Lecture 19-20: Pathogenesis

Bacteria – Host Interactions

  • Symbiosis: Living together.
  • Commensalism: One benefits, the other is not affected.
  • Mutualism: Both organisms benefit.
  • Parasitism: One benefits, the other is harmed.
  • Normal microbiota and hosts are usually mutually beneficial.
  • Normal microbiota often prevent colonization by pathogens.
  • Bacterial products are beneficial to the host.

Bacterial Infections

  • Steps of bacterial infections:
    • Attach to and invade host tissues.
    • Suppress host defenses.
    • Acquire nutrients from the host.
    • Reproduce in the host.
    • Transmit to a new host.

Infection and Infectious Diseases

  • Infection: A pathogen grows and multiplies within or on another organism.
  • Infection doesn't always cause disease.
  • Most infections are removed by the immune system.
  • Infectious disease: Because of the presence and multiplication of pathogens, part or all of the host can’t perform normal functions.

Host-Pathogen Interactions

  • Pathogenesis: Process by which microbes cause disease in a host.
  • Parasite: Bacteria, fungi, viruses, and protozoa that colonize and harm a host.
  • Pathogen: Agents of disease (bacterial, fungal, viral).
  • Pathogens and parasites enter and infect their animal and plant hosts in different ways.

Primary vs. Opportunistic Pathogens

  • Primary pathogens: Have the ability to penetrate host defenses; their whole life exists to be pathogens.
  • Opportunistic pathogens: Cause disease only in compromised hosts.
    • Immune system is defective.
    • A break in tissue lets the organism into a new site.
    • Loss of other microflora allows the organism to bloom.

Latent State and Pathogenicity

  • Some microbes enter a Latent State during infection.
    • The organism cannot be found by culturing.
    • Can remain dormant for years, then suddenly emerge to cause disease (e.g., Herpes virus, TB).
  • Pathogenicity: The ability of an organism to cause disease.
    • Defined in terms of infectivity and virulence.
    • Infectivity: How easily an organism causes disease.
    • Virulence: How severe the disease is.
      • Ebola virus: Incredibly virulent: fatal (70%).
      • Rhinovirus (colds): Highly infective but low virulence: not fatal.

Virulence Measurement

  • Virulence of an organism: Measure of the degree or severity of disease, determined by the genetic makeup of the organism.
    • Infectious dose = ID50: Number of organisms to colonize 50% of hosts; lower is worse.
    • Lethal dose = LD50: Number of organisms to kill 50% of hosts; lower is worse!
    • A lower LD_{50} indicates a more virulent organism.

Infection Cycle

  • For a disease to spread, a pathogen must pass from one host organism to another.
  • The route an organism takes to spread disease is known as the infection cycle.
    • Direct transmission: Direct contact (i.e., touch) or aerosolization (i.e., sneeze).
    • Indirect contact: Animal vectors (zoonotic diseases).

Infection Cycle - Transmission

  • Direct horizontal and airborne transmission
  • Indirect horizontal transmission
  • Vertical transmission
  • Arthropod vectors

The Chain of Infection Cycle

  • Susceptible Host: Elderly, infants, immunocompromised, anyone!
  • Portal of Entry: Mouth, nose, eyes, cuts in the skin.
  • Mode of Transmission: Direct contact, indirect contact, vectors.
  • Pathogen: Bacteria, virus, fungi, parasite.
  • Reservoir: People, animals, soil, food, water.
  • Portal of Exit: Coughing/sneezing, bodily secretions, feces.

Pathogen Portals of Entry

  • Pathogens use “portals of entry” best suited to their mechanism of pathogenesis.
  • Mode of entry depends on the pathogen.
    • Food-borne pathogens:
      • Ingested through the mouth.
      • Survive the stomach.
      • Colonize the intestine.
    • Airborne organisms:
      • Infect through the respiratory tract.
    • Mucosal surfaces
    • Wounds (skin)
    • Parenteral route: Injected into the organism bloodstream (i.e., insect or needle).

