(BTS 801) Quorum Sensing as a Potential Antimicrobial Target

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Biofilm and Quarum Sensing
M. Sc (P) Biotechnology
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Biofilms

• Biofilms are organised microbial systems
  consisting of cells associated with surfaces
• - Likely the most wide-spread mode of growth for
  bacteria in nature

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Anton van Leeuwenhook used a primitive microscope
to look at biofilms in 1684.
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Biofilms
In the natural world most bacteria aggregate as
biofilms - they form when bacteria adhere to
surfaces in aqueous environments and begin to
excrete a polysaccharide that can anchor them to
all kinds of material. The biofilm is held
together and protected by the polysaccharide
matrix. This matrix protects the cells within it
and facilitates communication among them through
biochemical signals. Bacteria living in a
biofilm usually have significantly different
properties from free-floating bacteria of the
same species, as the dense and protected
environment of the film allows them to cooperate
and interact in various ways.
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Biofilms

• A biofilm can be formed by a single bacterial
  species, but more often biofilms consist of many
  species of bacteria, as well as fungi, algae,
  protozoa, debris and corrosion products.
• Once anchored to a surface, biofilm
  microorganisms carry out a variety of detrimental
  or beneficial reactions (by human standards),
  depending on the surrounding environmental
  conditions.

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Advantages for Bacteria

• ? Creation of habitable niches
• ? Protection against
• - Physical forces (e.g. in flowing systems)
• - Phagocytosis by immune cells
• - Grazers (e.g. ciliates, amoeba)
• - Viruses
• ? Barrier against toxic substances
• ? Facilitates intercellular communication
• ? Close proximity of cells enables genetic
  exchange

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Disadvantages for mankind

• ? Immune system can not attack biofilms
• ? Antibiotics/antimicrobial agents fail
• ? Slow the flow of liquids or clog pipelines
• ? Accelerate corrosion of pipelines
• ? Risk for drinking water supply via pipes

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Effects of Biofilms

• Microbial biofilms on surfaces result in billions
  of dollars in losses yearly due to equipment
  damage, product contamination, energy losses and
  medical infections.
• Conventional methods of killing bacteria (such as
  antibiotics, and disinfection) are often
  ineffective with biofilm bacteria.
• The huge doses of antimicrobials required to rid
  systems of biofilm bacteria are environmentally
  undesirable  and medically impractical.
• Conversely, microbial processes at surfaces also
  offer opportunities for positive industrial and
  environmental effects, such as bioremediating
  hazardous waste sites, biofiltering industrial
  water, and forming biobarriers to protect soil
  and groundwater from contamination. 

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Pseudomonas strain S61 biofilms on glass slides
Staining technique (Congo red) in which the
bacterial cells stain dark red and the
exopolysaccharide stains orange-pink
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Biofilms grown in soil
14 day old biofilm
7 day old biofilm
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Structure of Biofilms

• Although bacteria can grow in a free-living or
  planktonic state it is common for them to
  adhere to surface by producing extracellular
  polysaccharides.
• The adherent bacteria produce microcolonies
  leading to an intricate three-dimensional
  structure.
• Biofilms survive so well because they have
  channels, like aqueducts, that transport water,
  oxygen, and nutrients to all the bacteria of the
  community.
• These channels also get rid of the bacterial
  wastes, making these biofilms seem almost as
  complex as a city.
• The complexity goes so deep that even the
  different regions of the biofilm have bacterial
  cells with different genetic information,
  physical characteristics, and duties for the
  community.

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Close-up of a microcolony
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Stages in biofilm formation

• Attachment of bacteria
• Irreversible binding by bacteria
• Formation of microcolonies
• Maturation of microcolonies
• Dispersal

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biofilm attachment
Maturation
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Biofilm-forming Bacteria
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Campylobacter jejuni

• Gram-negative bacteria
• genus was first discovered in the 1970s
• most common cause of gastroenteritis
• Microbial coloniser of surface waters
• Incidence in the U.S. is estimated at 30 to 60
  per 100,000 of the population.

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Legionella pneumophila

• Gram-negative bacteria
• Causes legionellosis, commonly known as
  Legionnaires disease
• Transmitted to host via aerosolisation and
  ingestion.
• Found in domestic households and large municipal
  buildings plumbing, air conditioning systems,
  etc.
• Occurs in biofilms where symbiotic relationships
  with other heterotrophs are evident

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Salmonella enteritidis

• Gram-negative bacteria
• Anaerobic
• Present predominantly in raw water and
  occssionally in potable water.
• Chlorine is effective in destroying this organism
  in the planktonic state.
• Forms dense, metabolically active biofilms.
• Often develop on stainless steel.

