Microbiology Exotoxins and Endotoxins Quiz

Pathogenicity of Bacteria

• Pathogenicity is the ability of a microorganism to cause disease
• Virulence is the degree of pathogenicity, quantified by the number of organisms needed to cause disease in 50% of those exposed (ID₅₀) or kill 50% of those exposed (LD₅₀)
• Bacterial pathogenesis involves bacterial exposure, colonization, evasion of the immune response, and causing infection.
• Bacteria gain entry to the host via various routes, including inhalation, oral, urogenital, and skin penetration.
• Bacterial adherence is important for colonization and is facilitated by pili, fimbriae, and surface adhesion molecules like fibronectin-binding proteins and hydrophobic cell walls

Bacterial Pathogenesis Summary

• Bacterial pathogenesis involves exposure, colonization, immune evasion and the negative effects it creates on the host resulting from infection.
• Exposure to bacteria happens from air, food, infected people and environments
• Bacteria colonize inside the host's organs, tissues and cells
• Bacteria evade the immune system through diverse strategies
• Bacteria harm the host by causing the infection

Virulence Factors

• A bacterial product or strategy that contributes to virulence (pathogenicity)
• Different bacteria have different virulence factors
• Virulence factors are categorized into those that promote colonization and invasion, promote immune evasion, damage the host (toxicity)

Colonization and Invasion

• Bacterial adherence is critical for preventing removal by mucus and fluids
• Adherence leads to attachment formation of a microcolony
• The mechanisms of bacterial adherence involve pili and fimbriae.  Pili are longer, involved in attachment and genetic exchange. Fimbriae are shorter, more numerous, facilitate colonization of host tissues.
• Surface adhesion molecules on bacterial cells attach to receptors on host cells (e.g., fibronectin binding proteins of Staphylococcus aureus
• Hydrophobic cell walls can adhere to host cell membranes, resistant to wash away by fluids

Invasiveness

• Invasive bacteria have the ability to invade host cells, penetrate mucosal surfaces, and spread from the initial infection site.
• Bacterial enzymes that facilitate invasiveness include collagenase, which degrades collagen, and hyaluronidase that break down hyaluronic acid.

Immune Evasion

• Polysaccharide capsules in some bacteria prevent phagocytosis, provide protection from acidic and proteolytic enzymes.
• Protein A present in Staphylococcus aureus cells, binds to IgG antibodies inhibiting opsonization/phagocytosis.
• Antigenic switching, is the ability of pathogens to change their proteins or polysaccharides that are on their surface. Bacteria can survive by avoiding detection by the host's immune system.
• Phase variation is a genetically reversible process to turn genes for surface antigens on/off
• Antigenic variation involves genetic recombination. This leads to permanent antigenic variations in surface structures.

Host Tissue Damage (Toxins)

• Exotoxins are secreted proteins by both gram-positive and gram-negative bacteria
• Exotoxins are some of the most poisonous substances,
• Some have two components (an A and B subunit, e.g., Tetanus toxin). The B subunit binds to the host cell, while the A subunit has the toxic effect.
• Exotoxins are inactivated by heating at 60^oC.
• Endotoxins are cell wall components of gram-negative bacteria. They are released by bacterial cell lysis
• Endotoxins trigger fever, shock, and thrombosis. The toxic effect of endotoxin is due to Lipid A.

Exotoxins vs. Endotoxins

• Exotoxins are secreted bacterial proteins, while endotoxins are part of the cell wall of gram-negative bacteria.