SURFACE WAVES IN A BACTERIAL COLONY AS AN ACTIVE NEMATIC FLUID

One of the characteristic collective phenomena displayed by the bacterium Myxococcus xanthus is its ability to self-organize into wave-like structures on the surface of dense colonies. Previous work on the three-dimensional characterization of these waves have demonstrated that they have wavelengths of several times a single cell length and amplitudes of 6 to 20 cell layers in height. We demonstrate that the wavelength can be modulated by changing the interfacial surface tension of the fluid that surrounds colonies through the use of surfactants and different substrates. Furthermore, we investigate the internal structure of waves using dual view light sheet microscopy. We image volumes of dense cells with high isotropic resolution and find that cells on the surface of the waves align their long axes with the 3D structure of the waves. These observations suggest that colonies of M. xanthus behave as a three-dimensional active nematic fluid that forms surface waves when affected by capillary forces from the surrounding fluid. Consequently, we present an active nematic model that incorporates substrate and capillary interactions with the cell colony to produce surface waves. This model recapitulates our measurements of wavelength modulation with respect to surface tension perturbations. This work not only comprehensively describes the governing mechanics of surface wave formation in M. xanthus colonies, but also extends our understanding of collective behaviors in a three-dimensional regime.