Virulence Factors

  • Virulence factors: Individual characteristics (encoded by genes) of pathogens that allow pathogens to invade hosts and cause disease.
  • All virulence factors enhance the disease-producing ability of the pathogen.
    • Facilitate bacterial attachment and invasion.
    • Promote bacterial growth.
    • Promote disease symptoms.
    • Evade host defense mechanisms.
  • Virulence factors don’t have to make the pathogen more lethal; they just help the pathogen cause disease.

Virulence Genes and Pathogenicity Islands

  • Virulence genes: Encode factors allowing a pathogen to invade a host (i.e., virulence factors).
  • Pathogenicity islands:
    • Section of the genome containing multiple virulence genes.
    • Often encode related functions (e.g., protein secretion system, toxin production).
    • Transferred as a block from other organisms (horizontal gene transfer).
    • Often flanked by phage or plasmid genes.
    • Often have a GC content different from the rest of the genome.

Microbial Attachment

  • Once bacteria are in the host, our bodies try to expel the invaders.
    • Mucosa, dead skin constantly expelled.
    • Urine expelled from the bladder.
    • Coughing, cilia in lungs.
    • Expulsion of intestinal contents.
  • Bacteria must adhere to host tissue if they want to colonize.
    • Pili (fimbriae): Hollow fibrils with tips to bind host cells.
    • Adhesins: Surface proteins that bind host cells.
    • Any microbial factor that helps attachment.

Pili and Disease

  • Different pili from different bacterial species have been classified based on phenotypes.
    • Type I Pili: Adhere to mannose residues on host cell surfaces (static).
    • Type IV Pili: Assemble on the cell surface, dynamic (continuously assemble and disassemble).

Type I Pili Assembly

  • Pili subunits are made in the cytoplasm, then secreted to the periplasm.
  • PapD chaperones the subunits to PapC.
  • PapC forms the pore for the pili units to assemble.
  • All the other proteins assemble in order: PapG – PapF – PapE – PapA, then the cap (PapH) is added.

Type IV Pili

  • PilA (Pilin) is made as a preprotein and inserted into the inner membrane.
  • PilD is a peptidase that removes a leader sequence from PilA preproteins before pilus assembly.
  • PilT and PilF are NTP-binding proteins that provide energy for retraction and assembly.
  • The secretin PilQ is required for the type IV pilus to cross the outer membrane.

Type IV Pili and Motility

  • Involved in gliding or twitching motility.
  • “Spider-Man motility”.

Non-Pili Adhesins

  • Other bacteria use non-pilus adhesins that mediate binding to host tissues:
    • Streptococcus pyogenes (causes Strep Throat): M protein.
    • Bordetella pertussis (causes Whooping Cough): Pertactin.

Biofilms and Infection

  • Bacteria can attach to surfaces, forming a biofilm.
  • Biofilms play important roles in chronic infections (cystic fibrosis, dental plaque, chronic otitis media, osteomyelitis, chronic wound infections).
  • Biofilms are often resistant to antibiotics.

Bacterial Toxins

  • Bacteria produce toxins to help them invade the host, damage host cells, or evade the immune system.
    • Exotoxins: Kill host cells, which releases nutrients; many microbes secrete exotoxins after attachment.
    • Endotoxins: Non-protein toxic compounds; hyper-activate host immune systems to harmful levels; present in gram-negative bacteria; part of lipopolysaccharide: released as a gram-negative cell breaks down.

Exotoxins vs Endotoxins

  • Exotoxin: Bacterium secrete exotoxins, cytotoxin kills host's cells
  • Endotoxin: Dead Gram-negative bacteria release endotoxin (lipid A) which induces effects such as fever, inflammation, diarrhea, shock, and blood coagulation.

Toxins Subvert Host Function

  • Five categories of protein exotoxins:
    1. Cell membrane disruption: Cause host cell membrane leakage.
    2. Block protein synthesis: Target eukaryotic ribosomes.
    3. Block 2nd messenger pathways.
    4. Superantigens overactivate the immune system.
    5. Proteases cleave host proteins.