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Biofilm Interactions

• Quorum sensing
• Interspecies interactions
• Symbiosis
• Population relationships
• Spatial
• Temporal
• Metabolic
• Genetic

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Warning Biofilms present!

• Biofilms may be 50 to 500 times more resistant to
  chemotherapy than planktonic bacteria of the same
  strain.
• Cause cosmetic degradation in toilet bowls
• Are the cause of flawed prints and malfunctioning
  machines during photo processing
• Infect implanted devices such as contact lens,
  catheters, prosthetic heart valves, and cardiac
  pacemakers.
• Cause such chronic infections as cystic
  fibrosis, pneumonia, biliary tract infections,
  osteomyelitis, and bacterial prostatitis.
• They are also the cause of dental plaque!!!

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Biofilms to the Rescue!

• Play an essential role in the processing of
  sewage water prior to its discharge into rivers
• Bioremediation
• Clean up groundwater
• Oil recovery
• Mine remediation

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How do biofilms form?

• The formation of a biofilm requires coordinated
  chemical signalling between cells.
• Unless an adequate number of neighbouring cells
  are present, the costs of biofilm production to
  an individual bacterium outweigh the benefits.
• A signalling process benefits the bacteria by
  allowing them to sense the presence of
  neighbouring bacteria and respond to varying
  conditions.
• The process by which a bacterium does this is
  called quorum sensing.

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Quorum sensing Biofilm formation involves more
than just bacteria attaching to a solid surface
individual organisms aggregate with their kin and
often congregate with members of other species.
Bacteria accomplish this through chemical
signaling mechanisms. When the local
extracellular concentration of the chemical
signal reaches a threshold level, indicating that
the population of microbes has reached a minimum
densitya quorumthe community of organisms
undergoes phenotypic changes. The process of
chemically sensing the population density is
called quorum sensing.
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Signal molecules produced by individual cells do
not have an effect until the bacterial population
density is sufficient to provide a concentration
of molecules, which then cross cell membranes and
activate the manufacture of such cellular
products as toxins, enzymes or surfactants.
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Quorum sensing

• The term 'Quorum Sensing' (QS) is used to
  describe the phenomenon whereby the accumulation
  of signalling molecules enable a single cell to
  sense the number of bacteria (cell density).
• In the natural environment, there are many
  different bacteria living together which use
  various classes of signalling molecules.

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Introduction

• Quorum sensing is cell to cell signaling
  mechanism that enables the bacteria to
  collectively control gene expression.
• This type of bacterial communication is achieved
  only at higher cell densities.
• Bacteria release various types of molecules
  called as autoinducers in the extracellular
  medium, these molecules are mediators of quorum
  sensing.
• When concentration of these signaling molecules
  exceed a particular threshold value, these
  molecules are internalized in the cell and
  activate particular set of genes in all
  bacterial population, such as genes responsible
  for virulence, competence, stationary phase etc .

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Cell density and quorum sensing
R gene I gene R protein I
protein
AHL diffuse in
Cell density
R gene I gene R protein I
protein

AHL diffuse out
AHL diffuse out
Time
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Quorum sensing controlled processes

• It occurs in various marine bacteria
• such as Vibrio harveyi and Vibrio fischeri.
• Takes place at high cell density.

• Bioluminescence
• Biofilm formation
• Virulence gene expression
• Sporulation
• Competence

• It iscompact mass of differentiated microbial
  cells, enclosed
• in a matrix of polysaccharides. Biofilm resident
  bacteria
• are antibiotic resistant. Quorum sensing is
  responsible for
• development of thick layered biofilm.

• QS upregulates virulence gene expression

• Virulence gene expression

• QS upregulates spore-forming genes in
• Bacillus subtilis

• It is ability to take up exogenous DNA
• QS Increase competence in Bacillus subtilis

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Quorum sensing molecules
Three types of molecules 1 Acyl-homoserine
lactones (AHLs) 2 Autoinducer peptides
(AIPs) 3 Autoinducer-2 (AI-2)
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Acyl-homoserine lactones (AHLs)

• Mediate quorum sensing in Gram-negative
  bacteria.
• Mediate exclusively intracellular communication.
• These are of several types depending on their
  length of acyl side chain.
• Able to diffuse through membrane.
• These are synthesized by an autoinducer synthase
  LuxI and recognized by a
• autoinducer receptor/DNA binding
  transcriptional activator protein LuxR.

