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Many strains from the Bacillus subtilis species complex exert strong plant growth-promoting activities. However, their efficacy in relevant conditions is variable, due in part to their inability to establish a strong interaction with roots in stressful environmental conditions. Adaptative laboratory evolution (ALE) is a powerful tool to generate novel strains with traits of interest. Many Bacillus evolved isolates, stemming from ALE performed with plants, possess a stronger root colonization capacity. An in-depth analysis of these isolates also allowed the identification of key features influencing the interaction with plant roots. However, many variables can influence the outcome of these assays, and thus, caution should be taken when designing ALE destined to generate better root colonizers., (© 2024 The Author(s). Microbial Biotechnology published by John Wiley & Sons Ltd.)

Horizontal gene transfer (HGT) by integrative and conjugative elements (ICEs) is an important mechanism in the spread of antibiotic resistance genes. However, little is known about the spatiotemporal dynamic of ICE propagation in bacterial biofilms, which are multicellular structures ubiquitous in natural and clinical environments. We report here that a high level of biofilm matrix production favors ICE Bs1 acquisition. Also, using a fluorescently marked ICE Bs1 , we observed that conjugation appears restricted to clusters of bacteria in a close neighborhood in which a high level of ICE Bs1 transfer occurs. These conjugative clusters are heterogeneously distributed in the biofilm, forming close to the air-biofilm interface. Importantly, we established that transconjugant cells are the main contributors to ICE Bs1 propagation in biofilms. Our findings provide a novel spatiotemporal understanding of ICEs propagation in biofilms, which should have an important role in our understanding of horizontal gene transfer in relevant settings. IMPORTANCE The transfer of mobile genetic elements between bacteria is the main cause of the spread of antibiotic resistance genes. While biofilms are the predominant bacterial lifestyle both in the environment and in clinical settings, their impact on the propagation of mobile genetic elements is still poorly understood. In this study, we examined the spatiotemporal propagation of the well-characterized ICE Bs1 . Using the Gram-positive Bacillus subtilis, we observed that the main actors of ICE Bs1 propagation in biofilms are the newly formed transconjugants that allow rapid transfer of ICE Bs1 to new recipients. Our study provides a better understanding of the spatiotemporal dynamic of conjugative transfer in biofilms.