It's complicated: relationships between integrative and conjugative elements and their bacterial hosts

Integrative and conjugative elements (ICEs) are typically found integrated in a bacterial host chromosome. They can excise, replicate, and transfer from cell to cell. Many contain genes that confer phenotypes to host cells, including antibiotic resistances, specialized metabolisms, phage defense, and symbiosis or pathogenesis determinants. Recent studies revealed that at least three ICEs (ICEclc, Tn916, and TnSmu1) cause growth arrest or death of host cells upon element activation. This review highlights the complex interactions between ICEs and their hosts, including the recent examples of the significant costs to host cells. We contrast two examples of killing, ICEclc and Tn916, in which killing, respectively, benefits or impairs conjugation and emphasize the importance of understanding the impacts of ICE–host relationships on conjugation. ICEs are typically only active in a small fraction of cells in a population, and we discuss how phenotypes normally occurring in a small subset of host cells can be uncovered.