Life History Analysis of Integrative and Conjugative Element Activation in Growing Microcolonies of Pseudomonas

Abstract

Integrative and conjugative elements (ICE) are in some ways parasitic mobile DNA that propagate vertically through replication with the bacterial host chromosome but at low frequencies can excise and invade new recipient cells through conjugation and reintegration (horizontal propagation). The factors that contribute to successful horizontal propagation are not very well understood. Here, we study the influence of host cell life history on the initiation of transfer of a model ICE named ICEclc in bacteria of the genus Pseudomonas. We use time-lapse microscopy of growing and stationary-phase microcolonies of ICEclc bearing cells in combination with physiological staining and gene reporter analysis in stationary-phase suspended cells. We provide evidence that cell age and cell lineage are unlikely to play a role in the decision to initiate the ICEclc transfer program. In contrast, cells activating ICEclc show more often increased levels of reactive oxygen species and membrane damage than nonactivating cells, suggesting that some form of biochemical damage may make cells more prone to ICEclc induction. Finally, we find that ICEclc active cells appear spatially at random in a microcolony, which may have been a selective advantage for maximizing ICEclc horizontal transmission to new recipient species.

The hidden life of integrative and conjugative elements

“…These might constitute selected evolved reactions from the ICE to ‘save’ itself from host cells that are in bad shape (e.g. SOS response), have been subject to some form of other biochemical damage (Reinhard and van der Meer 2014 ) or have lost their ICE (Fig. 6C ) (Babic et al.…”

“… 2011 ). These concepts are still very difficult to study at single cell level and in many cases we simply do not know the actual reason for the onset of ICE activation, which might even be completely stochastic (Reinhard and van der Meer 2014 ). Transfer rates in the environment are further controlled by a variety of processes, such as contact possibilities of donor to recipient cells, donor cells not reaching the appropriate physiological state for transfer because of poor environmental conditions (e.g.…”

“…Incidentally, we showed that the ICE might even have evolved to cope with regularly encountered recipient cell shortage in the environment. For example, although division of ICE clc transfer competent cells becomes strongly impaired, their limited division (two to three times) allows them to form small microcolonies that have a higher probability to encounter potential sparse recipient cells (Reinhard and van der Meer 2014 ) (Fig. 6A ).…”

Highly variable individual donor cell fates characterize robust horizontal gene transfer of an integrative and conjugative element

“…This limited cell division leads to the formation of small groups of tc donor cells, which are thought to benefit the overall transfer success of ICEclc by increasing the chance to contact recipients (31).…”

Transient Replication in Specialized Cells Favors Conjugative Transfer of a Selfish DNA Element

“…This fitness loss at population level is limited because of the small subpopulation size of tc cells, but these are highly effective in transferring ICE clc 14 . Beneficial for transfer is the induced formation by ICE clc of small tc cell groups that have an increased chance to contact recipients 15 .…”