Gastrointestinal Tract: Structure and Defenses

Overview of the Gastrointestinal (GI) Tract

The gastrointestinal tract, also known as the digestive or enteric tract, is a continuous tube extending from the mouth to the anus. It is responsible for the digestion and absorption of nutrients, as well as waste elimination.

• Main sections: Mouth, pharynx, esophagus, stomach, small intestine, large intestine, rectum, anus

• Accessory organs: Salivary glands, liver, gallbladder, pancreas (add digestive fluids and enzymes)

Defenses of the GI Tract

• Mucus layer: Coats all intestinal surfaces, providing a physical barrier to pathogens.

• Secretory IgA: Immunoglobulin found on most surfaces, neutralizing pathogens.

• Peristalsis: Muscular contractions move food and microorganisms through the tract, reducing colonization.

• Saliva: Contains lysozyme (breaks down bacterial cell walls) and lactoferrin (binds iron, inhibiting microbial growth).

• Stomach acidity: Gastric fluid is highly acidic and antimicrobial.

• Bile: Has antimicrobial properties.

• Gut-associated lymphoid tissue (GALT): Lymphoid tissue (e.g., tonsils, adenoids, Peyer’s patches, appendix) containing T and B cells, producing IgA and other immune responses.

• Commensal organisms: Especially in the large intestine, these microbes help prevent colonization by pathogens.

Normal Biota of the Gastrointestinal Tract

Oral Cavity

• Populated by over 600 species of microorganisms.

• Bacteria: Actinomyces, Lactobacillus, Neisseria, Prevotella, Streptococcus, Treponema, Veillonella

• Fungi: Candida albicans

• Protozoa: Trichomonas tenax, Entamoeba gingivalis

• Bacteria form dental plaque (biofilm), with alpha-hemolytic streptococci as early colonizers.

Stomach and Large Intestine

• Stomach: Once thought sterile, now known to host species such as Bacillus, Clostridium, Staphylococcus, Streptococcus.

• Large intestine: Contains up to 1011 microbes per gram. Key genera include Bacteroides, Bifidobacterium, Clostridium, Enterobacter, Escherichia, Fusobacterium, Lactobacillus, Peptostreptococcus, Staphylococcus, Streptococcus.

• Also contains fungi (Candida), protozoa, and archaea.

Functions of Normal Biota

• Protective function against pathogens

• Assist in digestion and nutrient production (e.g., E. coli synthesizes vitamin K)

• Support epithelial cell structure and immune system education

• Influence host susceptibility to obesity and autoimmune diseases

Diseases Affecting the Gastrointestinal Tract

Acute Diarrhea

Defined as three or more loose stools in a 24-hour period. It is a major cause of morbidity and mortality, especially in children in developing countries.

• Common symptoms: fever, abdominal pain, cramping, nausea, vomiting, dehydration

• Transmission: Often foodborne; contaminated food and water are major sources

• Most cases are self-limiting; some require antibiotic treatment (e.g., shigellosis)

Bacterial Causes of Acute Diarrhea

Salmonella

• Large genus; disease mainly caused by Salmonella enterica subspecies enterica

• Motile, ferment glucose, produce hydrogen sulfide, resistant to bile and dyes

• Normal biota in animals; transmitted via contaminated meat, eggs, and other foods

• High infectious dose (ID50); symptoms include vomiting, diarrhea, fever, sometimes septicemia

• Virulence factors: adherence to gut mucosa, endotoxin production

Shigella

• Gram-negative, nonmotile rods; human parasites

• Low infectious dose (10–200 organisms); transmitted via oral route and direct contact

• Symptoms: frequent, watery stools, fever, intense abdominal pain, dysentery (blood and mucus in stool)

• Virulence factors: invasion of large intestine mucosa, endotoxin, enterotoxin, Shiga toxin (inhibits protein synthesis)

Shiga-Toxin-Producing E. coli (STEC)

• Most virulent: E. coli O157:H7

• Symptoms: mild gastroenteritis to bloody diarrhea; can cause hemolytic uremic syndrome (HUS)

• Virulence: Shiga toxin (acquired via bacteriophage), effacement of enterocytes

• Transmission: undercooked beef, contaminated food/water

• Antibiotics contraindicated; supportive therapy preferred

Campylobacter

• Most common bacterial cause of diarrhea in the U.S.; mainly Campylobacter jejuni

• Symptoms: watery stools, fever, vomiting, abdominal pain; may last >2 weeks

• Transmission: contaminated water, milk, meat, chicken

• Complication: Guillain-Barré syndrome (neuromuscular paralysis)

Clostridium difficile

• Gram-positive, endospore-forming rod; normal biota

• Causes pseudomembranous colitis, often after broad-spectrum antibiotics

• Symptoms: abdominal cramps, fever, leukocytosis, pseudomembranes in colon

• Releases endospores; requires strict infection control

• Treatment: withdraw offending antibiotic, metronidazole or vancomycin

Vibrio cholerae

• Comma-shaped, motile rods; cause cholera

• Symptoms: abrupt vomiting, copious watery "rice-water" stools, rapid dehydration, shock

• Virulence: cholera toxin (CT) causes massive electrolyte and water loss

• Transmission: contaminated water/food; outbreaks linked to climate and water sources

• Treatment: oral rehydration therapy (ORT), antibiotics in severe cases

Protozoal and Viral Causes of Diarrhea

Cryptosporidium

• Intestinal protozoan; zoonotic

• Symptoms: gastroenteritis, chronic diarrhea in AIDS patients

• Transmission: oocysts in contaminated water/food; resistant to chlorine

• Diagnosis: ELISA, acid-fast staining

Rotavirus

• dsRNA virus; primary cause of severe diarrhea in children

• Transmission: fecal-oral, contaminated food/water

• Symptoms: watery diarrhea, vomiting, dehydration, shock

• Greatest risk: infants 6–24 months without maternal antibodies

Norovirus

• Second most common cause of foodborne hospitalizations in the U.S.

