Bacterial Pathogenesis Overview

Bacterial Pathogenesis

• Bacterial Pathogenesis is the process of how bacteria cause disease.

Molecular Koch's Postulates

• The gene or its products should be found in only in strains that causes disease.
• Gene should be isolated by cloning.
• Disruption of gene should should reduce or attenuate its virulence.
• Gene is expressed by bacterium at some point during infection.
• The fifth Postulate: Elimination of a disease causing microbe or prevention should eliminate or prevent disease.

Virulence

• Virulence is a measure of the how pathogenic a bacterium is; pathogenicity refers to the capacity of a microbe to produce disease.
• Virulence factors (or determinants): Gene products that allow a microorganism to establish itself within or on a host and enhance the potential to cause disease.

Measuring Virulence: Animal Models

• Ethical concerns are relevant when choosing animal models to test bacterial virulence.
• Rodent models are widely used, but there are benefits and disadvantages to using each.
• Knockout and knock-in mice: Mice with specific genes deleted or introduced, respectively.
• LD50 refers to the 50% lethal dose, the dose where 50% of animals die.
• ID50 refers to the 50% infectious dose, the number of bacteria needed to infect 50% of animals.

A Simple Animal Model

• An animal model is used to determine bacterial virulence.
• In a simple model, bacteria are inoculated into an animal, then the infected animal is sacrificed and the number of bacteria are measured.
• Competition assays can make LD50 and ID50 experiments more sensitive by seeing how mutant strains of bacteria compare to wild-type bacteria.

To Cause Disease, Microorganism must:

• Find and enter the host
• Colonize the host
• Resist host defenses
• Cause damage to, or malfunction of host tissue

Portals of Entry

• Ingestion - Fecal-oral route, contaminated food and water.
• Respiratory tract - Aerosols, contaminated hands
• Wounds - Scratches, bites
• STD - Sexual activity
• Medical devices - Catheters, contact lenses
• Normal flora movement or altered growth

Adhesion and Colonization

• Bacteria require mechanisms to attach to host cells and colonize a specific location.
• Pili are hair-like structures that play a role in adhesion.
• Flagella are used for motility.
• Capsule prevents the host immune system from recognizing the bacteria.

Resisting Host Defenses

• Bacteria have a repertoire of strategies to counter host defense mechanisms.
• Loss of bacterial antigen
• Overproduction of bacterial antigen binds up host antibodies, such as in the case of superantigens
• Capsules bind to serum proteins and inhibit complement activation.
• Altered bacterial antigen (antigenic variation) allows some bacteria to evade the host immune system.
• Molecular mimicry is when bacterial antigens mimic host molecules, effectively camouflaging the bacterium.

Immunoglobulin Resistance Strategies

• Protein A of S. aureus and Protein G of S. pyogenes bind to the Fc region of antibodies (immunoglobulins), preventing opsonization.
• Ig protease degrades immunoglobulins.
• Bind host materials

Escape from Phagocytosis

• Listeria monocytogenes: Secreted listeriolysin O (LLO) creates pores in phagosomal membranes, allowing the bacteria to escape into the cytoplasm, then multiply.

Penetration Strategies

• Salmonella: Delivers effector proteins into host cells to protect vacuoles and evade host immune response.

Other Strategies

• Blood clots: Bacteria can degrade clots with fibrinolysin.
• Mucus: Bacteria can produce enzymes to degrade mucus.
• Normal flora competition - Bacteria compete with other organisms and disrupt signals.
• Host signals - Some bacteria can disrupt host signals.
• Siderophores - Bacteria can acquire iron using siderophores.
• Adverse pH, low nutrients: Some bacteria have evolved the ability to survive in these environments.

Toxins

• Type I toxins bind to the cell surface and are not translocated into the cell.
• Type II toxins act on the cell membranes, such as phospholipases and pore-forming cytotoxins
• Type III toxins are typically A-B toxins with two functional components: an enzymatic domain (A) which inactivates intracellular targets and a binding component (B) which recognizes a surface receptor.

Endotoxin

• Lipopolysaccharide (LPS) is a toxin found in the outer membrane of Gram-negative bacteria.

Superantigens

• Superantigens (SAg) are Type I toxins from bacteria like Staphylococcus aureus, that bind to and stimulate a large number of T cells, leading to overproduction of cytokines and immune dysregulation.

Membrane Disruption (Type II Toxins )

• Alpha toxin from Staphylococcus aureus forms pores in cell membranes.
• Phospholipase C and hemolysins can remove polar head groups from phospholipids, disrupting the membrane.

Type III (A-B toxins)

• A-B toxins are typically synthesized as single polypeptide chains.
• The 'A' and 'B' domains separate by proteolysis.

Toxin Export

• Toxins are often produced inside the bacterial cell but act on the outside.
• Secretion systems are used to export toxins out of the cell:
  • General secretory system (sec)
  • Types I to VII secretion systems
  • Cytolysin mediated translocation (CMT)
  • ExPortal
  • T7SS

General Secretory System (Sec)

• Found in Gram-positive and Gram-negative bacteria.
• Proteins synthesized with signal sequences (leader peptides) are recognized by Sec proteins which help transport the protein across the membrane.
• Once exported, the signal sequence is cleaved off.

Secretion Systems

• Sec-dependent secretion systems: T2SS and T5SS
• Sec-independent secretion systems: T1SS, T3SS, T4SS, T6SS