Shedding light on bacterial fitness in a tug-of-war with liquid crystal emulsions.

Rapid and reliable quantification of bacterial dynamics at the cellular level is critical for pathogen sensing, live-dead bacterial assays, and monitoring of bacteria fitness and viability. Here, we demonstrate bacterial fitness quantification by capturing individual cells on topological defects of micro-scale liquid crystal emulsion droplets. The emulsion droplets are composed of phase-separated nematic liquid crystal and fluorocarbon components and exhibit an asymmetric mass distribution. A topological singularity in the director field of the liquid crystal phase localizes tailormade surfactants that tether a single bacterium per droplet. Active motion of the bacterium induces a tilt and azimuthal rotation of the droplet trap, which is counteracted by gravity acting on the droplet center of mass. By comparing the observed dynamics of a tethered bacterium's stochastic movement to a computational model of bacterial motion on spherical surfaces that is based on the classical Ornstein-Uhlenbeck process, 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 bacterial cell 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.