Physics Colloquium with Dr. Jay Tang

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Active Matter Physics of Bacterial Swarms

with Dr. Jay Tang Brown University

ABSTRACT: On nutrient-rich substrates, a droplet of motile bacteria can grow into a large swarm. Swarming is a prevalent form of bacterial collective motility, often viewed as a precursor to biofilms. The spreading of a bacterial swarm is facilitated by factors such as osmolytes and biosurfactants, secreted by the dense population of bacteria. Studying Pseudomonas aeruginosa, a human pathogen known to swarm and form biofilms, we identify and investigate a pattern that forms on the outer edge of a large circular area within hours of growth, i.e., a solid-like ring of dense bacteria. As the swarm expands, this ring stretches, cracks inside out, and breaks apart, leading to broken sections along the circumference and subsequently, finger-shaped protrusions. We developed a model based on the stress profile within the ring and applied cracking mechanics to predict how the number of cracks formed varies with the radius of the swarm. Our work demonstrates that solid-like behavior can play a dominant role in swarm morphology. This new mode of pattern development differs from those based on instability at the fluid front, which can also form fingers and have been modeled previously. Taken together, we now recognize that dense bacterial swarms straddle between the fluid and solid states, manifesting intricate properties of active matter with built-in growth and dynamics. Such dynamic states of bacteria are relevant to their physiological behaviors such as biofilm growth, resistance to antibiotics or other stress factors, and the spread of infection.

BIOGRAPHY: Professor Jay X. Tang received his BS in physics from Peking University and came to the US for graduate study through the China and the US Physics Examination and Application (CUSPEA) program established by Professor TD Lee, a Nobel Laureate in Physics. Prof. Tang received his PhD in Physics from Brandeis University. After postdoctoral training at Harvard Medical School, he became an assistant professor of physics at Indiana University in 1999. Since 2003, he has been a faculty member at Brown University. He is currently a professor of physics and engineering. Prof. Tang's research area is experimental biophysics, with topics covering biomolecular assembly, cell mechanics, motility of cells, bacterial swimming and swarming, and biofilms. Professor Tang has published over 100 original research papers and received research awards from the National Science Foundation, the National Aeronautics and Space Administration, and the National Institutes of Health. Prof. Tang is a Fellow of the American Physical Society. He serves as a Specialty Chief Editor for Frontiers in Soft Matter and an associate editor of MDPI Fluid Dynamics.