Quantifying bacterial fitness in a tug-of-war with bi-phase liquid crystal emulsions

Rapid and reliable quantification of bacterial dynamics is critical for pathogen sensing and monitoring of bacterial fitness and viability. We present a system for bacterial fitness quantification based on trapping individual bacterial cells to microscale liquid crystal bi-phase emulsion droplets. A topological singularity in the liquid crystal director field serves as an anchoring point to localize custom surfactants that tether bacteria to the apex of emulsion droplets. Active swimming of a tethered bacterium induces a tilt and azimuthal rotation of the droplet, which is counteracted by the droplet's gravitational restoring torque. This restoring torque can be tuned by changing the mass ratio of phase-separated nematic liquid crystal and fluorocarbon used to form the bi-phase emulsion droplets, resulting in an asymmetric mass distribution. By comparing the observed dynamics of a tethered bacterium's stochastic motion to predictions of a computational model that captures the run-and-tumble dynamics of bacteria on spherical surfaces, we quantify the fitness of bacteria subjected to starvation over several days. This pathogen fitness sensing concept, which relies on the scalable chemical design of single bacteria traps, a robust optical readout, and a theoretical understanding of bacterial dynamics on spherical surfaces, offers opportunities for rapid pathogen activity assessment, micro-biological sensing, and biologically powered micro-actuator systems.