Categories of Microbial Exotoxins

  • Various exotoxins, their organisms, modes of action, host targets, and associated diseases.
  • Examples include:
    • LT (E. coli): ADP-ribosyltransferase, targets G proteins, causes diarrhea.
    • Cholera toxin (Vibrio cholerae): ADP-ribosyltransferase, targets G proteins, causes cholera.
    • Tetanus toxin (Clostridium tetani): Zinc metalloprotease, targets VAMP/synaptobrevin, causes tetanus.
    • Damage Membranes - Perfringolysin O (Clostridium perfringens): Pore former, targets cholesterol, causes Gas gangrene.
    • Inhibit Protein Synthesis - Diphtheria toxin (Corynebacterium diphtheriae ): ADP-ribosyltransferase, targets Elongation factor 2, causes Diphtheria.

Categories of Microbial Exotoxins - Mechanisms

  • Damage cellular membranes/matrices
  • Inhibit protein synthesis
  • Activate second messenger pathways

Alpha Toxin

  • Category 1 of toxins.
  • The hemolytic alpha toxin is produced by Staphylococcus aureus.
  • Forms a transmembrane, seven-member pore in target cell membranes.
  • Disrupts cells (red blood cells) by forming pores in the membranes.

Shiga Toxin

  • Category 2 exotoxin.
  • Produced by Shigella flexneri and E. coli O157:H7.
  • Disrupts protein synthesis by destroying 28S rRNA found in Eukaryotic ribosomes.

AB Toxins

  • 2-component toxin:
    • B subunit: Binds to host cell and delivers A subunit to the cytoplasm.
    • A subunit: Has toxic activity (e.g., ADP-ribosyltransferase).
    • Examples: Diphtheria and Cholera toxins.

Cholera Toxin Mechanism

  1. The 5B:1A toxin complex binds the ganglioside GM1 on host membrane lipid rafts.
  2. The toxin is endocytosed.
  3. The phagosome containing CT is taken to the endoplasmic reticulum.
  4. The A1 subunit is removed from the B subunits and exported into the cytoplasm.
  5. The A1 peptide attaches an ADP ribose to an amino acid within the host G protein that regulates adenylate cyclase.
  6. Cyclic AMP levels rise and activate ion transport systems, causing an electrolyte imbalance; water from the cell follows the ions, causing diarrhea.

Endotoxin - Lipopolysaccharide

  • Lipopolysaccharide (LPS) is only made by gram-negative bacteria and is a component of the outer membrane
  • LPS contains endotoxin and Lipid A is released as bacteria die.
  • Massively activates host inflammatory response and can cause toxic shock.

Endotoxin - Lipopolysaccharide - Treatments

  • Need to be careful when administering treatments against infections (e.g., Neisseria meningitidis).
  • Too much treatment can cause a massive release of endotoxins.

Protein Secretion Pathways

  • Many pathogens use specific protein secretion pathways to deliver toxins.
  • The proteins may not kill the cell but redirect host signaling pathways in ways that benefit the microbe.
  • Many secretory systems have a structural resemblance to other innocuous systems.

Protein Secretion Pathways Types

  • Type I secretion system: General secretion pathway.
  • Type II secretion system: Use a pilus-like mechanism to push proteins out of the bacterial cell (similar to Type IV pili).
  • Type III secretion system: Molecular syringe to inject proteins from bacteria into the host cytoplasm.
  • Type IV secretion system: Uses proteins that resemble conjugation machinery to secrete proteins or DNA.

Type I Secretion System

  • General secretion system; one-step process of secretion.
  • Consists of 3 portions:
    • ATP-binding cassette protein.
    • Periplasmic protein: membrane fusion protein.
    • Outer membrane channel protein.
  • Hemolysin (α-toxin) is secreted through the type I secretion system.

Type II Secretion

  • Similar to Type IV pilus; modified for secreting proteins.
  • Can extend and retract.

Type III Secretion System (T3SS)

  • Injects proteins directly into the host cell, working just like a needle (one-step injection).
  • Injected proteins (type III effectors).
  • Essential for Gram-negative pathogens, e.g., E. coli, Salmonella, Yersinia pestis, Pseudomonads.

Type III Effectors (T3SE)

  • Salmonella injects over 13 toxins.
  • Induces tight attachment to the host and causes the host to engulf the bacteria.
  • Induces actin rearrangement in the cell.
  • Suppresses host immunity and causes diarrhea.

Type IV Secretion

  • Similar to the conjugation pilus.
  • Secretes both DNA and proteins.
  • Can extend and retract.
  • Can move proteins into or out of the cell.