter; ELISA for SLT

Helicobacter pylori & Peptic Ulcer Disease

• Prevalence: infects 30\text{–}50\% of people in developed nations
• Survival: produces urease → \text{urea}\to\text{NH}_3 (alkaline micro-niche)
• Virulence factors:
  • Flagella → burrow through mucus
  • Urease
  • Adhesins
  • Toxins (thin mucus, mucosal damage)
• Outcomes: gastritis, gastric/duodenal ulcers, ↑ gastric cancer risk
• Dx: biopsy + culture, urea breath test
• Rx: antimicrobials + bismuth subsalicylate

Clostridioides difficile–Associated Diarrhea (CDAD)

• Gram +, endospore-forming anaerobe; nosocomial predominance
• Etiology: prolonged broad-spectrum antibiotics → microbiota depletion
• Pathogenesis: spores germinate → toxins CdA & CdB → pseudomembranes
• Sx: mild diarrhea → fulminant colitis, perforation, death
• Dx: PCR, antigen assays, stool culture
• Rx: vancomycin, fidaxomicin, fecal microbiota transplant (FMT)

Bacillus cereus Gastroenteritis

• Large Gram +, spore-forming soil organism
• Spores survive cooking → germinate in food & secrete distinct toxins
  • Emetic toxin → nausea/vomiting
  • Diarrheal toxin → watery stools
• Symptom profile depends on toxin produced

Viral Hepatitis Overview

• Hepatitis = liver inflammation; etiologies include drugs & viruses (HAV–HEV)
• Hallmarks: jaundice, elevated liver enzymes, possible chronicity & cancer

Hepatitis C (HCV)

• ssRNA enveloped virus (Flaviviridae)
• Transmission: parenteral, blood transfusion, bodily fluids
• Immune evasion via rapid genetic variation → chronic infection in 85\%
• “Silent epidemic” (20-yr latency)
• Exceeds AIDS in U.S. mortality; leading reason for liver transplants

Comparative Table (Key Points)

• HAV: fecal-oral; acute; inactivated vaccine
• HBV: blood/sex; sometimes chronic; vaccine available; Rx interferon + nucleoside analogs
• HCV: blood; often chronic; no vaccine; protease/polymerase inhibitors treat
• HDV: coinfects with HBV; severe; HBV vaccine protective
• HEV: fecal-oral; high mortality in pregnancy; no vaccine (in U.S.)

Mycotoxin Food Poisoning

• Ergot (Claviceps purpurea) on grains
  • Alkaloids → vasoconstriction (gangrene) & hallucinations
• Aflatoxin (Aspergillus flavus) on peanuts, grains
  • Potent hepatocarcinogen; causes cirrhosis & cancer

Protozoan Infection – Giardiasis

• Agent: Giardia duodenalis (flagellated)
• Transmission: ingestion of cysts (water, person-to-person); infectious dose ≈ 10 cysts
• Life cycle: cyst → trophozoite excysts in SI → adheres via ventral disk → binary fission → encystment in colon → cysts shed
• Sx: prolonged greasy diarrhea, flatulence, weight loss; H_2S odor on breath/stool
• Dx: stool O&P microscopy, modern serologic fecal assays
• Rx: metronidazole, quinacrine, or nitazoxanide

Helminthic Diseases

Platyhelminths (Flatworms)

• Trematodes (flukes): oral & ventral suckers, external cuticle
• Cestodes (tapeworms):
  • Scolex (hooks/suckers)
  • Proglottids (reproductive segments)
  • Cuticle absorbs nutrients

Taeniasis & Cysticercosis

• Beef tapeworm Taenia saginata; Pork tapeworm Taenia solium
• Taeniasis: ingestion of cysticerci in undercooked meat → adult worms (can reach >5\,\text{m})
• Cysticercosis: ingestion of eggs → larvae invade tissues
• Neurocysticercosis: larvae in CNS (seizures)

Hydatid Disease (Echinococcus granulosus)

• Life cycle: dog (definitive host) sheds eggs → livestock/human ingest → hydatid cysts in liver/lungs/brain (up to 15\,L fluid)
• Cyst rupture → anaphylaxis
• Dx: serology, imaging (X-ray/CT/MRI)
• Rx: surgical removal + albendazole

Nematodes (Roundworms)

• Complete digestive tract; dioecious with sexual dimorphism; males sometimes have spicules

Pinworm (Enterobius vermicularis)

• Tiny nematode; nighttime perianal egg deposition → pruritus
• Dx: “scotch-tape” egg test
• Rx: pyrantel pamoate or mebendazole; treat family contacts

Hookworms (Necator americanus, Ancylostoma duodenale)

• Larvae in soil penetrate skin → bloodstream → lungs → swallowed → SI
• Adult attaches to mucosa & feeds on blood → anemia, lethargy, pica
• Prevention: sanitation, shoes; Rx mebendazole

Ascariasis (Ascaris lumbricoides)

• Massive global burden (8\times10^8–10^9 cases)
• Eggs ingested → larvae migrate via blood to lungs → coughed & swallowed → adults in SI (up to 30\,cm)
• Worms may exit via mouth, nose, or anus
• Dx: ova in feces
• Rx: mebendazole or albendazole

Miscellaneous / Practical Connections

• Pedestal formation by EHEC is a classic example of pathogen manipulation of host cytoskeleton; may be explored in laboratory practicals.
• Oral rehydration & fecal microbiota transplantation illustrate low-tech vs high-tech but equally life-saving interventions.
• Many GI pathogens showcase AB exotoxins (Shiga, cholera, typhoid) → pertinent for toxin structure/function exam questions.
• Emergence of antimicrobial resistance (e.g., C. difficile, Salmonella) underlines ethical imperative of antibiotic stewardship.
• Water safety, sanitation, and proper food handling remain foundational public-health measures against most diseases summarized above.