Biofilms

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Biofilms

• Dense aggregates of surface-adherant
  microorganisms embedded in an exopolysaccharide
  matrix.
• 65 of human bacterial infections involve
  biofilms!

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Streptoccus spp.

• S. mutans ubiquitous in oral microflora.
• Ecologic niche
• Biofilms can lead to dental caries
• Acid tolerant response
• Natural genetic competence
• Bacteriocin activity

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DENTAL CARIES demineralization of the tooth by
bacteria.
http//www.db.od.mah.se/car/data/cariesser.html
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Quorum sensing

• Cell-cell communication.
• Production, detection, and response to
  autoinducers (extracellular signaling molecules).
• Bacteria can monitor population density and
  coordinate gene expression.

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Quorum sensing

• Involved in regulation of biofilms, virulence,
  antimicrobial production, bioluminescence, etc.
• Gram positive and gram negative bacteria.
• Inter- and intra-specific communication!

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Henke and Bassler, 2004. Three parallel
quorum-sensing systems regulate gene expression
in Vibrio harveyi. Journal of Bacteriology 186
6902-6914.
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Biofilms

• Facilitate horizontal gene transfer?
• S. mutans biofilm transformation frequences
  10-600 x that of planktonic cells.
• Subpopulations in biofilms constantly competent
  for DNA uptake compared to 15-30 min. transient
  state in planktonic cells.

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Dental biofilms

• Oral environment stressors
• Nutrient shortage and excess
• Low pH
• High osmolarity
• Oxidation
• Host ingestion of antimicrobial agents.
• Genetic transformation for antimicrobial
  resistance or virulence factors may provide
  important selective advantages to bacteria in
  these environments.

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Acid Tolerance Response

• ATR experiments
• Exposure to pH 5.0-6.0 allows the bacteria to
  adapt and survive at 3.0-3.5.
• Filtrates from acid-adapted cells induced an ATR
  in cells not exposed to low pH.
• Bacteria grown at high density adapted faster
  than cells grown at low density.
• S. mutans biofilms more resistant to low pH and
  even grew at the low pH after a glucose pulse in
  contrast to planktonic cells.

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Biofilm survival advantage

• High population density.
• High concentration of signal molecules.
• Heterogenous adaptation possible.
• Bacteria sensing stress can signal more distant
  bacteria to adapt!

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Mucosal pathogens

• Bacterial concentrations are often lower on
  mucosal surfaces than in dental plaque due to
  bathing effects of secretions and desquamation.
• Quorum sensing still important
• S. pneumoniae quorum sensing mutant was less
  virulent than parent strain in mouse model.

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Bacteriocins

• Antimicrobial peptides generated by bacteria that
  target other bacteria.
• Quorum sensing pathways involved
• S. pyogenes has parts of quorum sensing apparatus
  but not CSP to enhance DNA uptake.
• A specialized niche to regulate bacteriocin
  activities in a cell density dependent manner to
  outcompete other bacteria on mucosal surfaces,
  but not involving DNA uptake? Other signals?

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http//cgi.darwinawards.com/cgi/frames.pl?/book/3c
hapter05.html
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-Squid-bacteria symbiosis involving quorum
sensing, trickery, and a daily love-hate
relationship!
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Symbiosis Strategy

• The players
• Euprymna scolopes a Hawaiian squid
• Vibrio fischeri bioluminescent bacteria

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Timing is everything

• At birth squid light organ has no bacteria.
• Within hours symbiotic colonization.
• Each morning gt90 of bacteria expelled from
  squid light organ.
• Each night bacteria multiply in squid light
  organ and bioluminesce for squid camouflage.

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