SUMMARY Several intracellular pathogens including Salmonella enterica and Mycobacterium tuberculosis require the virulence protein MgtC to survive within macrophages and to cause a lethal infection in mice. We now report that, unlike secreted virulence factors that target the host vacuolar ATPase to withstand phagosomal acidity, the MgtC protein acts on Salmonella's own F1Fo ATP synthase. This complex couples proton translocation to ATP synthesis/ hydrolysis and is required for virulence. We establish that MgtC interacts with the a subunit of the F1Fo ATP synthase, hindering ATP-driven proton translocation and NADH-driven ATP synthesis in inverted vesicles. An mgtC null mutant displays heightened ATP levels and an acidic cytoplasm whereas mgtC overexpression decreases ATP levels. A single amino acid substitution in MgtC that prevents binding to the F1Fo ATP synthase abolishes control of ATP levels and attenuates pathogenicity. MgtC provides a singular example of a virulence protein that promotes pathogenicity by interfering with another virulence protein.

Organisms must maintain physiological levels of Mg2+ because this divalent cation is critical for the stabilization of membranes and ribosomes, the neutralization of nucleic acids, and as a cofactor in a variety of enzymatic reactions. In this review, we describe the mechanisms that bacteria utilize to sense the levels of Mg2+ both outside and inside the cytoplasm. We examine how bacteria achieve Mg2+ homeostasis by adjusting the expression and activity of Mg2+ transporters, and by changing the composition of their cell envelope. We discuss the connections that exist between Mg2+ sensing, Mg2+ transport and bacterial virulence. Additionally, we explore the logic behind the fact that bacterial genomes encode multiple Mg2+ transporters and distinct sensing systems for cytoplasmic and extracytoplasmic Mg2+. These analyses may be applicable to the homeostatic control of other cations.

Bacteria can withstand killing by bactericidal antibiotics through phenotypic changes mediated by their preexisting genetic repertoire. These changes can be exhibited transiently by a large fraction of the bacterial population, giving rise to tolerance, or displayed by a small subpopulation, giving rise to persistence. Apart from undermining the use of antibiotics, tolerant and persistent bacteria foster the emergence of antibiotic-resistant mutants. Persister formation has been attributed to alterations in the abundance of particular proteins, metabolites, and signaling molecules, including toxin-antitoxin modules, adenosine triphosphate, and guanosine (penta) tetraphosphate, respectively. Here, we report that persistent bacteria form as a result of slow growth alone, despite opposite changes in the abundance of such proteins, metabolites, and signaling molecules. Our findings argue that transitory disturbances to core activities, which are often linked to cell growth, promote a persister state regardless of the underlying physiological process responsible for the change in growth.

Summary The synthesis of ribosomes is regulated by both amino acid abundance and the availability of adenosine triphosphate (ATP), which regenerates guanosine triphosphate (GTP), powers ribosomes, and promotes transcription of ribosomal RNA genes. We now report that bacteria supersede both of these controls when experiencing low cytosolic magnesium (Mg2+), a divalent cation essential for ribosome stabilization and for neutralization of ATP’s negative charge. We uncover a regulatory circuit that responds to low cytosolic Mg2+ by promoting expression of proteins that import Mg2+ and that lower ATP amounts. This response reduces the levels of ATP and ribosomes, making Mg2+ ions available for translation. Mutants defective in Mg2+ uptake and unable to reduce ATP levels accumulate non-functional ribosomal components and undergo translational arrest. Our findings establish a new paradigm whereby cells reduce the amounts of translating ribosomes to carry out protein synthesis.

Cellulose is the most abundant organic polymer on Earth. In bacteria, cellulose confers protection against environmental insults and is a constituent of biofilms typically formed on abiotic surfaces. We report that, surprisingly, Salmonella enterica serovar Typhimurium makes cellulose when inside macrophages. We determine that preventing cellulose synthesis increases virulence, whereas stimulation of cellulose synthesis inside macrophages decreases virulence. An attenuated mutant lacking the mgtC gene exhibited increased cellulose levels due to increased expression of the cellulose synthase gene bcsA and of cyclic diguanylate, the allosteric activator of the BcsA protein. Inactivation of bcsA restored wild-type virulence to the Salmonella mgtC mutant, but not to other attenuated mutants displaying a wild-type phenotype regarding cellulose. Our findings indicate that a virulence determinant can promote pathogenicity by repressing a pathogen's antivirulence trait. Moreover, they suggest that controlling antivirulence traits increases long-term pathogen fitness by mediating a tradeoff between acute virulence and transmission.biofilm | magnesium | ATP