Second-Generation Transfer Mediates Efficient Propagation of ICE<i>Bs1</i> in Biofilms.

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<i>Bs1</i> acquisition. Also, using a fluorescently marked ICE<i>Bs1</i>, we observed that conjugation appears restricted to clusters of bacteria in a close neighborhood in which a high level of ICE<i>Bs1</i> 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<i>Bs1</i> 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. <b>IMPORTANCE</b> 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<i>Bs1</i>. Using the Gram-positive Bacillus subtilis, we observed that the main actors of ICE<i>Bs1</i> propagation in biofilms are the newly formed transconjugants that allow rapid transfer of ICE<i>Bs1</i> to new recipients. Our study provides a better understanding of the spatiotemporal dynamic of conjugative transfer in biofilms.