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Bacteria adaptability is closely associated with horizontal gene transfer (HGT), often facilitated by mobile genetic elements that contribute to the plasticity of bacterial genomes. While the mechanisms underlying HGT are well-documented, the corresponding regulatory systems and the impact of environmental signals on gene transfer are less understood.

Here, we focus on the Integrative and Conjugative Element ICEclc. We investigate the environmental signals influencing its activation. ICEclc carries genes responsible for the degradation of pollutants, notably the aromatic compound 3-chlorobenzoate (3CBA). Its activation is limited to a small subset of ‘transfer competent' (tc) cells. Interestingly, the activation rate peaks in stationary phase for cells grown with 3CBA (2-5% of cells), and appears to be specific, as few tc cells are detected with the related compound benzoate. Growth with 5 mM 3CBA also induces a high level of intracellular oxidative stress, which is not observed with benzoate.

Our primary hypothesis is that the high ROS levels detected in 3CBA-grown cells influence tc cell formation, either directly through the activation of ICEclc regulators or indirectly through other cellular factors such as RpoS. Specifically, we propose that cells experiencing higher-than-average oxidative stress are more likely to initiate the tc program. To test this hypothesis, we used double-reporter strains, with one fluorescent gene fused to an ICEclc promoter to serve as a proxy for its activation, and the other fused to one of six selected oxidative stress response promoters. We quantified both fluorescence signals at the single-cell level using flow cytometry as a function of growth substrate, and employed time-lapse microscopy to assess a potential temporal link between oxidative stress and tc appearance. Our results suggest that tc cells preferentially appear in cells with a higher oxidative stress response, which could be linked to a higher oxidative stress response experienced by their lineage ancestors.