HISTORY-DEPENDENT BEHAVIOUR IN THE GALACTOSE REGULON OF ESCHERICHIA COLI

Abstract

Bacteria routinely respond to environments that change with nutrient availability being a key factor contributing to these changes. Nutrient acquisition is a top priority for all microorganisms, driving selection of species that can quickly respond to changes in the nutrients surrounding them. When nutritional changes are predictable, bacteria can evolve to anticipate them. In these cases, an organism’s past environment can shape its response to its current environment, an example of a history-dependent behaviour (HDB). While, HDBs appear to regulate a variety of biological processes, the selective pressures under which they evolve remain poorly understood. Here, I explore how bacteria evolve new HDBs using a directed evolution approach. In each experiment, bacteria were exposed to a short pulse of one carbon source, followed by a longer incubation in a second, unrelated carbon source. The goal was to select for strains that learned to use the first carbon source to anticipate the second. In the first trial, passaged strains grew faster in the second carbon source regardless of prior exposure to the first carbon source, indicating the evolution of a constitutive response. A second trial, designed to suppress constitutive phenotypes through new stimuli and stronger counterselection, instead revealed a pre-existing HDB related to galactose metabolism. This behaviour was found to be linked to growth phase, by which cells transferred to galactose media from exponential phase exhibited a growth advantage relative to those transferred from stationary phase, a phenotype not observed for any other carbon source tested. Growth phase-dependent priming of galactose metabolism was lost in backgrounds deficient in the galactose repressors, galR and galS, indicating their involvement in regulating this behaviour. This unique response to galactose builds on our v understanding of how bacteria use past environments to shape their responses to new stimuli. Although the adaptive rationale for this HDB remains unclear, it likely reflects the natural history of E. coli, whose primary niche (the mammalian gut) has shed light on how HDBs related to carbon metabolism have evolved in this organism.