Signal Transduction in Quorum Sensing Systems

• Quorum Sensing

  • communication between bacteria
  • cell density dependent interaction between inter or intraspecies
  • Autoinducers: specific signaling molecule in quorum sensing that binds to specific activator protein and triggers transcription of specific genes
• Autoinducers in Bacteria

  • Gram-negative: acyl homoserine lactone (AHLs)
  • can passively diffuse through cell walls
  • linked to a fatty acid chain
  • Gram-positive: through peptides that are actively transported via the ABC transporter system
• Categories of QS Systems

  based on types of employed autoinducers

  1. AHL system: in gram-negative
  2. N-acyl homoserine lactones
  3. AIP (previously AI-1): Gram-positive; Autoinducing Peptides
  4. AI-2 and -3 Systems: both in gram positive and negative
  5. interspecies signal exchange
  6. AI-3: pyrethroids
• Critical Mass

  • concentration (cell density); whole population can initiate a concerted action
  • excess: downregulates autoinducer synthesis
• Critical Mass

  • when EC concentration < IC concentration: cells continue to replicate
  • when balance: replicated cells produce AI
  • EC > IC: intracellular AI stops moving out of the cell

  Result: Higher AI concentration downregulates itself by inhibiting its own transcription factor – EC < IC again (repeat process)
• Examples of Quorum Sensing

  1. Sporulation
  2. Bioluminescence
  3. Virulence
  4. Conjugation
  5. Competence
  6. Biofilm Formation
• Bioluminescence

  • production of light by a living organism
  • LUX gene cluster
  • Luciferase

  
• Vibrio fischeri

  • free-living; can colonize light organs of squid

  
• Vibrio harveyi

  • pathogen of wild and cultured marine fish and invertebrates

  
• Lux gene cluster


  LuxR: inhibits production of genes involved in exopolysaccharide and siderophores, and type III secretion

  • stimulates production of genes involved in bioluminescence proteolysis

  At low cell density: LuxOP is high; at high: LuxOP is low

  CqsS: AI of CAI-1

  LuxS: AI of AI-2

  LuxM: AI of AI-1

  
• Biofilm Formation

  • production of external extrapolyssaccharide; contributes to surface adherence
  • complex communities of microorganisms
  • self protection
  • may be irreversible but cannot be removed by gentle rinsing
  • LuxR: regulate the formation of biofilm in gram-negative bacteria

  
• Swimming Motility and Biofilm Formation in P. aeruginosa

  • FleS and FleR in PAO1: regulates biofilm formation in a c-di-GMP dependent manner
  • c-di-GMP manner is mediated by TF FleQ
  • Swimming motility: is regulated by FleS and FleR in descrepant manner
  • FleR: key regulator to independently control the biosynthesis of flagella