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Psl trails guide exploration and microcolony formation in Pseudomonas aeruginosa biofilms

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Abstract

Bacterial biofilms are surface-associated, multicellular, morphologically complex microbial communities1,2,3,4,5,6,7. Biofilm-forming bacteria such as the opportunistic pathogen Pseudomonas aeruginosa are phenotypically distinct from their free-swimming, planktonic counterparts7,8,9,10. Much work has focused on factors affecting surface adhesion, and it is known that P. aeruginosa secretes the Psl exopolysaccharide, which promotes surface attachment by acting as ‘molecular glue’11,12,13,14,15. However, how individual surface-attached bacteria self-organize into microcolonies, the first step in communal biofilm organization, is not well understood. Here we identify a new role for Psl in early biofilm development using a massively parallel cell-tracking algorithm to extract the motility history of every cell on a newly colonized surface16. By combining this technique with fluorescent Psl staining and computer simulations, we show that P. aeruginosa deposits a trail of Psl as it moves on a surface, which influences the surface motility of subsequent cells that encounter these trails and thus generates positive feedback. Both experiments and simulations indicate that the web of secreted Psl controls the distribution of surface visit frequencies, which can be approximated by a power law. This Pareto-type17 behaviour indicates that the bacterial community self-organizes in a manner analogous to a capitalist economic system18, a ‘rich-get-richer’ mechanism of Psl accumulation that results in a small number of ‘elite’ cells becoming extremely enriched in communally produced Psl. Using engineered strains with inducible Psl production, we show that local Psl concentrations determine post-division cell fates and that high local Psl concentrations ultimately allow elite cells to serve as the founding population for initial microcolony development.

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Figure 1: Efficiency of surface coverage by bacterial trajectories and correlation with Psl trails.
Figure 2: Visit frequency distribution and its effect on bacterial movement.
Figure 3: Local Psl levels determine post-division cell fates.

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Acknowledgements

K.Z., B.S.T., M.R.P. and G.C.L.W. are supported by the US National Institutes of Health (NIH 1RO1HL087920). K.Z. and G.C.L.W. also acknowledge support from the US National Science Foundation (NSF DMR1106106) and a UCLA Transdisciplinary Seed Grant. B.S.T., J.J.H. and M.R.P. also acknowledge support from the NIH (R01AI077628, R01AI081983, R56AI061396) and NSF (MCB0822405). B.S.T. is supported by the Cystic Fibrosis Foundation Postdoctoral Fellowship (TSENG11F0). J.J.H. was supported by a postdoctoral fellowship from the Natural Sciences and Engineering Research Council of Canada. B.B. and E.L. acknowledge support from the NSF under DMR-1006430 (E.L.) and DGE-0824162 (B.B.). The authors would like to thank J. Copic for discussions and R. J. Siehnel for technical assistance. We dedicate this paper to the memory of M. Shannon.

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Contributions

G.C.L.W., M.R.P. and K.Z. conceived the project. K.Z., B.S.T., M.R.P. and G.C.L.W. designed studies. K.Z. and B.S.T. performed experimental measurements. K.Z. and G.C.L.W. performed image analysis. F.J. helped in performing image analysis. M.L.G. helped in collecting experimental data. B.S.T., J.J.H. and M.R.P. constructed strains. B.B. and E.L. designed the model and performed computer simulations. K.Z., B.S.T., B.B., E.L., M.R.P. and G.C.L.W. wrote the paper. All authors discussed the results and commented on the manuscript.

Corresponding authors

Correspondence to Erik Luijten, Matthew R. Parsek or Gerard C. L. Wong.

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The authors declare no competing financial interests.

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Supplementary Information

This file contains Supplementary Figures 1-16, Supplementary Methods, Supplementary Tables 1-3 and Supplementary References. (PDF 1308 kb)

Evolution of surface coverage by bacterial trajectories

The left panels show the bright field videos while the right panels show the corresponding evolution of surface coverage by bacterial trajectories (WT (top row) and ΔpslD (bottom row)). In the right panels, red and black regions indicate used and fresh surfaces, respectively. For clarity, cells that reside on fresh surfaces are colored yellow, whereas cells that reside on used surfaces are colored purple. (MOV 30225 kb)

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Zhao, K., Tseng, B., Beckerman, B. et al. Psl trails guide exploration and microcolony formation in Pseudomonas aeruginosa biofilms. Nature 497, 388–391 (2013). https://doi.org/10.1038/nature12155

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