Mucosal LO3

Commensal Bacteria and Microbiota

  • Definition of Commensal Microbes / Microbiota:

    • Microbiota refers to the microbes living in and on our bodies.

    • Interdependence of intestinal microbiota and the host is highly significant and manifests in various ways.

  • Effect of Microbiota on Health:

    • Influences nutrient uptake and drug metabolism.

    • Alters risk of cancer and impacts cognitive function.

    • Studies show microbiota transplant from healthy to unhealthy individuals can treat diseases such as Crohn's disease.

    • Highlights correlation between microbiota and various body systems.

Bodily Systems and Commensal Bacteria

  • Importance of Gut Microbes:

    • Essential for vitamin synthesis and dietary fiber digestion.

    • Maintains gut epithelial integrity and stimulates immune responses that produce mucus and tight junctions.

    • Regulation of inflammatory responses.

  • Variety of Microbes:

    • A diverse range of microbes exist in and on our bodies, varying by anatomical site (e.g., skin vs. gut).

    • Predominantly bacterial species, but also includes other types of microbes.

  • Predominant Gut Bacterial Species:

    • Vermiculitis and Bacteroidetes are identified as the most predominant in the gut.

Factors Affecting Microbiota Composition

  • Birth Method:

    • Vaginal delivery vs. cesarean section affects microbiota composition.

    • Studies indicate inoculation of cesarean-delivered babies with vaginal secretions to favorably change microbiota.

  • Age and Microbiota Diversity:

    • Aging influences types and diversity of gut microbes.

  • Genetics:

    • Genetic predisposition can affect microbiota composition.

  • Geography and Environment:

    • Living conditions (rural vs. urban) and environmental exposure influence microbiota.

  • Antibiotic Use:

    • Antibiotics target not just pathogenic bacteria but also beneficial commensals, causing an imbalance.

  • Diet:

    • Diet plays a crucial role in shaping gut microbiota through the nutrients available to microbes.

    • Interrelation between age, diet, and microbiota diversity.

    • High fiber diet supports beneficial microbes; low fiber diet may promote inflammatory microbiota.

Immune Modulation by Commensal Microbes

  • Stimulation of Immune Responses:

    • Commensal microbes stimulate Paneth cells to secrete antimicrobial substances.

    • Goblet cells produce mucin glycoproteins, contributing to mucus formation, which creates an anti-inflammatory environment.

    • Cellular Mechanism:

      • Macrophages and dendritic cells sample commensal antigens, presenting them to lymphocytes in mesenteric lymph nodes.

  • T Cell Differentiation:

    • Dendritic cells equipped with TGF-beta and retinoic acid promote differentiation into T regulatory cells (Tregs) and Th17 cells.

    • Dendritic cells influence T cell homing back to the gut.

  • Short Chain Fatty Acids (SCFAs):

    • Produced by gut bacteria from dietary fiber and serve as energy sources for colonic epithelium.

    • SCFAs promote Treg cell proliferation and enhance immune responses against pathogens.

    • SCFAs also function as ligands for GPR43, influencing further immune modulation.

NF-kappa B Pathway Regulation

  • Significance of NF-kappa B:

    • NF-kappa B is a crucial transcription factor within immune responses.

    • Regulates genes associated with both innate and adaptive immunity.

  • Pathway Details:

    • Mechanism: When pathogens bind to Toll-like receptors, they induce the production of IRAK1, which degrades I kappa B, releasing NF-kappa B to enter the nucleus and promote inflammatory cytokine transcription.

    • Commensals can modulate this pathway in various ways to prevent excessive inflammation.

Dietary Impact on Microbiota and Immune System

  • Good vs. Bad Diets:

    • High-fiber, healthy diets foster anti-inflammatory microbiota.

    • Western diets (high fat, low fiber) promote inflammation and dysbiosis, resulting in negative health outcomes.

  • Concluding Concepts:

    • The interaction between commensal bacteria and the immune system is significant for the body’s health and disease prevention.

    • Maintaining a diverse and well-functioning microbiota is crucial.

    • Mucosal immunity is characterized by a tolerance to non-harmful antigens while responding appropriately to pathogens.

    • Protective epithelial barriers play a vital role in establishing an anti-inflammatory environment.

    • T cell and B cell responses are adapted to promote tolerance and neutralization of antigens, respectively.

    • Understanding these relationships helps elucidate the role of commensal microbes in health and disease management.