The cation diffusion facilitator family protein EmfA confers resistance to manganese toxicity in Brucella abortus 2308 and is an essential virulence determinant in mice
Johnsrude, Matthew; Pitzer, Joshua E.; Martin, Daniel W.; Roop, R. Martin
Brucella abortus is a Gram-negative bacterium that causes abortion and infertility in food animals and a chronic debilitating febrile disease in humans known as brucellosis. As with all pathogenic bacteria, the Brucella spp. require sufficient metal nutrition during the course of an infection. Host-mediated ‘metal withdrawal’ defenses actively restrict the bioavailability of metals which requires invading bacteria to employ high affinity metal acquisition systems to overcome these metal-limiting conditions. While obtaining sufficient metals during host infection is critical to the survival of these bacteria, avoiding metal toxicity is equally important. Excess accumulation of one metal relative to others can lead to protein mis-metallation when surplus metal ions outcompete other metal species for their native binding sites. To prevent metal toxicity, bacteria respond to high intracellular metal concentrations by means of metal-responsive transcriptional regulators that downregulate metal import systems, and exporters that remove excess intracellular metal. Manganese (Mn) is an essential micronutrient for Brucella strains, and the purpose of this study was to better define the cellular components that maintain Mn homeostasis and prevent Mn toxicity in these bacteria. The Mn-responsive repressor Mur downregulates the expression of mntH, the gene encoding the sole high affinity Mn importer in Brucella in response to increased intracellular levels of Mn. But phenotypic analysis of a B. abortus mur mutant suggests that Mur plays a minimal role in preventing Mn toxicity. Instead, an ortholog of the cation diffusion facilitator (CDF) type metal exporter EmfA, which prevents Mn toxicity in the close phylogenetic relative Rhizobium etli, is critical for preventing Mn toxicity in Brucella. The experimental findings of this study indicate that EmfA-mediated resistance to Mn toxicity plays a critical role in the virulence of Brucella strains, and suggests that the primary function of EmfA may be to maintain the proper intracellular balance of Mn in these bacteria during the course of infection.