• Transmission: fecal-oral, contaminated food/water

• Symptoms: profuse watery diarrhea, vomiting, fever (3–5 days)

• Treatment: rehydration

Food Poisoning (Intoxication)

Results from ingestion of preformed toxins in food. Symptoms are rapid in onset (1–6 hours) and often violent.

• Staphylococcus aureus exotoxin: Heat-stable enterotoxin; associated with unrefrigerated foods. Symptoms: cramping, nausea, vomiting, diarrhea. Self-limiting.

• Bacillus cereus exotoxin: Two forms: emetic (vomiting, linked to fried rice) and diarrheal (linked to meats/vegetables). Prevention: proper food handling.

• Clostridium perfringens: Also causes food poisoning (not detailed above).

• Lasts longer than 14 days; may be infectious or noninfectious (e.g., irritable bowel syndrome, ulcerative colitis)

• Opportunistic infections in AIDS: Cryptosporidium, Mycobacterium avium

Chronic Diarrhea

Protozoal Causes of Chronic Diarrhea

• Cyclospora cayetanensis: Fecal-oral transmission via contaminated produce/water; not person-to-person.

• Giardia lamblia: Flagellated protozoan; causes greasy, foul-smelling stools, abdominal pain, flatulence. Transmitted via cysts in water/food. Treatment: tinidazole or nitazoxanide.

• Entamoeba histolytica: Amoeba causing amoebiasis; symptoms include dysentery, abdominal pain, weight loss. Transmitted via cysts in contaminated food/water. Treatment: metronidazole or chloroquine.

Oral Cavity Infections

Dental Caries (Tooth Decay)

• Most common infectious disease in humans

• Caused by acid production from bacterial fermentation of carbohydrates (mainly Streptococcus mutans and others)

• Progresses from enamel to dentin to pulp, causing pain

• Prevention: restrict sucrose, brush/floss, fluoride in water/toothpaste

• Treatment: removal of decayed tissue, restoration with fillings

Periodontal Diseases

• Includes gingivitis (gum inflammation) and periodontitis (destruction of supporting structures)

• Caused by polymicrobial biofilms; calculus formation triggers inflammation

• Treatment: removal of calculus/plaque, oral hygiene, surgery, antibiotics

Diseases of the GI Tract: Viral and Bacterial

Mumps

• Viral infection of salivary glands (parotitis); swelling of cheeks

• Complications: orchitis, epididymitis in males

• Transmission: salivary/respiratory secretions

• Prevention: MMR vaccine

Gastritis and Gastric Ulcers

• Caused by Helicobacter pylori (curved gram-negative rod)

• Symptoms: burning abdominal pain, ulcers, possible bleeding

• Virulence: urease enzyme neutralizes stomach acid

• Transmission: oral-oral or fecal-oral; acquired early in life

• Treatment: antibiotics (clarithromycin, metronidazole) plus acid suppressors

Hepatitis

General Features

• Inflammation of the liver; causes jaundice (bilirubin accumulation)

• Caused by viruses (HAV, HBV, HCV, HDV, HEV), drugs, alcohol, autoimmune diseases

Hepatitis A and E

• Single-stranded, nonenveloped RNA viruses; transmitted fecal-oral

• Usually self-limited; HEV dangerous in pregnancy and immunosuppressed

• Prevention: HAV vaccine, improved sanitation

Hepatitis B and D

• HBV: enveloped DNA virus; HDV: enveloped RNA virus (requires HBV for replication)

• Transmission: blood, sexual contact, vertical

• Symptoms: fever, malaise, liver dysfunction, possible chronic infection, cirrhosis, cancer

• Prevention: recombinant HBV vaccine; required for healthcare workers

• Treatment: interferon, antivirals (not curative); passive immunization for exposure

Hepatitis C

• RNA virus; "silent epidemic" due to long asymptomatic period

• Transmission: blood, injecting drug use, vertical

• High rate of chronic infection; major cause of liver transplants and cancer in the U.S.

• Treatment: sofosbuvir and ledipasvir (can cure infection); no vaccine available

Helminthic Diseases of the GI Tract

General Features

• Three main groups: nematodes (roundworms), trematodes (flukes), cestodes (tapeworms)

• Symptoms often due to host immune response (eosinophilia)

• Transmission: fecal-oral, skin penetration

• Diagnosis: O&P test (ova and parasites in stool), blood eosinophilia, serology

• Treatment: antihelminthic drugs; no vaccines available

Selected Helminthic Diseases

Helminth Pathogenesis and Diagnosis

• Adaptations: specialized mouthparts, enzymes, protective cuticle

• Definitive host: where adult worm resides; intermediate host: larval stages

• Diagnosis: stool O&P, blood eosinophilia, serology, travel history

Prevention and Treatment

• No vaccines; periodic antihelminthic treatment in endemic areas

• Drugs target worm metabolism or movement; resistance can occur

• Surgery may be required in some cases

Summary Table: Selected GI Pathogens and Features

Additional info: This summary integrates and expands upon the provided lecture notes, adding definitions, classifications, and tables for clarity and exam preparation. For more detailed mechanisms, refer to standard microbiology and anatomy & physiology textbooks.