The ICE-flox Integrative Conjugative Element of Legionella pneumophila.

Abstract

Legionella pneumophila is an accidental human pathogen that causes the bacterial pneumonia Legionnaire’s Disease. The bacteria are ubiquitous in freshwater environments and are spread by aerosolization of contaminated water from the built environment. Accessory traits carried on mobile genetic elements diversify the L. pneumophila and may contribute to persistence in stressful environments. One class of mobile elements are Integrative Conjugative Elements (ICEs) which encode cargo genes as well as type IV secretion system (T4SS) transfer apparatuses to direct their own transmission among a bacterial population. In this dissertation, I demonstrate that ICE-βox enhances L. pneumophila resistance to oxidative stresses encountered in vitro (such as bleach) and in macrophages. Specifically, this mobile element protects L. pneumophila from the toxic activities of the macrophage phagocyte oxidase. In addition to cargo genes predicted to repair oxidative damage, ICE-βox encodes a paralog of the master L. pneumophila life cycle regulator csrA. Bioinformatic analyses of 34 L. pneumophila ICE-associated T4SS reveals four families based on apparatus composition. Each T4SS family is genetically and phylogenetically linked with a distinct csrA paralog, suggesting functional interactions. Indeed, the ICE-βox csrA paralog csrT can repress ICE-βox traits as well as motility of the host bacterium. Finally, a preliminary epidemiologic survey identified ICE-βox in a majority of built environment L. pneumophila isolates. Accordingly, the hypothesis that chlorine-based disinfectants enrich for ICE-βox and increase L. pneumophila resilience and virulence warrants testing. By understanding the fitness advantages, regulation and prevalence of ICE-βox, disinfection protocols can be designed to eradicate persistent L. pneumophila and reduce its risk to humans.