Metabolic Cues and Regulatory Proteins that Govern Legionella Pneumophila Differentiation and Virulence.

Abstract

In aquatic environments, the gram-negative bacterium Legionella pneumophila likely resides within biofilm communities. However, when protozoa ingest L. pneumophila, the microbes can establish an intracellular niche protected from digestion. Moreover, if humans inhale bacteria-laden aerosols, L. pneumophila can survive and replicate within alveolar macrophages to cause the pneumonia, Legionnaires’ disease. To persist within these diverse niches, L. pneumophila alternates between at least two phenotypic phases: a non-infectious, replicative form required for intracellular growth and an infectious, transmissive form that enhances survival in the extracellular milieu. By exploiting our knowledge of the L. pneumophila biphasic life cycle, we have uncovered novel metabolites and regulatory proteins that govern differentiation. Previous studies established that the LetA/LetS two-component system regulates host transmission through the regulator CsrA, while the stringent response coordinates transcription by producing the alarmone ppGpp. Through genetic, transcription and phenotypic analyses we determined that the architecture of the LetA/LetS two-component system enables L. pneumophila to customize its genotypic and phenotypic profiles. Moreover, our data suggest that the model of the flexible LetA/LetS regulon may be applicable to many two-component systems, and may equip microbes with a mechanism to fine-tune their traits. Since L. pneumophila resides within many different environments, we predicted that various metabolites cue its developmental switch. Using phenotypic microarrays we deduced that perturbations in fatty acid biosynthesis activate the stringent response via an interaction between SpoT and acyl carrier protein, which leads to L. pneumophila differentiation. We propose that by coupling phase differentiation to its metabolic state, L. pneumophila can adapt to environmental fluctuations or stresses encountered in its host, thereby enhancing its overall fitness. Data suggest that nicotinic acid can activate microbial two-component systems, and consequently, modulate the genes and phenotypes governed by these regulatory proteins. By comparing the transcriptional profiles of replicative and transmissive phase L. pneumophila to genes regulated by nicotinic acid, we identified a cohort of genes that are unique for this phenotypic modulator. Taken together, our data suggest that L. pneumophila acclimates to environmental stresses by monitoring metabolic fluctuations and employing a regulatory cascade that enables the bacteria to coordinate an appropriate response.