AHL core molecule
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Acyl-homoserine lactones (AHLs) cont.AHL
mediated quorum sensing cycle
LuxI
AI
AI
LuxR

RNA polymerase
Transcription
promoter target genes
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Autoinducer peptides

• These are small peptides, regulate gene
  expression in Gram-positive
• bacteria such as Bacillus subtilis,
  Staphylococcus aureuas etc.
• Recognized by membrane bound histidine kinase
  as receptor.
• Regulates competence and sporulating gene
  expressions.

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Autoinducer peptides cont AIPs
signaling mechanism in Bacillus subtilis
In Bacillus subtilis QS is mediated by two AIPs
1 ComX involve in competence
development 2 CSF (competence and
sporulation factor) regulates spore
formation
Christopher et al.,2005
Figure ComX and CSF pathway in Bacillus subtilis
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Autoinducer-2 (AI-2)

• Involve in interspecies communication among
  bacteria.
• Present in both Gram () and Gram (-) bacteria.
• Chemically these are furanosylborate diester.

S-ribosyl-homocysteine (SRH)
LuxS
4,5-dihydroxyl-2,3 pentanedione (DPD)
Cyclization
Autoinducer-2 (AI-2)
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Autoinducer-2 (AI-2) contAI-2 controlled
processes

• Induces mini cell formation
• Induces expression of stationary phase genes
• Inhibition of initiation of DNA replication

Figure AI-2 signaling in E. coli
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Quorum sensing in bacterial pathogenesis

• QS is involved in expression of virulence genes
  in various bacteria,
• indicating the possible role of quorum sensing
  as a drug target.
• Several QS system mutant bacteria show the
  heavily reduced pathogenicity.
• Pseudomonas aeruginosa mutant in synthesis of
  autoinducer molecules
• shows heavy reduction in pathogenesis.

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Quorum sensing in bacterial
pathogenesis contQuorum sensing in P. aeruginosa

• In P. aeruginosa QS molecules are synthesized
  by two autoinducer
• synthase LasI and RhlI

LasI
3-O-C12 -HSL (AI)
AI
LasR

Transcription
RNA polymerase
promoter target virulence
genes
AI
RhIR

RNA polymerase
C4-HSL(AI)
RhlI

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Quorum sensing in P. aeruginosa cont..

• In an in-vivo study, using two strains P.
  aeruginosa PAO1 (virulent), and PAOR (lasI and
  rhII double mutant, avirulent), it was seen that
  rats infected with PAOR are much immunologically
  active and number of P. aeruginosa also reduced.

POA1
POAR
Wu et al., 2001
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Quorum Sensing
AIs are sensed by two major mechanisms 1. AI
diffuses into cytosol and is bound by a cytosolic
regulatory proteins (eg. LuxR) once bound the
regulator changes conformation and either
activates or represses genes (eg., Vibrio
fischeri) 2. Sensor kinase on cytoplasmic
membrane senses AI and transmits the signal to a
response regulator through a phosphorylation
cascade (Two-component signal transduction
system) (eg., Vibrio harveyi)
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Quorum Sensing Mechanisms

• Autoinducer diffusion and binding to LuxR
• homolog directly

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Quorum sensing in bioluminescent bacteria
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Bioluminescence first quorum sensing system
discovered
light
Vibrio fischeri lux operon
Luciferase
Divergent transcription
Homoserine lactone (Autoinducer)
Provided by J. Foster
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C4 RhlI
C12 LasI
http//www.apsnet.org/education/AdvancedPlantPath/
LabExercises/BacteriaSignaling/Images/fig1.gif
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Quorum Sensing Mechanisms

• Autoinducer sensed by sensor kinase and signal
• relayed to response regulator by phosphorylation
• (two-component system)

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Two-component signal transduction systems can
activate or repress gene transcription
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Two-component systems
Transmitter
Sensor kinases often exist as dimers and are
often also phosphatases
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No recognition of AI in cytoplasm
(phosp form represses)
From B. Bassler
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Low Cell Density
LuxQ
LuxN
LuxU
P
P
s54
sRNAs/Hfq
LuxR
No Light Production
Provided by J. Nordstrom
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High Cell Density
LuxQ
LuxN
LuxU
P
P
LuxO
LuxR
luxCDABE
Luminescence
Provided by J. Nordstrom
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The End