F DELAVAT¹, S MITRI¹, S PELET¹, JR VAN DER MEER<sup>1

1</sup>Department of Fundamental Microbiology - University of Lausanne - Switzerland

Integrative and Conjugative Elements (ICEs) are widespread mobile DNA. ICE are integrated in the host genome and are maintained through vertical descent. Occasionally, they excise and form a circular intermediate, which can conjugate to recipient cells and re-integrate into the genome at one or more specific sites. It is poorly understood what the consequences of such occasional excision and conjugation are for "donor" cells that harbor the ICE.
We have previously shown how a subpopulation of 3 to 5% of cells of Pseudomonas knackmussii B13 in stationary phase conditions becomes "transfer competent" (tc) for conjugation of an ICE called ICEclc. In this study, we developed a specific single-cell tracking tool to study the fate of individual donor cells during ICEclc transfer. The tool is based on differential activation of fluorescent proteins coupled to ICEclc promoters and can differentiate initial activation of tc development in cells, ICEclc excision, transfer and integration. Results of time-lapse microscopy from both donors alone or donors mixed with recipients showed that while ICEclc is activated when cells enter in stationary phase, excision and transfer predominantly occur when cells obtain new carbon substrate. Although tc donor cells can still divide a few times, division becomes very irregular and half of the individual tc donors arrest growth altogether or lyse. ICEclc also shows a variety of fates in donor cells: in some but not all cells ICEclc excises, in some it replicates during or after transfer, whereas in others it is lost. A probabilistic model was developed to explain the different cell fates. Interestingly, the model predicts lower ICE-fitness if the proportion of cells activating tc development increases from 5% to 20%, or if activation occurs already during growth and not in stationary phase. Our results highlight a new facet of ICE life-style and shed a new light on bacterial cellular differentiation during horizontal gene transfer.