Gastrointestinal Hepatobiliary Viewed in its simplest form, the gastrointestinal tract is a hollow digestive tube that extends through the center of the body from the mouth to the anus. The walls of the tube are lined with a diverse number of epithelial cells that function well at transmembrane secretion and absorption and maintain the barrier that protects the host from microbial pathogens. The barrier consists of the intact mucosal surface and a population of resident immune cells. The primary function of the gastrointestinal system is digestion and nutrient uptake.
The epithelial cells have a relatively short life, with most cells living between 48 and 72 hours. Because of the constant turnover of cells, it is difficult for pathogens to colonize the gastrointestinal tract. However, the disadvantage of the high rate of cell turnover is that the epithelial cells are more susceptible to mutagenic compounds and tumor formation.
All of the liquid and solid material ingested, along with bacteria, is carried through the tube. Bacteria colonize the areas of the tube that offer a suitable environment for growth. Soon after birth, a microbiota is established in each part of the tube. The oral cavity and the colon are at opposite ends of the tube and are heavily colonized with bacteria. The central part of the tube, the stomach, duodenum, jejunum, and the proximal half of the ileum, are lightly colonized.
Each portion of the gastrointestinal tract has special defense mechanisms that protect it from pathogenic microorganisms. When pathogenic microorganisms or their toxins breach these defense mechanisms, disease can occur.
Defense Mechanisms of the Gastrointestinal Tract
- An unbroken mucosal epithelium lines all parts of the gastrointestinal system. The epithelial cells are continually sloughed off and replaced. If cell replacement is impeded (e.g., radiation therapy or cancer chemotherapy), ulceration of the mucosa can occur. The ulcer is no longer lined with epithelial cells and the surface of the ulcer can be infected. The infection can damage the blood vessels in the wall of the gastrointestinal tract, causing septicemia and fever.
- The glycocalyx is a glycoprotein and polysaccharide layer that covers the surface of the epithelial cells. This layer presents a thick physical barrier that prevents pathogens from attaching to the epithelial cells and serves as a chemical trap that binds microorganisms of the normal flora.
- The normal pH of the stomach is < 4 (i.e., acidic). This acidity spills into the small intestine and establishes a pH gradient that prevents most bacteria from colonizing the stomach, duodenum, jejunum, and the upper half of the ileum. Therefore, most ingested pathogens never reach the intestinal tract alive.
- Bile solubilizes lipids and inactivates organisms that have a lipid envelope. Most enveloped viruses are inactivated, and many bacteria are unable to grow at a high bile salt concentration.
- Peristalsis contributes to the health of the gut by aiding in fluid absorption, maintaining appropriate dilution of indigenous enteric microflora, and ridding the host of pathogenic microorganism.
- Peyer patches are unencapsulated patches of lymph follicles in the mucosa and submucosa and provide a homing site for lymphocytes. M cells lining the intestine process antigens and present antigens to the lymphocytes in the Peyer patches. The intestinal mucosa is in a constant state of “physiologic inflammation.” The lamina propria is a thin layer of connective tissue that lies just below the intestinal epithelium. It contains capillaries and a central lacteal (lymph vessel) as well as numerous neutrophils, macrophages, plasma cells, and lymphocytes. After invasive infections, a vigorous inflammatory reaction ensues, resulting in many white blood cells entering the lumen of the intestine from the lamina propria.
- Most of the microbiota of the gastrointestinal tract is composed of anaerobic bacteria (over 99%). Many bacterial species have not been fully characterized because they have yet to be cultured. The microbiota competes with pathogens for nutrients and epithelial cell receptor sites and keeps them from causing disease. About 60% of the dry mass of the feces is bacterial. Most bacteria in the microbiota of the colon are part of the following phyla Firmicutes, Bacteroidetes, Actinobacteria and Proteobacteria. Most bacterial belong to the following genera Bacteroides (30% of all the bacteria in the colon), Clostridium, Faecalibacterium, Eubacterium, Ruminococcus, Peptococcus, Peptostreptococcus and Bifidobacterium. Other genera, such as Escherichia and Lactobacillus, are less numerous.