Biofilms

• Dense aggregates of surface-adherant microorganisms embedded in an exopolysaccharide matrix.

• 65% of human bacterial infections involve biofilms!

Streptoccus spp.

• S. mutans – ubiquitous in oral microflora.

• Ecologic niche:

– Biofilms can lead to dental caries– Acid tolerant response– Natural genetic competence– Bacteriocin activity

http://www.db.od.mah.se/car/data/cariesser.html

DENTAL CARIES – demineralization of the tooth by bacteria.

Quorum sensing

• Cell-cell communication.

• Production, detection, and response to autoinducers (extracellular signaling molecules).

• Bacteria can monitor population density and coordinate gene expression.

Quorum sensing

• Involved in regulation of biofilms, virulence, antimicrobial production, bioluminescence, etc.

• Gram positive and gram negative bacteria.

• Inter- and intra-specific communication!

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

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.

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.

Acid Tolerance ResponseATR 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.

Biofilm survival advantage

• High population density.

• High concentration of signal molecules.

• Heterogenous adaptation possible.

• Bacteria sensing stress can signal more distant bacteria to adapt!

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.

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?

http://cgi.darwinawards.com/cgi/frames.pl?/book/3chapter05.html

-Squid-bacteria symbiosis involving quorum sensing, trickery, and a daily love-hate relationship!

Symbiosis Strategy

The players:

• Euprymna scolopes – a Hawaiian squid

• Vibrio fischeri – bioluminescent bacteria

Timing is everything…

• At birth – squid light organ has no bacteria.

• Within hours – symbiotic colonization.

• Each morning – >90% of bacteria expelled from squid light organ.

• Each night – bacteria multiply in squid light organ and bioluminesce for squid camouflage.