Microbiology of Infectious Diseases Affecting the Gastrointestinal Tract

CHAPTER 20: Gastrointestinal Tract: Structure and Function

Overview of the GI Tract

The gastrointestinal (GI) tract is a continuous tube extending from the mouth to the anus, responsible for nutrient delivery and waste elimination. It is also known as the digestive or enteric tract.

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

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

Normal Biota of the Gastrointestinal Tract

Microbial Communities

The GI tract harbors a complex community of microorganisms, especially in the oral cavity and large intestine. These microbes play essential roles in health and disease.

• Oral cavity: Contains bacteria and fungi living on teeth and soft tissues.

• Stomach: Once thought sterile due to low pH; now known to contain diverse microbes, some transient, some resident.

• Large intestine: Home to billions of microbes, providing protective and digestive functions (e.g., Escherichia coli synthesizes vitamin K).

• Accessory organs: Normally free of microorganisms.

Diseases of the Gastrointestinal Tract Caused by Microorganisms (Nonhelminthic)

Acute Diarrhea

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

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

• Transmission: Often via contaminated food or water; food safety and hygiene are critical for prevention.

• Treatment: Most cases are self-limiting; antibiotics are contraindicated except for specific agents (e.g., shigellosis).

Major Bacterial Causes of Diarrhea

Pathogen	Key Features
Salmonella enterica	High infectious dose; associated with contaminated meat, eggs, and animal contact
Shigella	Low infectious dose; causes dysentery (bloody diarrhea); person-to-person spread
E. coli O157:H7 (EHEC)	Produces shiga toxin; can cause hemolytic uremic syndrome (HUS); associated with undercooked beef and raw produce
Campylobacter jejuni	Most common bacterial cause in the U.S.; found in poultry, water, and unpasteurized milk
Clostridium difficile	Antibiotic-associated colitis; major cause of hospital-acquired diarrhea
Vibrio cholerae	Causes cholera; severe watery diarrhea ('rice water stool'); rapid dehydration

Salmonella

Salmonella enterica is the primary species of concern. Transmission occurs via ingestion of contaminated animal products or contact with infected animals.

• Symptoms: Range from mild gastroenteritis to severe typhoid fever (caused by S. Typhi).

• Prevention: Avoiding contaminated food; proper cooking and hygiene.

• Treatment: Fluid and electrolyte replacement; antibiotics only for severe cases.

Shigella

Shigella species cause shigellosis, characterized by frequent, bloody stools and intense abdominal pain. The infectious dose is very low (10–200 organisms).

• Transmission: Fecal-oral route, direct contact; outbreaks in crowded settings (e.g., daycares, prisons).

• Prevention: Good hygiene and sanitation.

• Treatment: Supportive; antibiotics in severe cases (controversial for bloody diarrhea).

Enterohemorrhagic E. coli (EHEC, O157:H7)

This strain produces shiga toxin, leading to bloody diarrhea and potentially hemolytic uremic syndrome (HUS), which can cause kidney failure.

• Transmission: Undercooked beef, raw produce, and person-to-person contact.

• Prevention: Proper cooking of meat, washing produce, and hygiene.

• Treatment: Supportive; antibiotics are contraindicated.

Campylobacter

Campylobacter jejuni is the leading bacterial cause of diarrhea in the U.S. It is transmitted via contaminated food and water, especially poultry and unpasteurized milk.

• Symptoms: Watery diarrhea, fever, abdominal pain, sometimes recurring symptoms.

• Prevention: Sanitary food and water handling.

• Treatment: Rehydration and electrolyte therapy.

Clostridium difficile

Clostridium difficile causes antibiotic-associated colitis, especially in hospitalized patients. It can lead to severe inflammation and pseudomembranous colitis.

• Symptoms: Diarrhea, abdominal cramps, fever, and in severe cases, colonic perforation.

• Prevention: Judicious use of antibiotics; infection control in healthcare settings.

• Treatment: Withdrawal of causative antibiotics, fluid replacement, and sometimes fecal transplant.

Vibrio cholerae (Cholera)

Vibrio cholerae causes cholera, a severe diarrheal disease with rapid fluid loss. Outbreaks are common after natural disasters in areas with poor sanitation.

• Symptoms: Profuse watery diarrhea ('rice water stool'), vomiting, dehydration, shock.

• Transmission: Fecal-oral, contaminated water and food.

