String theory: Unraveling the influence of saline environments on bacterial community structuring

Bacteria often grow in saline environments such as tissues/gels in hosts and bodies of water (e.g., salty lakes, marshes, and the ocean) in the environment. While lab studies typically focus on bacterial cells in idealized solutions of primarily monovalent salts, these saline environments often have an abundance of multivalent ions. Hence, we ask: How do multivalent ions change how bacteria grow? Here, we report that at sufficiently high concentrations of divalent salts, immotile E. coli do not grow as random dispersions of disconnected cells, as in the monovalent salt case; instead, they grow into string-like colonies comprising single cells arranged end-to-end. We conjecture that this unusual colony morphology arises due to divalent cation-mediated crosslinking of lipopolysaccharides at the surface of adjacent cells after division. These results highlight the pivotal role of environmental factors in reshaping bacterial colonies, with potential implications for their collective functioning. As an example of this point, we also present agent-based simulations that demonstrate how growth in string-like colonies alter the rates at which bacteriophages can locate and kill cells.