Integrative Conjugative Element ICE-βox Confers Oxidative Stress Resistance to Legionella pneumophila In Vitro and in Macrophages

Integrative conjugative elements (ICEs) are mobile blocks of DNA that can contribute to bacterial evolution by self-directed transmission of advantageous traits. Here, we analyze the activity of a putative 65-kb ICE harbored by Legionella pneumophila using molecular genetics, conjugation assays, a phenotype microarray screen, and macrophage infections. The element transferred to a naive L. pneumophila strain, integrated site-specifically, and conferred increased resistance to oxacillin, penicillin, hydrogen peroxide, and bleach. Furthermore, the element increased survival of L. pneumophila within restrictive mouse macrophages. In particular, this ICE protects L. pneumophila from phagocyte oxidase activity, since mutation of the macrophage NADPH oxidase eliminated the fitness difference between strains that carried and those that lacked the mobile element. Renamed ICE-βox (for β-lactam antibiotics and oxidative stress), this transposable element is predicted to contribute to the emergence of L. pneumophila strains that are more fit in natural and engineered water systems and in macrophages.