Digestive System Anatomy, Microbiota, and Infections

Anatomy and Normal Microbiota of the Digestive System

• The digestive tract includes the oral cavity, pharynx, esophagus, stomach, small intestine, and large intestine, all of which host a normal microbiota crucial for maintaining health.
• The body defends against pathogen colonization through:
  • The continuous movement of materials through the gastrointestinal canal.
  • A protective mucus layer.
  • The presence of normal microbiota.
  • Harsh chemical conditions in the stomach and small intestine.
• In the oral cavity, infections or microbial toxins can lead to:
  • Tooth decay.
  • Periodontal disease.
  • Various types of ulcers.
• Infections and intoxications in the gastrointestinal (GI) tract can manifest as:
  • Nausea.
  • Vomiting.
  • Diarrhea.
  • Fever.
• Localized inflammation in the GI tract can result in:
  • Gastritis (inflammation of the stomach).
  • Enteritis (inflammation of the small intestine).
  • Gastroenteritis (inflammation of the stomach and small intestine).
  • Hepatitis (inflammation of the liver).
  • Colitis (inflammation of the colon).
• Dysentery occurs when epithelial cells of the colon are damaged.
• Foodborne illness arises from infections or intoxications caused by pathogens or toxins ingested through contaminated food or water.

Microbial Diseases of the Mouth and Oral Cavity

• Dental caries (cavities), tartar, and gingivitis are caused by the overgrowth of oral bacteria, typically: $Streptococcus$ and $Actinomyces$ species due to poor dental hygiene.
• Gingivitis can progress to periodontitis as $Porphyromonas$, $Streptococcus$, and $Actinomyces$ species spread.
• Acute necrotizing ulcerative gingivitis can occur when the following are involved: $Prevotella intermedia$, $Fusobacterium$ species, and $Treponema vicentii$.
• Herpes simplex virus type 1 can cause herpetic gingivostomatitis, characterized by lesions in the mouth and throat.
• Other oral infections:
  • Oral thrush: A fungal infection caused by an overgrowth of $Candida$ yeast.
  • Mumps: A viral infection of the salivary glands caused by the mumps virus, a paramyxovirus.

Bacterial Infections of the Gastrointestinal Tract

• Major bacterial causes of GI illness include: $Salmonella$ spp., $Staphylococcus$ spp., $Helicobacter pylori$, $Clostridium perfringens$, $Clostridium difficile$, $Bacillus cereus$, and $Yersinia$ bacteria.
• $C. difficile$ is a significant cause of hospital-acquired infections.
• $Vibrio cholerae$ causes cholera, a severe diarrheal illness.
• $H. pylori$ is associated with peptic ulcers.
• $Salmonella enterica$ serotypes can cause typhoid fever, which is more severe than salmonellosis.
• Rehydration and supportive therapies are common general treatments for GI illnesses.
• Careful antibiotic use is essential to:
  • Minimize the risk of $C. difficile$ infections.
  • Treat antibiotic-resistant infections effectively.

Viral Infections of the Gastrointestinal Tract

• Common viral causes of gastroenteritis are: rotaviruses, noroviruses, and astroviruses.
• Hepatitis can be caused by hepatitis viruses A, B, C, D, and E.
• Hepatitis viruses vary in:
  • Transmission modes.
  • Treatment approaches.
  • Potential for causing chronic infections.

Protozoan Infections of the Gastrointestinal Tract

• Protozoan infections of the intestine include: giardiasis, cryptosporidiosis, amoebiasis, and cyclosporiasis.
• Transmission typically occurs through contaminated food and water.
• Treatment is specific to the causative agent, necessitating accurate diagnosis.
• Diagnosis often involves microscopic examination of stool or biopsy specimens, along with other diagnostic methods.

Helminthic Infections of the Gastrointestinal Tract

• Helminths (parasitic worms) commonly cause intestinal infections transmitted to humans through contaminated soil, water, or food.
• Symptoms are often mild, but severe complications can occur.
• $Ascaris lumbricoides$:
  • Eggs are transmitted via contaminated food or water.
  • Larvae hatch in the intestine, travel to the lungs, then to the pharynx, are swallowed, and return to the intestines to mature.
  • Causes ascariasis.
• $Necator americanus$ and $Ancylostoma doudenale$:
  • Cause hookworm infection.
  • Larvae penetrate the skin from soil contaminated by dog or cat feces.
  • They travel to the lungs and are then swallowed to mature in the intestines.
• $Strongyloides stercoralis$:
  • Transmitted from soil through the skin to the lungs and then to the intestine.
  • Causes strongyloidiasis.
• $Enterobius vermicularis$:
  • Nematode pinworms transmitted by the fecal-oral route.
  • After ingestion, they travel to the colon and cause enterobiasis.
• $Trichuris trichiura$:
  • Transmitted through soil or fecal contamination.
  • After ingestion, eggs travel to the intestine where larvae emerge and mature, attaching to the walls of the colon and cecum.
  • Causes trichuriasis.
• $Trichinella$ spp.:
  • Transmitted through undercooked meat.
  • Larvae emerge from cysts in the meat and mature in the large intestine.
  • They can migrate to the muscles and form new cysts, causing trichinosis.
• $Taenia$ spp. and $Diphyllobothrium latum$:
  • Tapeworms transmitted through undercooked food or the fecal-oral route.
  • $Taenia$ infections cause taeniasis.
  • Tapeworms use their scolex to attach to the intestinal wall.
  • Larvae may also move to muscle or brain tissue.
• $Echinococcus granulosus$:
  • Cestode transmitted through eggs in the feces of infected animals, especially dogs.
  • After ingestion, eggs hatch in the small intestine, and the larvae invade the intestinal wall.
  • They travel through the circulatory system to form dangerous cysts in internal organs, causing hydatid disease.
• Flukes:
  • Transmitted through aquatic plants or fish.
  • Liver flukes interfere with the bile duct.
  • Intestinal flukes develop in the intestines, where they attach to the intestinal epithelium.