Microbial Infections of the Digestive System and Associated Immune Responses

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The digestive tract contains specialized immune structures that protect against pathogens entering via ingestion.

Gut-associated lymphoid tissue (GALT): Lymphoid tissue in the gut that initiates immune responses to pathogens.
Lymph nodes: Filter lymph and house immune cells.
Peyer's patches: Aggregated lymphoid nodules in the small intestine that monitor intestinal bacteria populations.

Oral health is closely linked to microbial biofilms and dietary habits.

Dental plaque: Biofilm on teeth that attracts cariogenic bacteria, leading to dental caries (tooth decay).
Key organism: Streptococcus mutans is the most important cariogenic bacterium in the oral cavity.
Dietary sugar: Increased table sugar intake correlates with higher rates of dental caries.

Gingivitis: Caused by Streptococci, actinomycetes, and anaerobic gram-negative bacteria.
Periodontitis: Caused by Porphyromonas.
Acute necrotizing ulcerative gingivitis (Trench mouth): Caused by Prevotella intermedia.

Foodborne illnesses are often caused by bacterial toxins or direct infection.

Staphylococcal food poisoning: Caused by Staphylococcus aureus enterotoxin (an intoxication, not infection).
Heat resistance: The enterotoxin is very heat resistant and survives boiling.
Source tracing: Phage typing is used to trace contamination sources.

Pathogenesis: Salmonella uses M cells in the intestine to enter lymphatics.
Causative agent: Salmonella enterica (salmonellosis); commonly found in commercial chicken and eggs.
Typhoid fever: Caused by Salmonella serotype Typhi and Paratyphi; some patients become chronic carriers (harboring bacteria in the gallbladder).

Cholera: Caused by Vibrio cholerae, which produces cholera toxin leading to massive electrolyte and water loss ("rice water stools").
Symptoms: Shock, collapse, organ failure, and death if untreated.
Treatment: IV fluids and oral rehydration therapy.
Sanitation: Outbreaks increase with poor sanitation and sewage disposal.
Noncholera Vibrios: Vibrio parahaemolyticus associated with raw oysters and crustaceans.

Several pathogenic varieties of E. coli cause gastroenteritis, each with distinct mechanisms and epidemiology.

Type	Features
Enteropathogenic (EPEC)	Diarrhea in developing countries; forms pedestals on host cells
Enteroinvasive (EIEC)	Shigella-like dysentery
Enterotoxigenic (ETEC)	Produces enterotoxin similar to cholera toxin; main cause of traveler's diarrhea
Enteroaggregative (EAEC)	Only in humans; watery diarrhea via enterotoxin
Enterohemorrhagic (EHEC)	Produces Shiga-like toxin; main reservoir is cattle; causes most outbreaks

Other causes of traveler's diarrhea: Salmonella, Shigella, and Campylobacter (leading cause of foodborne illness).

Peptic ulcer disease: Caused by Helicobacter pylori, which survives stomach acid by producing urease.
Yersinia gastroenteritis: Transmitted via pork products.
Clostridium perfringens gastroenteritis: Associated with meats and stews containing animal intestinal contents.
Clostridium difficile-associated diarrhea: Most deadly intestinal infection; triggered by extended antibiotic use; can lead to life-threatening colitis.

Mumps: Targets parotid glands; prevented by MMR vaccine; transmitted via saliva and respiratory secretions.

Virus	Genome/Envelope	Transmission	Features
Hepatitis A	ssRNA, non-enveloped	Fecal-oral	Acute; vaccine available
Hepatitis B	dsDNA, enveloped	Blood/body fluids	Acute/chronic; cirrhosis/cancer; vaccine available
Hepatitis C	ssRNA, enveloped	Blood/body fluids	Chronic in 85%; cirrhosis/cancer; no vaccine
Hepatitis D	Satellite virus	Blood/body fluids (needs HBV)	Progressive liver damage
Hepatitis E	ssRNA, non-enveloped	Fecal-oral	Similar to HAV; poor sanitation

Hepatitis B forms: Acute (often subclinical), chronic (10% of cases), HBeAg indicates active replication.

Rotavirus: Common in children; low mortality; prevented with live oral vaccine.
Norovirus: Most common foodborne viral illness; detected by PCR/EIA; treated with oral rehydration.

Giardiasis: Caused by Giardia duodenalis; associated with untreated water exposure.
Cryptosporidiosis: Caused by Cryptosporidium parvum and C. hominis; resistant to chlorination.
Amebic dysentery: Caused by Entamoeba histolytica.

Taeniasis: Adult tapeworm infection of the intestine (Taenia saginata – beef, Taenia solium – pork).
Cysticercosis: Infection by ingesting eggs of Taenia solium; larvae can lodge in tissues (ophthalmic cysticercosis – eye; neurocysticercosis – CNS).
Pinworm: Enterobius vermicularis; causes perianal itching.
Hookworms: Necator americanus and Ancylostoma duodenale; attach to intestinal wall, feed on blood, cause anemia and pica; infect via skin contact with contaminated soil.
Ascariasis: Caused by Ascaris lumbricoides; most common helminth infection worldwide (800 million to 1 billion infected); complex life cycle involving migration through lungs and intestines.
Whipworm: Trichuris trichiura; 429–800 million infected; causes anemia, malnutrition, and growth retardation.
Trichinellosis: Caused by Trichinella spiralis; acquired by eating undercooked pork or other meats containing encysted larvae.

Oral rehydration therapy: Essential for reversing dehydration in gastrointestinal diseases by replacing lost water and electrolytes.
Sanitation: Critical for preventing waterborne and foodborne diseases such as cholera and hepatitis A/E.
Vaccination: Effective for preventing mumps, hepatitis A, and hepatitis B.

Eggs shed in feces are ingested by another person.
Eggs hatch into larvae, which migrate to the bloodstream and lungs.
Larvae move to the throat, are swallowed, and mature into adults in the intestine.
Adult worms may emerge from the anus, mouth, or nose.

Global impact: 800 million to 1 billion people infected worldwide.

Oral rehydration therapy typically uses a solution containing sodium, glucose, potassium, and bicarbonate or citrate to maximize water absorption.
Chronic carriers of Salmonella Typhi can shed bacteria for years, posing a public health risk.
Hepatitis B surface antigen (HBsAg) and e antigen (HBeAg) are important markers for infection and infectivity.