Strong Confinement of Swarming Bacteria on Surfaces

Bacterial swarming is a rapid, collective movement of bacteria over a surface powered by rotating flagella [1]. Unlike swimming, swarming takes place in thin liquid films, constraining bacteria to two dimensions. So far, it always has been assumed that the film is thicker than the diameter of a bacterium, but its exact thickness has never been measured. Here we present a novel method for measuring film thickness with tens of nanometer precision. For Bacillus subtilis colonies grown on agar gels, we find film thicknesses as thin or even thinner than the diameter of a single bacterium. For thicknesses thinner than a single bacterium, surface tension forces are expected to be on the order of tens of nanonewtons. Considering swimming bacteria experience drag force of about half a piconewton [2], surface associated bacteria face orders of magnitude stronger resistive force even if we assume small frictional coefficients of between bacteria and the agar gel surface. It remains unclear how flagella driven motility can be achieved under such strong confinement.
[1] Kearns, D. "A field guide to bacterial swarming motility." Nature Reviews Microbiology 8(9) (2010): 634.
[2] Chattopadhyay, S. et al. "Swimming efficiency of bacterium Escherichia coli." PNAS 103(37) (2006): 13712-13717.