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Sticky bacteria and the growth of biofilms on surfaces - Rosalind Allen

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Sticky bacteria and the growth of biofilms on surfaces - Rosalind Allen

  1. 1. Sticky bacteria and the growth of biofilms on surfaces Rosalind Allen School of Physics and Astronomy, University of Edinburgh QLSB II, Como, 20th June 2016
  2. 2. Edinburgh University (Physics) Gavin Melaugh UT Austin (Physics) Vernita Gordon Jaime Hutchison U. Copenhagen (Microbiology) Thomas Bjarnsholt Kasper Kragh U. Nottingham (Evolutionary Biology) Steve Diggle Yasuhiko Irie Aled Roberts
  3. 3. Bacterial biofilms http://labrat.fieldofscience.com/2011/04/social-evolution- in-bacteria-sgm-series.html Communities of bacteria growing on surfaces Cause chronic infections and industrial biofouling Also: A beautiful example of multicellular self-assembly 40µm Copenhagen and Austin groups
  4. 4. Fundamental questions: •How do bacteria interact in biofilms? physical forces, metabolic interactions, cooperative interactions •What controls biofilm spatial and genetic structure? rough versus smooth surface, clonal versus genetically diverse •How does evolution happen in biofilms? •Can we learn about multicellularity more generally?
  5. 5. Pseudomonas aeruginosa Rod-shaped cells, common in soil Causes hospital-acquired infections and chronic infection in cystic fibrosis patients E. Banin et al, PNAS 102: 11076 (2005) Produces `sticky’ extracellular polymers Engages in many cooperative behaviours Model organism for biofilm formation
  6. 6. Lifecycle of a P. aeruginosa flow-cell biofilm http://biofilmbook.hypertextbookshop.com • Individual cells attach • Transient formation of microcolonies • Production of exopolymers • Proliferation • Dispersal
  7. 7. But P. aeruginosa can aggregate even in liquid Alhede et al, PLOS ONE (2011) Aggregates < 105 cells in stationary phase cultures of P. aeruginosa Also seen in vivo at sites of chronic infection Lung tissue from cystic fibrosis patient, courtesy of K. Kragh How does the picture of biofilm development change if growth starts from preformed aggregates? Our approach: Combine flow cell microscopy with computer simulations
  8. 8. Experiments: Track biofilm growth in a flow cell and Kasper Kragh Chamber is exposed to slow nutrient flow 3D images of biofilm as it grows Can image over several days
  9. 9. Bacteria are represented as spherical particles Food is represented as a continuum concentration field Bacteria consume food, grow and divide Bacteria push each other out of the way Food diffuses and is consumed by bacteria Computer simulations: Model growth of individual cells with iDynoMics
  10. 10. Specific questions • Do initial aggregates affect final biofilm spatial structure? • Do cells in aggregates outcompete isolated cells? • What are the mechanisms involved?
  11. 11. Experiments Seed flow cells with stationary phase culture Locate aggregates and track their fate by voxel counting Compare growth of aggregate to that of cells far from an aggregate Simulations Quantifying the fate of cell aggregates Start with circular aggregate, surrounded by “single cells” Track fate of aggregate Vary density of surrounding cells, nutrient concentration, etc
  12. 12. Experimental results P. aeruginosa PA01 GFP in M9 + phosphate buffer + 0.3mM glucose Do initial aggregates affect final biofilm structure? 24 h 24 h Yes they do Kasper Kragh Copenhagen Jaime Hutchison UT Austin
  13. 13. Simulations: what is the fate of an initial aggregate? Aggregate has a strong effect on final biofilm structure This seems to be due to competition for nutrients Gavin Melaugh Edinburgh
  14. 14. Cells in aggregate can outcompete single cells at high competition Average progeny from aggregate / average progeny from single cell Density of surrounding cells (per micron) Cells in aggregate outcompete single cells Simulations: fate of aggregate depends on competition from surrounding cells
  15. 15. Key mechanism is competition for nutrients But cells at top of aggregate have better access to nutrients -> being in an aggregate can be advantageous at high competition Cells in aggregate have a fitness cost because nutrients are limited in centre -> being in an aggregate is disadvantageous at low competition
  16. 16. Experimental results: Fate of aggregate depends on level of competition Aggregate can outcompete surrounding cells but only at high competition Medium competition Cells in aggregate Single cells Cells in aggregate Single cells High competitionLow competition Single cells Cells in aggregate Single cells grow faster Cells in aggregate grow faster
  17. 17. •Pre-formed aggregates can drastically affect biofilm development •This depends on level of competition K. Kragh, et al. mBio 7, e00237-16 (2016) G. Melaugh, et al. PloS One 11, e0149683 (2016) Why does this matter? •Biofilm shape rougher biofilms easier to penetrate with antimicrobials? rougher biofilms evolve slower? current work, Gavin Melaugh, Edinburgh • Evolution of cooperative behaviour Kin selection: less genetic mixing -> more potential for cooperation
  18. 18. Conclusions Bacterial biofilms are a beautiful example of multicellular self-assembly Pre-formed aggregates can change our picture of biofilm development Stickiness caused by polymer is crucial in aggregate formation Density-dependent potential can be a way to simulate polymer production Ongoing questions What are the pathways to aggregate formation? Are aggregates a first step in evolution of multicellularity?
  19. 19. Edinburgh University (Physics) Gavin Melaugh UT Austin (Physics) Vernita Gordon Jaime Hutchison U. Copenhagen (Microbiology) Thomas Bjarnsholt Kasper Kragh U. Nottingham (Evolutionary Biology) Steve Diggle Yasuhiko Irie Aled Roberts

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