Gastro-Intestinal Infections and the Microbiome Overview

Gastro-Intestinal (GI) Infections

GI infections can be caused by various factors including psychogenic (related to psychological issues) and neurogenic (related to nerves) causes, such as vagal hypermotility, which can lead to generalized hyperactivity of the alimentary tract. Surgical interventions like vagotomy (cutting the vagus nerve) and gastrectomy (removal of the stomach) can significantly affect GI function. Various endocrine disorders, such as hyperthyroidism and adrenal cortical insufficiency, as well as secreting tumors like carcinoid tumors, can also precipitate GI symptoms.

Types of GI Infections

GI infections can be categorized into:

• Inflammatory: Diseases such as regional enteritis and ulcerative colitis fall under this category.
• Mechanical: These infections can be caused by fecal impactions, foreign bodies, neoplasms, intussusception (a condition where a part of the intestine telescopes into itself), extraluminal compression, and angulation.
• Chemical: This includes ingestion of poisons or cathartics that lead to irritation.
• Bacterial: Influential pathogens include Salmonella, Shigella, Staphylococcus, Streptococcus, E. coli, and Clostridia.
• Parasitic: Pathogens like Entamoeba histolytica (amoebiasis), Trichinella (trichinosis), and Ascaris (ascariasis) fall into this category.
• Food Sensitivity: This encompasses drug-induced or dietary irritations, which can lead to conditions like food intolerance, allergic reactions, and intestinal diseases such as celiac disease and Whipple's disease.

GI Overview and Microbiome

The GI system includes a variety of organs involved in digestion, the absorption of nutrients, and the elimination of waste. Key components include:

• Pharynx
• Salivary Glands (parotid, submandibular, sublingual)
• Esophagus
• Liver
• Stomach
• Gallbladder
• Pancreas
• Small Intestine (divided into duodenum, jejunum, distal ileum)
• Large Intestine (which includes the colon, cecum, appendix, rectum, and anus)

Microbiome

The human gut harbors a complex ecosystem of bacteria, with a predominance of anaerobic species. More than 400 species have been identified in human feces, with varying densities across different sections of the GI tract:

• Stomach and Upper Gut: Sparse flora with less than 10^4 organisms/ml.
• Large Intestine: Contains 10^11 bacteria/g of stool, dominated by Bacteroides, Clostridia, and other anaerobes.

The GI Journey - Absorptive Features

The small intestine possesses several features that facilitate a large absorptive surface area:

• Mucosal Folds: Increases surface area and regulates flow.
• Villi: Small projections that absorb nutrients.
• Microvilli: Tiny hair-like structures on the villi for increased absorption capacity.
  On average, the small intestine measures about 7 meters long, despite being narrower than the large intestine, which is about 1.5 meters long. The primary function of the large intestine is to absorb water and salts from unabsorbed food residue, turning it into fecal matter.

Microbial Communities and Host Defenses

The gut microbiome plays a crucial role in maintaining health by:

• Suppression of pathogenic microorganisms through the production of antimicrobial substances such as bacteriocins and short-chain fatty acids.
• Immunity: Host defenses include physical barriers, immune responses (like phagocytosis), and antibody production (IgA).
• Maintaining a balance in the gut microbiota is vital, as disturbances (e.g., through antibiotic use) can lead to overgrowth of harmful bacteria, such as Clostridium difficile, causing severe colitis.

Gastrointestinal Diseases and Infections

Gastrointestinal infections lead to significant morbidity and mortality worldwide, particularly in children within developing countries. The infectious dose of various pathogens varies, affecting susceptibility and disease severity:

• Shigella: MID of 10^1-2
• Campylobacter jejuni: MID of 10^2-6
• Salmonella: MID of 10^5
• Escherichia coli: MID of 10^8
• Vibrio cholerae: MID of 10^8
• Giardia lamblia: MID of 10^1-2
• Entamoeba histolytica: MID of 10^1-2

Viral Gastrointestinal Diseases

Most viral infections like those caused by noroviruses and rotavirus tend to be asymptomatic, but can present with watery diarrhea and, in severe cases, especially in younger patients, lead to significant dehydration.

Takeaway Points

• The gut microbiome's health is essential for not only digestion but also immune function and overall well-being.
• Antibiotic use and dietary changes can significantly alter the microbiome, emphasizing the importance of maintaining a balanced gut flora.
• Deep understanding of the GI system and its microbiome is vital for tackling various infections, potential diseases, and achieving successful interventions.