Current research projects:

  • Single-cell motility and physiology near surfaces

  • Bacterial self-organization and collective motion at mesoscales

  • Dynamics and pattern formation during bacterial colony spreading

  • Collective antibiotic tolerance of bacteria in communities

Bacteria self-organize to drive material transport in colonies

Bacteria normally rely on diffusion for material transport, such as nutrient uptake or signal exchange. Diffusion is slow if the distance is long. Many bacterial species use rotating flagella for locomotion. While they move, flagellated bacteria stir their fluidic environment and generate microscale flows. An interesting question is, can bacteria make use of the flagellum-driven flows for long-distance transport? Here we show that flagellated bacteria in colonies can achieve this goal via self-organization (Movie 1 , Movie 2 , Movie 3). This new mechanism of long-range active transport may bring profound effect on the physiology of bacterial communities in structured environments with heterogeneous nutrient distribution, such as those commonly found in natural and clinical settings. The finding also highlights the physiological significance of phenotypic variability in microbial communities.

References:
  1. Haoran Xu, Justas Dauparas, Debasish Das, Eric Lauga, Yilin Wu (2019) Self-organization of swimmers drives long-range fluid transport in bacterial colonies. Nature Communications. 10, 1792.

Collective oscillation without inherent local oscillators

Our lab discovered a new form of biological collective motion: Millions of motile cells in dense bacterial suspensions can self-organize into highly robust collective oscillatory motion, while individuals move in an erratic manner. This ‘weak synchronization’ phenomenon presents a novel mechanism of oscillatory behavior in multicellular systems and constitutes a new type of ordered active matter. We used a series of experiments to demonstrate that the self-organized collective oscillatory motion may result from spontaneous symmetry breaking mediated by purely local interactions between individual cells; this idea, emanating from our experimental findings, has found support from a mathematical model consisting of Vicsek-type self-propelled particles developed our theoretical collaborators.

Collective oscillatory behavior is ubiquitous in nature and it plays a vital role in many biological processes. Collective oscillations in multicellular systems studied to date often arise from long-range coupling between individual cells that display inherent oscillations. In stark contrast, the collective oscillation we discovered does not require long range coupling, nor even inherent oscillation of individual cells. Instead, it emerges from averaging large numbers of erratic but weakly-coupled trajectories of single bacteria. The collective oscillation may have profound effect on the formation and structure of bacterial biofilms, and will provide new insights for understanding the physics of self-organization in non-equilibrium systems.

References:
  1. Chong Chen*, Song Liu*, Xiaqing Shi, Hugues Chaté, Yilin Wu (2017) Weak synchronization and large-scale collective oscillation in dense bacterial suspensions. Nature. doi:10.1038/nature20817. (*co-first authors)

Cohesive swimming of bacteria in two-dimensional confinement

Characterizing bacterial interactions in 2D confinement will help to understand diverse microbial processes, such as bacterial swarming and biofilm formation. We discovered a novel form of interaction between flagellated bacteria in 2D confinement: When two or multiple nearby cells align their moving directions, they tend to swim side-by-side cohesively without direct cell body contact, as a result of short-range hydrodynamic interaction. We further found that cells in cohesive swimming move with higher directional persistence, which can increase the effective diffusivity of cells by ~3 times as predicted by computational modeling. The higher directional persistence may promote bacterial dispersal in unsaturated soils and in interstitial space during infections. See: [Movie 1]. [Movie 2]. [Movie 3].

References:
  1. Ye Li*, He Zhai*, Sandra Sanchez, Daniel B. Kearns, Yilin Wu. (2017) Non-contact cohesive swimming of bacteria in two-dimensional liquid films. Phys. Rev. Lett. 119, 018101 (*co-first authors)


Past research (before joining CUHK):

When grown on moist surfaces, many flagellated bacteria are able to form a densely packed colony in which millions or more cells move across the surface in a manner called swarming; see this Movie. The swarming colony is enclosed by a thin layer of fluid produced by cells themselves, which provide the milieu to support cell motility. The spreading of swarm fluid is critical to bacterial colonization. Our flow measurements and modeling suggest that osmotic flows sustain the spreading of swarm fluid and fuel the expansion of swarming colonies.

References:
  1. Wu Y & Berg, H. C. (2012) Water reservoir maintained by cell growth fuels the spreading of a bacterial swarm. PNAS 109(11), 4128–4133.
  2. Ping L, Wu Y, Hosu BG, Tang JX, Berg HC (2014) Osmotic pressure inside a bacterial swarm. Biophysical Journal 107:871-878.

Swarms of flagellated bacteria are swimming within a thin layer of fluid. The swarm fluid is essential for the operation of flagellar motility and for the transport of nutrients or chemical signals. Swarm fluid is only a few microns in thickness, posing great challenges to probe its motion. We found a unique way to generate microscopic bubbles and used these bubbles as tracers for thin-film flows. Using these bubble tracers we discovered a micro-scale river running clockwise (when viewed from above) around the edge of bacterial swarms, which may provide an avenue for long range communication.

References:
  1. Wu Y, Hosu BG, Berg HC (2011) Microbubbles reveal chiral fluid flows in bacterial swarms. PNAS 108(10): 4147-51.

Collective motion is ubiquitous in biological systems. It is of interest to physics and engineering. Despite the vast differences in length scale and propulsion mechanism, collective motion across biological systems shares a similar feature: Global order arises spontaneously from local interactions between individuals that do not have access to global information. To understand how the traits of individuals give rise to the emergent behavior at population level, we turned to microorganisms because they have a simple behavioral repertoire. We chose to study the collective motion of myxobacteria (Myxococcus xanthus), a well characterized soil bacterium that forms beautiful fruiting bodies. Our modeling results suggested that myxobacteria evolved an unusual behavior to organize efficiently in a crowded environment. The cells reverse moving directions regularly, which facilitates local orientational ordering. We predicted an optimal reversal frequency for this orientational ordering in agreement with the experimental value.

References:
  1. Wu Y, Jiang Y, Kaiser D, Alber M (2007) Social interactions in myxobacterial swarming. PLoS Comput Biol 3(12): e253.
  2. Wu Y, Kaiser D, Jiang Y, Alber M (2009) Periodic reversal of direction allows myxobacteria to swarm. Proc Natl Acad Sci USA 106(4): 1222–1227.