09: Pathogenesis of Bacterial Infections

Question 1

Describe the gram negative bacterial cell wall.

Answer 1

• Lipopolysaccharides (LPS):
  
  • Sugar chains with different amounts of charge (allows for interaction with aminoglycosides [class of antibiotics] and for phagocytes to engulf the organism)
  • Note that virulence of LPS varies (immunogenicity).
  • May have smooth or rough LPS, with different virulence.
  • Organisms can shed LPS (thus, inflammatory response possible even in absence of organism).
• Porins:
  
  • Important for antibiotics to pass through, bind to prorteins located on the inner cell wall.
  • Mutants can turn off genes that encode for these porins (resistance).
  • Porins oftentimes closed.
• Periplasmic space
  
  • Beta lactamase resides here, can destroy antibiotics.

Question 2

What factors contribute to the pathogenesis of bacterial infections?

Answer 2

Host-pathogen interactions:

• Properties of the pathogen: Pathogen-associated molecular patterns (PAMPs; e.g., endotoxins, flagella, DNA) and toxins (availability of receptors dictates host susceptibility).
• Properties of the host: Innate/adaptive immunity, “intensity” of innate response, genetic factors regulating host signaling.

Question 3

What bacterial gene products facilitate infection?

Answer 3

• Pili (fimbriae): attachment; basis for vaccines, but high genetic variability
• Siderophores: iron scavenging pigments
• Flagella: motility
• LPS: immunostimulatory
• Type III secretion system: injects toxins into host cell
• Exopolysaccharides (biofilms): immunomodulatory
• Pyocyanin: anti-oxidant

Question 4

Describe the simplified steps to an immune response to bacterial infection.

Answer 4

1. Macrophage bumps into bacterium, becomes activated –> produces chemokines and other cytokines; activates calcium flux which signals surrounding epithelial cells –> produce pro-inflammatory cytokines (particularly IL-6).
2. Neutrophils phagocytose bacteria, however if they lyse, cause harmful inflammatory response.
3. Dendritic cells sample antigen and take to the T-cells.
4. T-cell recruited after 24 hours via macrophage or dendritic cell.

Question 5

Identify the various bacterial virulence factors.

Answer 5

• Flagellum
• Pilus
• Alginate/biofilm
• Non-pilus adhesins
• Extracellular products:
  
  • Proteases (irritate epithelial cells and stimulate pro-inflammatory cytokines).
  • Hemolysins (lyse RBCs, providing organism with source of iron).
  • Exotoxin (potent inhibitor of protein synthesis).
  • Exoenzyme (modifies GTPases, allows bacteria to invade cells).
  • Pyocyanin: allows bacteria to resist oxidative stress in airway.

Question 6

Compare Toll and Nod-like receptors.

Answer 6

Receptors activate innate immune signaling.

• Toll: Pattern recognition receptors; surface displayed; intracellular.
• Nod (nucleotide binding oligomerization domain): cytosolic; linked to inflammasome.

Question 7

Describe the motility function and immunostimulatory response of flagella.

Answer 7

Motility function:

• Universal joint (rotor); sigma factor rpoN controls transcription in response to specific carbon sources to produce smooth spinning or random tumbling.
• Have ligand for non-opsonic (without antibody) phagocytosis; enables ingestion by macrophages.

Immunostimulatory response:

• Recognized by TLR5 –> NFkB signaling (proinflammatory).
  • TLR5 is basolateral in gut epithelium (only responds to invasive organisms)
  • Apical display in human airway cells
  • TLR5 polymorphisms in humans –> disease susceptibility
• Activates the NLRC4 inflammasome, causing pathology.
  • Primarily activated in macrophages (no NLRC4 in mucosal epithelia cells); lead to production of IL-1 beta and IL-18
  • Release of of macrophage components: DNA, ROS
  • Depletion of alveolar macrophages decreases pathology.

Question 8

Describe inflammasome signaling.

Answer 8

• Flagellin or T3SS (type 3 secretion system) rod subunit (bacterial products) activate the NLRC4 inflammasome (composed of ASC and NLRC4).
• Inflammosome cleaves pro-capsase-1 to active caspase-1, which stimulates macrophage pyroptosis (form of programmed cell death; results in the release of pathogen associated molecular patterns [PAMPs] and cytokines (IL-1ß and IL-18) that activate released pro-inflammatory immune cell mediators [HMGH-1]).
• Research demonstrates that inhibition of the inflammasome leads to reduced mortality in mouse models infected with P. aeruginosa.

Question 9

Give an example of an advantageous TLR4 polymorphism.

Answer 9

• Polymorphisms have been related to susceptibility to Gram-negative infections and septic shock.
• However, one polymorphism in African/Asian/European populations has eveloved as a protective allele against malaria.
• Nonetheless, this same allele causes increased susceptibility to severe bacterial infections.

Question 10

Describe the Type III secretion system.

Answer 10

• Toxins target the host (are injected intracellularly) and open cell-cell junctions, thus allowing paracellular movement of organisms.
  
  • Protein toxins are ADP-ribosylators, which modify GTPases, leading to inhibition of the Rho cascade (small GTPases important in regulation of cytoskeleton).
• ExoS: ADP ribosylation enzyme: targets Ras GTPase activation –> alters integrity of tight junctions, enables invasion.
• ExoU: potent phospholipase –> destroys tissues

Question 11

Describe the Quorum Sensing System.

Answer 11

• Allow for bacteria to coordinate gene expression (e.g., slime gene) according to the density of their local population.
• Secretion of small molecules (homoserine lactones, cyclic diGMP) are taken up by surrounding organisms along with a transcriptional activator –> initiates gene expression of community.