Anomalous size-dependence of bacterial diffusion in a micropillar array

Microorganisms are endowed with phenotypic variations, including diversities in sizes within the same species, which are crucial to their adaptation to living habitats. Here, we investigate how such size variations affect the transport of bacteria in a structured medium, using an array of microscale pillars and a smooth-swimming mutant of Escherichia coli. In contrast to the common belief that a smaller object can navigate solid obstacles more efficiently, we find that the long-time diffusion of individual E. coli actually decreases with decreasing cell size. By varying the pillar geometries, we determine that such anomalous diffusion is governed by the cell size relative to the pillar curvature: cells with smaller sizes relative to the pillar radius are more easily attracted to the pillar surface and are thus effectively “trapped.” We show that such an attractiveness can be well characterized by a size-dependent residency time that the bacterium spends near the pillar surfaces. We develop an agent-based model that purely relies on the geometry of the micropillars, bacteria and residency time. The numerical model agrees reasonably well with our experimental observations, suggesting that solid structures can affect bacterial transport by purely geometric mechanisms.