Speaker
Description
When a bacterial population expands by growth, access to new territory and nutrients at the expanding front becomes the determinant of the competition. In (quasi) two-dimensional (2D) range expansion, bacterial populations spread across flat surfaces. When fluorescently labelled bacteria of different colours are inoculated, random fluctuations lead to the formation of distinct sectors even without differences in their fitnesses, and the sectors at the front coarsen as the population expands [1]. This range expansion setup provides a flexible framework to test the microbial competition in a structured environment.
We present how a mixture of bacteria E. coli and temperate phage λ form a pattern [2]. When λ phages infect an E. coli cell, it can choose either the lytic cycle to kill the cell and produce several progeny phages or the lysogeny, where the lambda phage integrates its genome to E. coli genome, provide immunity to further infection by λ phage and replicate its genome as E. coli replicates. We use a lattice model to study what is necessary to produce the experimentally observed pattern and how the infection can keep up with the expanding front. Especially we find that mechanical pushing due to growing cells inside the colony is a necessary factor to consider.
We then briefly present our recent work, demonstrating that elongated cells always take over the range expansion when competing against round cells, even if they start as a minority [3]. Numerical simulation and single-cell observation show that the collective nematic ordering of elongated cells by mechanical interaction is the key to the takeover mechanism: Groups of locally aligned bacteria form "nematic arms", bridging the central region of the colony to the expanding front. Once at the front, bacteria align parallel to it and block shorter bacteria from accessing nutrients and space. Overall, our results illustrate the importance of mechanical interactions in microbial competition in expanding colonies.
[1] O. Hallatschek, P. Hersen, S. Ramanathan, DR Nelson. Genetic drift at expanding frontiers promotes gene segregation. Proc. Nat. Acad. Sci. 2007, 104(50):19926-30.
[2] Stephenson, G., Nadig, A., Parmar, T., Krishnamurthy, V., Mitarai, N., Krishna, S. and Thutupalli, S., 2022. Co-existence and extinction due to surfing viral infections in a spatially expanding bacterial colony. In APS March Meeting Abstracts (Vol. 2022, pp. T05-007).
[3] N. van den Berg, K. Thijssen, T. T. Nguyen, A. Sarlet, M. Cordero, A. G. Vázquez, N. Mitarai, A. Doostmohammadi, L. Jauffred. Emergent collective alignment gives competitive advantage to longer cells during range expansion. BioRxiv: 2024.01.26.577059; doi: https://doi.org/10.1101/2024.01.26.577059
