Second-generation transfer mediates efficient propagation of ICEBs1 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 ICEs propagation in bacterial biofilms, which are multicellular structures ubiquitous in the natural and clinical environment. Using a fluorescently marked ICEBs1, we report here that its propagation in biofilms is favored in recipient cells expressing the biofilm matrix. Also, conjugation appears restricted to clusters of bacteria in a close neighborhood in which a high level of ICEBs1 transfer occurs. These conjugative clusters are heterogenously distributed in the biofilm, forming close to the air-biofilm interface. Importantly, we established that transconjugant cells are the main contributors to ICEBs1 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. ImportanceThe 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 integrative and conjugative element (ICE) ICEBs1. Using the Gram-positive B. subtilis, we observed that the main actors of ICEBs1 propagation in biofilms are the newly formed transconjugants that allow rapid transfer of ICEBs1 to new recipients. Our study provides a better understanding of the spatiotemporal dynamic of conjugative transfer in biofilms.