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