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Psl trails guide exploration and microcolony formation in Pseudomonas aeruginosa biofilms.

Zhao K, Tseng BS, Beckerman B, Jin F, Gibiansky ML, Harrison JJ, Luijten E, Parsek MR, Wong GC - Nature (2013)

Bottom Line: Bacterial biofilms are surface-associated, multicellular, morphologically complex microbial communities.This Pareto-type behaviour indicates that the bacterial community self-organizes in a manner analogous to a capitalist economic system, 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.

View Article: PubMed Central - PubMed

Affiliation: Department of Bioengineering, University of California, Los Angeles, California 90095, USA.

ABSTRACT
Bacterial biofilms are surface-associated, multicellular, morphologically complex microbial communities. Biofilm-forming bacteria such as the opportunistic pathogen Pseudomonas aeruginosa are phenotypically distinct from their free-swimming, planktonic counterparts. 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'. 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 surface. 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-type behaviour indicates that the bacterial community self-organizes in a manner analogous to a capitalist economic system, 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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Visit frequency distribution and its effect on bacterial movementa. Visit frequency map of WT for the first 15.7 hours post inoculation, when microcolonies were just starting to form (example outlined by black square). b. Bright-field image for WT at t ~ 15.7 hours. c. Visit frequency distribution from (a). Solid line shows a power-law decay with exponent –2.9. Green arrow indicates where the curve begins to deviate from this power law. d. Visit frequency distributions for ΔPpsl/PBAD-psl at arabinose concentrations 0% (Δ), 0.1% (□), 1% (○). e. Simulation results of visit frequency distributions at Psl deposition rates (arbitrary units, see Supplementary Methods) 0 (*), 10–5 (+), 5 × 10–5 (×). In (d) and (e), each data set is normalized by the total number of visits (roughly the same as for (a)) and solid lines show power-law decay. f. Schematic graph showing that distributions with steep slopes are more egalitarian, while those with shallow slopes are more hierarchical. g./h. Fitted power-law exponents of visit frequency distributions from experiments at different arabinose concentrations (g) and simulations at different Psl deposition rates (h). i. Fluorescent lectin-stained image showing hierarchical distribution of Psl (ΔPpsl/PBAD-psl at 1% arabinose). j. Psl map from simulations (Psl deposition rate 5 × 10–5). Scale bars are 10 μm.
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Figure 2: Visit frequency distribution and its effect on bacterial movementa. Visit frequency map of WT for the first 15.7 hours post inoculation, when microcolonies were just starting to form (example outlined by black square). b. Bright-field image for WT at t ~ 15.7 hours. c. Visit frequency distribution from (a). Solid line shows a power-law decay with exponent –2.9. Green arrow indicates where the curve begins to deviate from this power law. d. Visit frequency distributions for ΔPpsl/PBAD-psl at arabinose concentrations 0% (Δ), 0.1% (□), 1% (○). e. Simulation results of visit frequency distributions at Psl deposition rates (arbitrary units, see Supplementary Methods) 0 (*), 10–5 (+), 5 × 10–5 (×). In (d) and (e), each data set is normalized by the total number of visits (roughly the same as for (a)) and solid lines show power-law decay. f. Schematic graph showing that distributions with steep slopes are more egalitarian, while those with shallow slopes are more hierarchical. g./h. Fitted power-law exponents of visit frequency distributions from experiments at different arabinose concentrations (g) and simulations at different Psl deposition rates (h). i. Fluorescent lectin-stained image showing hierarchical distribution of Psl (ΔPpsl/PBAD-psl at 1% arabinose). j. Psl map from simulations (Psl deposition rate 5 × 10–5). Scale bars are 10 μm.

Mentions: We used cell-tracking algorithms to determine the bacterial visit frequency for each surface pixel for the entire community (see Methods). A surface visit map (Figure 2a) of all WT cells within the field of view (Figure 2b, ~15.7 hours after inoculation) shows that a large fraction of the surface was never visited. Of the surface areas traversed by bacteria, most were traversed once. In fact, the visit frequency distribution (histogram of the number of pixels with N bacterial visits) was measured to be a monotonically decreasing function of N. The precise form of the function is complex; however, for N ranging from a few visits to over a hundred visits, the distribution is approximately described by a power law with exponent of −2.9 ± 0.1 (Figure 2c), which serves as a simple metric for the distribution.


