Biofilms Flashcards
(29 cards)
Biofilm general structure
•enclosed in extracellular polysaccharide substances (EPS)
•adhere to an inert (eg medical devices) or living tissue
•structure forms ‘mushroom’ shape and channels carrying fluid and nutrients to the community
Dental plaque biofilm initiation
•biofilm initiated by primary/1° colonisers
•adhere via adhesins to glycoprotein receptors on tooth surface (salivary pellicle)
•1° colonisers are normal flora= Strep sanguis, Strep gordonii
•unattached= planktonic
•attached= sessile
Dental plaque co-aggregation
•adherence of 2° colonisers to 1° colonisers via co-aggregation
•adhesins on 1° and 2° recognise each other’s CHO receptors
•2° colonisers can be normal flora (Strep mitis) or pathogens (actinobacillus)
‘corn cob’ formation on tooth surface
•co-aggregation interactions lead to corn cob formation
•binding of streptococci to a filamentous bacteria
•eg Fusobacterium nucleatum
•bacteria become embedded in extracellular matrix
•bacterial extracellular polysaccharide (EPS) and host salivary glycoproteins
Calculus formation
•salivary Ca2+, H2PO4- become calcified plaque mass (calculus or tartar)
•bacterial toxins within calculus lead to chronic inflammation
•periodontal disease
On a healthy tooth
•plaque levels are low
•early colonisers are predominantly Gram pos aerobic cocci
•eg streptococci
High plaque levels
•create an anaerobic environment
•shifts bacterial flora
•less aerobic gram pos, more gram negative anaerobic rods/spirals/filaments
•opportunistic pathogens to create periodontal disease
Examples of pathogens in periodontal disease
•Fusobacterium nucleatum (slender rod or spindle shape, associated with colorectal cancer)
•Porphyromonas gingivalis (rod)
•Treponema denticola (spirochaete)
•Actinobacillus (filamentous)
Clinical significance of biofilms
Sessile biofilm bacteria are more resistant
•antibiotics
•antibody/complement mediated lysis
•phagocytosis
Extracellular capsule of biofilm
•diffusion barrier to antibodies, complement, antibiotics
•inhibits macrophage binding
•protects from hydrolytic enzymes
•difficult to treat, chronic infections
Phenotypic changes within biofilm that increases antibiotic tolerance
•decreased antibiotic diffusion through capsule
•increased expression of genes, eg antibiotic efflux pump
•bacteria deep in biofilm are quiescent
•around 1% of them are ‘persister cells’ which are dormant
What are the 5 stages of P. aeruginosa biofilm development
- Planktonic cells attach to surface and form a monolayer, flagella with sad genes
- Cell proliferation to form a micro colony, type IV pili and twitching motility, sad genes
- Quorum sensing, lasi dependent signalling, mature biofilm forms
- Physical detachment by shear forces
- Program of mobilisation to planktonic cells
Which opportunistic pathogens cause infections in cystic fibrosis patients?
•Pseudomonas aeruginosa
•Burkholderia cepacia
•Staphylococcus aureus
In the initial attachment of P. aeruginosa, which gene(s) is responsible for adhesion
•sad genes
•surface adherence deficiency
•mutants don’t have ability to adhere and colonise
What do sad genes encode?
•flagella synthesis
•type IV pili
•Ps1 exopolysaccharides
What’s the purpose of the flagella in P aeruginosa biofilms?
•Fla- mutants cannot form biofilms
•motility is essential for initial adherence to surface as it overcomes electrostatic repulsion between bacterium and host
•flagella is also an adhesin
•the flagella recognises asialoGM1 receptors
Explain the positive feedback loop of microcolony formation of P aeruginosa
•cells deposit trail of Ps1 exopolysaccharides
•other cells detect these and linger around them
•cells form colony at Ps1-rich sites
Explain the role of type IV pilus in microcolony formation
•type IV pilus mediates twitch motility
•polymerisation of pilus subunits into base -> extension
•depolymerisation -> retraction back into cell
•extension of pilus and binding to substrate followed by retraction of pilus will drag bacteria along
High density bacteria increase the expression of which exopolysaccharides?
•Ps1
•Pe1
•alginate
What does alginate do in a biofilm?
•overproduced by mucoid strains often isolated from CF lungs •contributes to structural stability of biofilm
What does Ps1 do in P aeruginosa biofilm?
•role in biofilm initiation (adhesion) and maturation
•Ps1 interacts with extracellular DNA to form web of fibres
What does Pe1 do in P aeruginosa biofilm?
•early stage of biofilm formation, helps structure mature biofilm
What are some examples of QS functions?
•bioluminescence
•biofilm production
•sporulation
•competence for transformation
•virulence factors
How does QS signal high density in gram negatives?
•Luxl/LuxR system
•uses acyl-homoserine lactones (AHL) as signal molecule (autoinducer)
•Luxl (=AHL synthase)
•AHL diffuses out of cell
•concentration of AHL increases in proportion to cell density
•AHL freely diffuses into cells
•at certain threshold, it induces gene expression
•LuxR (=transcriptional regulator protein) binds to AHL
•when bound, LuxR binds to specific promoters and activates transcription of target genes (capsule etc)
P aeruginosa AHL systems
•each bacterial species has a unique AHL
•only members of same species can respond to signal
•P aeruginosa has 2 AHL systems: las and rhl
•transcription is activated (or repressed) at high cell density
