Self-organization of bacterial biofilms is facilitated by extracellular DNA

Abstract

Twitching motility-mediated biofilm expansion is a complex, multicellular behavior that enables the active colonization of surfaces by many species of bacteria. In this study we have explored the emergence of intricate network patterns of interconnected trails that form in actively expanding biofilms of Pseudomonas aeruginosa. We have used high-resolution, phase-contrast time-lapse microscopy and developed sophisticated computer vision algorithms to track and analyze individual cell movements during expansion of P. aeruginosa biofilms. We have also used atomic force microscopy to examine the topography of the substrate underneath the expanding biofilm. Our analyses reveal that at the leading edge of the biofilm, highly coherent groups of bacteria migrate across the surface of the semisolid media and in doing so create furrows along which following cells preferentially migrate. This leads to the emergence of a network of trails that guide mass transit toward the leading edges of the biofilm. We have also determined that extracellular DNA (eDNA) facilitates efficient traffic flow throughout the furrow network by maintaining coherent cell alignments, thereby avoiding traffic jams and ensuring an efficient supply of cells to the migrating front. Our analyses reveal that eDNA also coordinates the movements of cells in the leading edge vanguard rafts and is required for the assembly of cells into the "bulldozer" aggregates that forge the interconnecting furrows. Our observations have revealed that large-scale self-organization of cells in actively expanding biofilms of P. aeruginosa occurs through construction of an intricate network of furrows that is facilitated by eDNA.collective behavior | t4p | type IV pili | tfp | swarming

Mechanically-driven phase separation in a growing bacterial colony

“…Bacteria are known to selforganize into multicellular communities, commonly known as biofilms, in which microbial cells live in close association with a solid surface or liquid-air interface and are embedded in a self-produced extracellular matrix. Extracellular polymeric substances (EPSs) play an important role in determining the structural and mechanical architecture of a biofilm (7)(8)(9)(10)(11)(12). Generally, the collective dynamics of bacterial colony involves a complex interplay of various physical, chemical, and biological mechanisms, such as growth and differentiation of cells, production of EPSs, the collective movement of cells determined by interacting physical forces and chemical cues, e.g., chemotaxis, motility, cellcell signaling, adhesion, and gene regulation (13)(14)(15)(16)(17)(18)(19).…”

Emergent pattern formation in an interstitial biofilm

“…However, because pilus retractions pull the cell a distance on the order of the cell length [18], errors in spatial sampling lead to ineffective EPS trail following. Our results suggest that while EPS excretion plays an important role in morphogenesis by enhancing the movement rate within trails, these trails cannot form without the mechanism suggested by Gloag et al [12], whereby the bacteria actively remodel the topography of their environment. Without this effect, trail-following by the preferential binding of T4P to EPS is not robust.…”

“…when γ = 0). However, this is true only in a narrow subset of the parameter space where the EPS degradation and deposition rates are fine-tuned and the pilus binding affinity to the bare surface is close to zero -conditions that are not justified given that the bacteria are known to form furrows in the systems of interest [12], and that T4P are known to bind readily to a large variety of biological and abiotic surfaces [27].…”

“…Since there are no long-range chemical gradients in our simulations, we make the minimal assumption that bacteria reverse polarity several times during a division cycle, which can range from about 30 min to 200 min depending on conditions (see for example [22,23]). Visual inspection of the microscopy data in [12] indicates that bacteria range from about 3 -7 µm in length and are about 1 µm in girth. Based on the observations and limitations discussed above, the following parameters can be roughly established: t rev , σ rev , l max , l min , r pili , φ, and w. The observed unobstructed movement rate of 0.5 µm/s is established by balancing t ret , | F p |, and µ.…”

“…With biological parameters fixed, those relating to the environment can be systematically tuned and the results compared to experimental observations [12,24]. The environmental parameters are: γ, k s , β s , µ, P min , and β p .…”