Microbe Human Interactions Lecture

Microbe-Human Interactions

Symbioses

  • Two organisms living in close interaction.

  • The relationship is required by one or both organisms.

  • Classification of Symbiotic Microbe-Human Interactions:

    • Beneficial to microbes.

    • Classified based on the effect on the human (host):

      • Mutualism

      • Commensalism

      • Parasitism

Mutualism

  • A symbiotic relationship where both organisms benefit.

  • Examples: Normal microbiota in humans.

    • Normal microbiota gains:

      • Access to nutrients.

      • Favorable stable environmental conditions.

    • Humans benefit through:

      • Production of digestive enzymes & vitamins.

      • Training of the immune system responses.

      • Protection against pathogens (microbial antagonism).

Microbial Antagonism

  • Relationship between normal microbiota and pathogens that's nonsymbiotic.

  • Normal microbiota harm potential pathogens by:

    • Blocking access to host cell surfaces.

    • Competing for nutrients.

    • Creating unfavorable environments through waste products or antimicrobial chemicals.

Commensalism

  • One organism benefits while the other (host) is unaffected.

  • Normal microbiota present in low numbers neither benefit nor harm the human host.

Normal Microbiota by Location

  • Abundant Locations:

    • Skin and adjacent mucous membranes.

    • Upper respiratory tract.

    • Gastrointestinal tract and mouth.

    • Urethra, external genitalia, vagina, external ear canal, external eye.

  • Low Numbers:

    • Lungs, bladder, breast milk, amniotic fluid, fetus.

  • None Detected:

    • Brain, bloodstream.

Acquiring Normal Microbiota

  • In Utero:

    • The womb is probably not sterile and has its own microbiota.

  • Birth:

    • Vaginal vs C-section deliveries influence the initial microbiome.

  • Milk:

    • Breast milk and formula contain different microbes.

  • Caregivers:

    • Family, siblings, and others share microbes with the baby.

  • Environment:

    • Babies acquire microbes from environmental contact.

Parasitism

  • One organism benefits at the expense of the other (human).

  • Pathogenic microbes are responsible for diseases that harm humans.

Synergism

  • A nonsymbiotic relationship between multiple organisms that is beneficial but not required.

  • Example: Microbes growing in biofilms.

  • Associated with chronic diseases like:

    • Chronic ear infections.

    • Dental caries, gingivitis, periodontal disease.

Infections

  • Occur when microorganisms successfully multiply within a host.

  • Infections do not classify as diseases if no damage is inflicted on the host.

  • Three Steps to Establish Infection: (not detailed in provided content)

Infectious Disease

  • Any deviation from a state of health caused by:

    • Microbes or their products.

    • Infections leading to direct or indirect damage to host tissue/organs.

  • Transmissibility of Infectious Diseases:

    • Communicable (e.g., cryptosporidiosis) vs. noncommunicable (e.g., histoplasmosis).

Pathogens and True Pathogens

  • Pathogen: Microorganism capable of causing damage to a host.

  • True Pathogens: Can cause disease in healthy individuals under favorable circumstances.

Opportunistic Pathogens

  • Cause disease in hosts with weakened defenses (e.g., compromised immune systems).

  • Normal microbiota can cause disease when accessing invasive tissues/areas.

Pathogenicity

  • The ability of a pathogen to cause disease.

Virulence

  • The capacity of a pathogen to cause severe disease.

Examples of Pathogens

  • Candida albicans: Low pathogenicity, moderate-high virulence.

  • Cryptosporidium spp.: High pathogenicity, low virulence.

Virulence Factors

  • Characteristics that facilitate a pathogen's ability to cause disease.

  • Pathogenicity and virulence depend on virulence factors.

Bacterial Virulence Factors

  • Adhesion Factors:

    • Attach to host cells.

    • Examples include fimbriae, capsules, and adhesion proteins.

  • Evasion of Host Defenses:

    • Mechanisms include resisting phagocytosis, antigenic variation, and intracellular life.

  • Damage to Host:

    • Direct damage via nutrients, exoenzymes, and exotoxins.

    • Indirect damage through endotoxins and superantigens.

Exoenzymes and Toxins

  • Exoenzymes:

    • Secreted enzymes that directly damage hosts (e.g., coagulase, kinase, hyaluronidase).

  • Exotoxins:

    • Secreted proteins causing direct cellular damage.

      • Cytotoxins: (e.g., Diphtheria toxin)—inhibit protein synthesis.

      • Neurotoxins: (e.g., Botulinum toxin)—cause paralysis by blocking nerve impulses.

      • Enterotoxins: (e.g., Cholera toxin)—cause severe diarrhea.

      • Membrane-Disrupting Exotoxins: Cause host cell lysis (e.g., hemolysins).

Endotoxins

  • Components of Gram-negative bacteria causing indirect damage through host immune reaction (Lipid A in LPS).

  • Indirectly causes issues such as high fever and shock.

Superantigens

  • Secreted proteins that trigger inappropriate host defenses leading to severe symptoms.

    • Can be associated with conditions like toxic shock syndrome (e.g., Staphylococcus aureus).

Viral Virulence Factors

  • For Attachment: Spikes that bind tightly to host cell receptors.

  • Evasion-Evading Host Defenses:

    • Intracellular replication and antigenic variation.

  • Damage to Host:

    • Includes lysis of host cells and resource theft.