Bacterial Growth Dynamics at Fluid-granular Interfaces*

Bacteria in nature often live in both unconstrained fluid environments and in spatially-constrained granular media. Previous studies show that bacteria exhibit different growth and competitive dynamics than what is observed in unconstrained environments. These studies are typically conducted in artificially created spaces, such as microfluidic devices, which feature highly simplified geometries. However, in nature, bacteria often have the ability to move from one environment to another, such as from the sand at the bottom of a pond into the water above, or vice versa. Therefore, we aim to investigate how bacterial growth proceeds in environments that replicate the highly heterogeneous geometries they confront in nature. To do so, we created a lab-controlled environment that features a column of unconstrained fluid over a pile of granular materials, which creates an environment where living space, nutrients, and movement are limited. We utilize a fluorescent strain of E. coli to observe growth dynamics. We observe the bacteria on micro- and macro-scales using microscopy and microbiology lab techniques. We found that when given the option of unconstrained fluid and highly constrained granular materials, our E. coli strain preferentially attaches to granular media. As a result, they fail to utilize all the nutrients in their environment and thus grow less in the presence of rather than in the absence of granular media. We find that bacteria in the presence of granular materials grow to at most 70% of the size of a population grown with the same amount of growth media but without granular materials. Furthermore, we found that cells are resistant to mechanical shearing (shaking) and viscous forces implying that cells adhere to the granular material by self imposed forces despite the presence of abundant nutrients available elsewhere.