In-depth Notes on Human Microbiota and Disease

Bacterial Microbiota in Humans

  • Common Bacteria Found in Humans
    • Streptococcus viridans
    • Neisseria sicca
    • Candida albicans
    • Streptococcus salivarius
    • Helicobacter pylori
    • Bacteroides fragilis
    • Escherichia coli
    • Lactobacillus casei
    • Lactobacillus gasseri
    • Staphylococcus epidermidis
    • Others include variants of Bacteroides and Lactobacillus.

Host/Microbe Relationships

  • Types of Interactions
    • Symbiosis: Generally beneficial interaction
    • Mutualism: Both host and microbe benefit
    • Commensalism: Microbe benefits, host not significantly affected
    • Parasitism: Microbe harms host
  • Majority of bacteria associated with humans are either commensals or mutuals.

Microbiota and Microbiome

  • Microbiota:

    • Collection of microbial symbionts found in/on humans and other organisms
    • Contains bacteria, archaea, protozoa, fungi, and viruses
    • Represents approximately 90% of human cells; gut microbiota weight comparable to human brain
    • Essential for metabolic, immunologic, and hormonal equilibrium.

  • Microbiome:

    • The combined genomes of microbes in a particular environment
    • 99% of genes necessary for homeostasis are derived from microbiome.

Gut-Brain Axis

  • Communication Pathways
    • Involves neurotransmitters like serotonin, dopamine
    • Vagus nerve stimulation impacts gut and brain interactions
    • Includes effects from gut microbial metabolites like short-chain fatty acids (e.g., butyrate, which affects gut barrier integrity).

Eubiosis vs. Dysbiosis

  • Eubiosis:

    • A balanced microbial community supporting health
    • Characterized by an intact mucosal barrier and healthy gut microbiota

  • Dysbiosis:

    • Microbial imbalance causing potential harm and inflammation
    • Linked to several health issues including metabolic disorders and inflammation.

Infections and Diseases

  • Host Response to Microbes
    • Pathogenicity: Ability of a microbe to cause disease
    • Virulence: Degree of pathogenicity (measured by ID50 and LD50)
    • ID50: infection dose for 50% of subjects
    • LD50: lethal dose for 50% of subjects
  • Pathogen types include:
    • Primary Pathogens: Cause diseases through rapid reproduction and immune evasion
    • Opportunistic Pathogens: Cause disease in individuals with weakened defenses.

Mechanisms of Pathogenicity

  • Common Virulence Factors
    • Pili: Structures aiding attachment and movement (Type I and Type IV pili)
    • Exoenzymes: Degrade host tissues facilitating infection (hyaluronidase, collagenase)
    • Toxins:
    • Endotoxins: Components of Gram-negative bacteria cell walls (lipid A)
    • Exotoxins: Secreted toxins that damage host cells directly (e.g., cholera toxin, diphtheria toxin).

Conclusion and Applications

  • Understanding the human microbiome is vital for enhancing health outcomes and developing therapeutic strategies.
  • Ongoing research focuses on how microbiomes relate to various diseases, including metabolic disorders and autoimmune diseases, utilizing advanced genomic techniques to identify patterns and correlations.