Bacterial and Viral Virulence Factors

Definitions of Key Terms

  • Virulence factor:

    • A molecule produced by a microorganism that enables it to colonize, infect, invade host cells, and evade the immune response, ultimately leading to disease.

  • Bacterial capsule:

    • A polysaccharide and protein layer that surrounds the outer membrane of some bacterial cells.
    • Function in evading phagocytosis:
    • Masks the antigens on the bacterial surface to prevent detection by phagocytes.

  • Antigenic variation:

    • The alteration of surface proteins by pathogens, permitting them to avoid recognition by memory T and B cells, thus facilitating immune evasion and persistence of infection.

  • Viral adhesin:

    • A protein or glycoprotein that binds to specific cellular receptors on host cells, determining the tropism (host range and specific cell types the virus can infect).

  • Tropism:

    • Refers to the specific types of cells or tissues that a virus can infect, based on the presence of cell-specific receptors and viral adhesins.

Mechanisms of Immune Evasion

Antigenic Variation in Influenza Viruses

  • Antigenic drift:

    • Involves small mutations in hemagglutinin (HA) or neuraminidase (NA) proteins that allow the virus to partially evade established immune responses.

  • Antigenic shift:

    • Occurs when multiple viruses infect the same cell and exchange genetic material, or when a virus infects a new host species.
    • Results in new viral subtypes with different surface proteins; thus, existing immune responses are ineffective against these variants.

Virulence Factors of Pathogens

Streptococcus pneumoniae

  • Encapsulated strains vs. Non-encapsulated strains:
    • Encapsulated strains are more virulent due to the capsule's role as a physical and chemical barrier against the host immune response.
    • The capsule can hinder phagocyte binding, enabling better colonization and invasion of tissues, whereas non-encapsulated strains are more easily destroyed by the immune system.

Bacterial Proteases

  • Role in resisting immune defenses:
    • Proteases degrade proteins, including antibodies, cytokines, and antimicrobial peptides, thereby subverting immune responses.

Coagulase (Staphylococcus aureus)

  • Function in infection:
    • An enzyme that converts fibrinogen to fibrin, promoting clot formation around bacteria.
    • Provides protection from the immune system and facilitates chronic infection by preventing immune detection.

Kinases: Streptokinase and Staphylokinase

  • Contribution to bacterial spread:
    • These enzymes degrade clots, liberating bacteria to spread and invade deeper tissues by breaking down connective tissue.

Sequential Expression of Enzymes in Infection

  • Timing of coagulase and kinase expression:
    • Early expression of coagulase protects the bacteria by activating the host's blood clotting mechanism.
    • Later expression of kinase allows bacteria to escape from the clot and disseminate to surrounding cells and tissues.

Intracellular Survival Mechanisms

  • Bacteria surviving inside macrophages post-phagocytosis:
    • The virulence factor responsible is phospholipases that degrade phagosomal membranes, permitting pathogen escape and replication.

Persistence Mechanisms in Neisseria gonorrhoeae

  • Reason for repeated infections despite immune development:
    • The mechanism involved is antigenic variation, allowing the pathogen to evade recognition by the host's immune system.

Impact of Antigenic Variation on Immunity

Influenza Virus Shift Dynamics

  • Mechanism for immunity evasion:

    • Antigenic shift occurs via genetic exchange between two influenza viruses, resulting in new antigenic variants that are not recognized by existing immune responses.

  • Impact on vaccination:

    • Recurrent antigenic drift, involving point mutations, leads to gradual changes spreading in circulating virus strains, undermining the efficacy of prior vaccinations.

Specific Examples of Virulence Factors

Staphylococcus aureus Enzymes

  • Initial and subsequent functions:
    • Coagulase: Promotes blood clotting, creating a protective fibrin clot around the bacterium.
    • Staphylokinase: After establishment, it digests the clot allowing bacterial spread to nearby cells.

Borrelia burgdorferi and Antigenic Variation

  • Role of surface proteins (OspA and OspC):

    • OspA: Mediates attachment to the tick midgut facilitates persistence.
    • OspC: Upregulated when the tick feeds, crucial for survival and establishing infection in mammals.

  • VlsE antigenic variation system:

    • Alters amino acid sequences of surface-exposed epitopes, generating antigenic diversity allowing for long-term immune evasion.

  • Adaptive immunity response:

    • Although effective against dominant antigens initially, repeated alterations mean the immune response no longer matches, leading to persistent infection despite ongoing T- and B-cell activity.

Recap of Immune Evasion Strategies by Pathogens

  • Diverse virulence factors enable both bacteria and viruses to escape immune detection, prolong infections, and complicate treatment strategies. Understanding these methods is crucial for developing therapeutic interventions.