Events

4:00-5:00pm Chen 130
5:00-5:30pm Socializing (snacks and drinks will be provided)

"Pore-scale dynamics of bacterial populations in unsaturated porous media"

Soil harbors much of Earth's bacterial life, governing plant growth, nutrient cycling, and the land's climate response. Yet bacteria live inside an opaque, three-phase medium of grains, water, and air, which most measurements destroy or flatten to two dimensions. We build transparent soil mimics from refractive index-matched grains that reproduce soil pore geometry, and image bacteria in the pore space by confocal microscopy. Pores and cells are segmented in the same volume, linking geometry to occupancy. We quantify growth and settling in saturated packings, and the reorganization of populations during drying, as the liquid–air interface sweeps cells into the smallest pores.

"Desiccation driven mutations in a model soil taxon identified from metagenomic data"

Soil microbes are vital to agriculture and regularly experience desiccation in the environment. At the phylum level, desiccation is widely understood to select for monoderm over diderm bacteria, which have thicker cell walls. However, many physiological parameters contribute to desiccation tolerance, and selection at the strain level is less well studied. Leveraging metagenomic data from a 10-year precipitation experiment, I have identified amino acid mutations enriched under desiccation in members of the Bradyrhizobium genus, including within the model strain B. diazoefficiens USDA 110. Molecular genetics will enable further experimentation to determine the functional relevance of these mutations.

Special ChE Seminar

Nir Livne (Hebrew University)
Faculty host: Sujit Datta

"Chemotaxis drives rapid bacterial community formation"

Abstract: It is often beneficial for bacteria to live in communities, but how do these communities form? I will show that chemotaxis—the directed movement of motile bacteria along chemical gradients—enables rapid community formation. By locally modifying their chemical environment, cells generate gradients that they themselves chemotactically follow, effectively attracting one another; this self-attraction drives the spontaneous condensation of a population into macroscopic focal points composed of hundreds of thousands of individuals. In swimming E. coli, such condensates can form far faster than the timescale of cell multiplication, allowing populations to collectively mitigate stress and seed biofilm-like aggregates. Yet swimming proves to be a double-edged sword: as cells accumulate, their density becomes high enough to trigger correlated swimming and active turbulence. This collective motion generates macroscopic fluid flows with an outward component directed down the cell-density gradient, broadening the condensate and fundamentally limiting further accumulation. In porous environments, however, these flows are suppressed, allowing cells to self-organize into extremely dense communities. Preliminary results suggest that under growth conditions these communities further develop into stable, macroscopic structures and may improve the cells' antibiotic tolerance.

Biography: Nir Livne is a Milner Fellow and physics PhD candidate at the Hebrew University, studying bacterial chemotaxis in Prof. Ady Vaknin's lab. He completed his B.Sc. in physics through Amirim (the Faculty of Science's excellence program) and his M.Sc. in physics at the Hebrew University, and is a recipient of the Racah Prize for Excellence in Teaching.