Intracellular phase for an extracellular bacterial pathogen: MgtC shows the way.

is an extracellular pathogen known to impair host phagocytic functions. However, our recent results identify MgtC as a novel actor in virulence, which plays a role in an intramacrophage phase of this pathogen. In agreement with its intracellular function, gene expression is strongly induced when the bacteria reside within macrophages. MgtC was previously known as a horizontally-acquired virulence factor important for multiplication inside macrophages in several intracellular bacterial pathogens. MgtC thus provides a singular example of a virulence determinant that subverts macrophages both in intracellular and extracellular pathogens. Moreover, we demonstrate that MgtC is required for optimal growth in Mg deprived medium, a property shared by MgtC factors from intracellular pathogens and, under Mg limitation, MgtC prevents biofilm formation. We propose that MgtC has a similar function in intracellular and extracellular pathogens, which contributes to macrophage resistance and fine-tune adaptation to the host in relation to the different bacterial lifestyles. MgtC thus appears as an attractive target for antivirulence strategies and our work provides a natural peptide as MgtC antagonist, which paves the way for the development of MgtC inhibitors.