• Prevention: Water purification, sewage treatment, vaccination in endemic areas.

• Treatment: Oral rehydration therapy (ORT) is life-saving.

Rotavirus and Other Viral Gastroenteritis

Rotavirus is the leading cause of severe diarrhea in infants and young children worldwide. Other viruses (norovirus, adenovirus, astrovirus) also cause gastroenteritis.

• Symptoms: Watery diarrhea, vomiting, fever, dehydration.

• Transmission: Fecal-oral, contaminated food, water, and fomites.

• Prevention: Oral live virus vaccine for rotavirus; hygiene for other viruses.

• Treatment: Oral rehydration therapy.

Food Poisoning (Acute Diarrhea with Vomiting)

Food poisoning is caused by ingestion of preformed toxins from bacteria such as Staphylococcus aureus, Bacillus cereus, and Clostridium perfringens.

• Symptoms: Rapid onset (1–6 hours) of severe nausea, vomiting, and diarrhea.

• Transmission: Consumption of contaminated, improperly stored food.

• Treatment: Supportive; antibiotics are not indicated.

Chronic Diarrhea

Protozoan Causes

Chronic diarrhea (lasting more than 14 days) can be caused by protozoa such as Giardia lamblia, which is transmitted via contaminated water or close contact with infected individuals.

• Symptoms: Long-lasting diarrhea, abdominal pain, flatulence, greasy stools.

• Prevention: Avoid drinking untreated water; no human vaccine available.

Diseases of the GI Tract Caused by Viruses

Mumps

Mumps is a viral infection causing painful swelling of the parotid salivary glands. It is preventable by the MMR vaccine.

• Symptoms: Fever, malaise, parotitis, and sometimes complications in other organs.

• Prevention: MMR vaccine at 12–15 months and booster at 4–6 years.

Gastritis and Gastric Ulcers

Helicobacter pylori is the causative agent of gastritis and peptic ulcers. It is transmitted person-to-person, likely via oral-oral or fecal-oral routes.

• Symptoms: Burning abdominal pain, ulcers, possible bleeding.

• Treatment: Antibiotics and acid suppressors.

Hepatitis Viruses

Hepatitis A Virus (HAV)

HAV is transmitted primarily by the fecal-oral route and causes acute, self-limited hepatitis. Vaccination is effective for prevention.

• Symptoms: Jaundice, fever, malaise, swollen liver.

• Prevention: Immunization, hygiene, and safe food/water practices.

Hepatitis B Virus (HBV)

HBV is transmitted via blood, sexual contact, and perinatally. It can cause chronic liver disease and cancer. Healthcare workers are at high risk and vaccination is required.

• Symptoms: Fever, malaise, liver dysfunction, risk of chronic infection.

• Prevention: Vaccination, safe practices, screening blood products.

Hepatitis C Virus (HCV)

HCV is mainly transmitted through blood contact and can lead to chronic hepatitis, cirrhosis, and liver cancer. There is no vaccine.

• Symptoms: Often asymptomatic initially; chronic infection can cause severe liver disease.

• Prevention: Blood screening, harm reduction for drug users.

Diseases of the GI Tract Caused by Helminths

Overview of Helminthic Infections

Helminths are parasitic worms classified as nematodes (roundworms), trematodes (flukes), and cestodes (tapeworms). They are acquired via fecal-oral route or skin penetration.

• Diagnosis: Identification of eggs, larvae, or adult worms in stool or tissue samples.

• Prevention: Hygiene, proper cooking, and avoiding contaminated water/food.

• Treatment: Antihelminthic medications; no vaccines available.

Enterobius vermicularis (Pinworm)

The most common worm infection in children in temperate zones. Transmission is via ingestion of eggs from contaminated surfaces or self-inoculation.

• Symptoms: Anal itching, disrupted sleep, abdominal discomfort.

• Diagnosis: Adhesive tape test for eggs.

Taenia solium (Pork Tapeworm)

Humans acquire infection by eating undercooked pork containing cysts. Adult worms can reach several meters in length.

• Symptoms: Mild abdominal symptoms; passage of proglottids in stool.

• Prevention: Proper cooking of pork.

Trichinosis

Caused by Trichinella species, transmitted by eating undercooked pork or wild game. The parasite forms cysts in muscle tissue.

• Symptoms: Muscle pain, neurological symptoms.

• Prevention: Proper cooking of meat.