Psl trails guide exploration and microcolony formation in Pseudomonas aeruginosa biofilms.

Zhao K, Tseng BS, Beckerman B, Jin F, Gibiansky ML, Harrison JJ, Luijten E, Parsek MR, Wong GC - Nature (2013)

Visit frequency distribution and its effect on bacterial movementa. Visit frequency map of WT for the first 15.7 hours post inoculation, when microcolonies were just starting to form (example outlined by black square). b. Bright-field image for WT at t ~ 15.7 hours. c. Visit frequency distribution from (a). Solid line shows a power-law decay with exponent –2.9. Green arrow indicates where the curve begins to deviate from this power law. d. Visit frequency distributions for ΔPpsl/PBAD-psl at arabinose concentrations 0% (Δ), 0.1% (□), 1% (○). e. Simulation results of visit frequency distributions at Psl deposition rates (arbitrary units, see Supplementary Methods) 0 (*), 10–5 (+), 5 × 10–5 (×). In (d) and (e), each data set is normalized by the total number of visits (roughly the same as for (a)) and solid lines show power-law decay. f. Schematic graph showing that distributions with steep slopes are more egalitarian, while those with shallow slopes are more hierarchical. g./h. Fitted power-law exponents of visit frequency distributions from experiments at different arabinose concentrations (g) and simulations at different Psl deposition rates (h). i. Fluorescent lectin-stained image showing hierarchical distribution of Psl (ΔPpsl/PBAD-psl at 1% arabinose). j. Psl map from simulations (Psl deposition rate 5 × 10–5). Scale bars are 10 μm.
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Related In: Results  -  Collection

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Figure 2: Visit frequency distribution and its effect on bacterial movementa. Visit frequency map of WT for the first 15.7 hours post inoculation, when microcolonies were just starting to form (example outlined by black square). b. Bright-field image for WT at t ~ 15.7 hours. c. Visit frequency distribution from (a). Solid line shows a power-law decay with exponent –2.9. Green arrow indicates where the curve begins to deviate from this power law. d. Visit frequency distributions for ΔPpsl/PBAD-psl at arabinose concentrations 0% (Δ), 0.1% (□), 1% (○). e. Simulation results of visit frequency distributions at Psl deposition rates (arbitrary units, see Supplementary Methods) 0 (*), 10–5 (+), 5 × 10–5 (×). In (d) and (e), each data set is normalized by the total number of visits (roughly the same as for (a)) and solid lines show power-law decay. f. Schematic graph showing that distributions with steep slopes are more egalitarian, while those with shallow slopes are more hierarchical. g./h. Fitted power-law exponents of visit frequency distributions from experiments at different arabinose concentrations (g) and simulations at different Psl deposition rates (h). i. Fluorescent lectin-stained image showing hierarchical distribution of Psl (ΔPpsl/PBAD-psl at 1% arabinose). j. Psl map from simulations (Psl deposition rate 5 × 10–5). Scale bars are 10 μm.
Mentions: We used cell-tracking algorithms to determine the bacterial visit frequency for each surface pixel for the entire community (see Methods). A surface visit map (Figure 2a) of all WT cells within the field of view (Figure 2b, ~15.7 hours after inoculation) shows that a large fraction of the surface was never visited. Of the surface areas traversed by bacteria, most were traversed once. In fact, the visit frequency distribution (histogram of the number of pixels with N bacterial visits) was measured to be a monotonically decreasing function of N. The precise form of the function is complex; however, for N ranging from a few visits to over a hundred visits, the distribution is approximately described by a power law with exponent of −2.9 ± 0.1 (Figure 2c), which serves as a simple metric for the distribution.

Bottom Line: Bacterial biofilms are surface-associated, multicellular, morphologically complex microbial communities.This Pareto-type behaviour indicates that the bacterial community self-organizes in a manner analogous to a capitalist economic system, 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.

View Article: PubMed Central - PubMed

Affiliation: Department of Bioengineering, University of California, Los Angeles, California 90095, USA.

ABSTRACT
Bacterial biofilms are surface-associated, multicellular, morphologically complex microbial communities. Biofilm-forming bacteria such as the opportunistic pathogen Pseudomonas aeruginosa are phenotypically distinct from their free-swimming, planktonic counterparts. 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'. 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 surface. 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-type behaviour indicates that the bacterial community self-organizes in a manner analogous to a capitalist economic system, 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.

Show MeSH
Related in: MedlinePlus