Social behavior and decision making in bacterial conjugation.

Bacteria frequently acquire novel genes by horizontal gene transfer (HGT). HGT through the process of bacterial conjugation is highly efficient and depends on the presence of conjugative plasmids (CPs) or integrated conjugative elements (ICEs) that provide the necessary genes for DNA transmission. This review focuses on recent advancements in our understanding of ssDNA transfer systems and regulatory networks ensuring timely and spatially controlled DNA transfer (tra) gene expression. As will become obvious by comparing different systems, by default, tra genes are shut off in cells in which conjugative elements are present. Only when conditions are optimal, donor cells-through epigenetic alleviation of negatively acting roadblocks and direct stimulation of DNA transfer genes-become transfer competent. These transfer competent cells have developmentally transformed into specialized cells capable of secreting ssDNA via a T4S (type IV secretion) complex directly into recipient cells. Intriguingly, even under optimal conditions, only a fraction of the population undergoes this transition, a finding that indicates specialization and cooperative, social behavior. Thereby, at the population level, the metabolic burden and other negative consequences of tra gene expression are greatly reduced without compromising the ability to horizontally transfer genes to novel bacterial hosts. This undoubtedly intelligent strategy may explain why conjugative elements-CPs and ICEs-have been successfully kept in and evolved with bacteria to constitute a major driving force of bacterial evolution.

Icm/Dot-dependent inhibition of phagocyte migration by Legionella is antagonized by a translocated Ran GTPase activator.

The environmental bacterium Legionella pneumophila causes a severe pneumonia termed Legionnaires' disease. L. pneumophila employs a conserved mechanism to replicate within a specific vacuole in macrophages or protozoa such as the social soil amoeba Dictyostelium discoideum. Pathogen-host interactions depend on the Icm/Dot type IV secretion system (T4SS), which translocates approximately 300 different effector proteins into host cells. Here we analyse the effects of L. pneumophila on migration and chemotaxis of amoebae, macrophages or polymorphonuclear neutrophils (PMN). Using under-agarose assays, L. pneumophila inhibited in a dose- and T4SS-dependent manner the migration of D. discoideum towards folate as well as starvation-induced aggregation of the social amoebae. Similarly, L. pneumophila impaired migration of murine RAW 264.7 macrophages towards the cytokines CCL5 and TNFα, or of primary human PMN towards the peptide fMLP respectively. L. pneumophila lacking the T4SS-translocated activator of the small eukaryotic GTPase Ran, Lpg1976/LegG1, hyper-inhibited the migration of D. discoideum, macrophages or PMN. The phenotype was reverted by plasmid-encoded LegG1 to an extent observed for mutant bacteria lacking a functional Icm/Dot T4SS.Similarly, LegG1 promoted random migration of L. pneumophila-infected macrophages and A549 epithelial cells in a Ran-dependent manner, or upon 'microbial microinjection' into HeLa cells by a Yersinia strain lacking endogenous effectors. Single-cell tracking and real-time analysis of L. pneumophila-infected phagocytes revealed that the velocity and directionality of the cells were decreased, and cell motility as well as microtubule dynamics was impaired. Taken together, these findings indicate that the L. pneumophila Ran activator LegG1 and consequent microtubule polymerization are implicated in Icm/Dot-dependent inhibition of phagocyte migration.