Microbes in motion: a characterization of the surface properties conducive to bacterial swarms



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Under the right conditions, bacteria can collectively move across a surface in a whirling, flagella-driven motion reminiscent of flocking birds or schooling fish. Known as a bacterial swarm, this phenomenon is exhibited by many different species and is the fastest way bacteria can colonize a surface. Yet, the prevalence of bacterial swarms in the wild is unknown, in part because the surface properties conducive to swarming have not been well characterized. This has made it difficult to identify the natural surfaces that might be conducive to swarming. In particular, the amount of liquid required for swarming and the effects of swarming biproducts on surface conditions have been misunderstood. The surface tension and stiffness of substrates used in swarming experiments have never been measured, nor has the sensitivity of swarms to gradual changes in their local environments, although it’s clear from experiments that even slight changes can stop a swarm. In the work presented below, I use confocal microscopy to measure the liquid profile at the front of swarms for four representative species, showing that swarms generally move along dry surfaces using only the liquid that collects around them via capillary action. I then use shadowgraphy to track the spread of surfactants from colonies of Bacillus subtilis and show that it occurs in two phases and that the rate of surfactant expansion is highest on moist and thin surfaces. To measure the effects of surfactants on a substrate, I use a custom-build cantilever and a nanoindenter to measure the changes in surface tension between surfactant-free agar gel and surfactant-covered agar gel and find that the surfactant produced by B. subtilis can reduce surface tension by nearly 40%. I also use nanoindentation to measure the Young’s Modulus of substrates conducive to swarming in B. subtilis and find that at around 8 kPa, B. subtilis cells sharply transition to a different kind of collective surface motility called sliding. Together, these results characterize the surface properties conducive to bacterial swarms.



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