Brucella is an intracellular pathogen able to persist for long periods of time within the host and establish a chronic disease. We show that soon after Brucella inoculation in intestinal loops, dendritic cells from ileal Peyer's patches become infected and constitute a cell target for this pathogen. In vitro, we found that Brucella replicates within dendritic cells and hinders their functional activation. In addition, we identified a new Brucella protein Btp1, which down-modulates maturation of infected dendritic cells by interfering with the TLR2 signaling pathway. These results show that intracellular Brucella is able to control dendritic cell function, which may have important consequences in the development of chronic brucellosis. A key determinant for intracellular pathogenic bacteria to induce infectious diseases is their ability to avoid recognition by the host immune system. Although most microorganisms internalized by host cells are efficiently cleared, Brucella behave as a Trojan horse causing a zoonosis called brucellosis that affects both humans and animals. Here we show that pathogenic Brucella are able to target host cell defense mechanisms by controlling the function of the sentinels of the immune system, the dendritic cells. In particular, the Brucella TIR-containing protein (Btp1) targets the Toll-like receptor 2 activation pathway, which is a major host response system involved in bacterial recognition. Btp1 is involved in the inhibition of dendritic cell maturation. The direct consequence is a control of inflammatory cytokine secretion and antigen presentation to T lymphocytes. These bacterial proteins are not specific for Brucella and have been identified in other pathogens and may be part of a general virulence mechanism used by several intracellular pathogens to induce disease.

We used flow cytometry and confocal immunofluorescence microscopy to study the localization of Salmonella typhimurium in spleens of infected mice. Animals were inoculated intragastrically or intraper...

Recent advances in the understanding of the Brucella intracellular trafficking and bacterial factors required for a successful intracellular lifestyle

Intracellular pathogens have developed different mechanisms which enable their survival and replication within the host cells. Some survive and replicate within a membrane-bound vacuole modified by the bacteria to support microbial growth (e.g. Salmonella enterica serovar Typhimurium), whereas others escape from the vacuole into the host cell cytosol, where they proliferate (e.g. Listeria monocytogenes). In this study a Salmonella strain carrying a mutation in sifA which is released from the vacuole was used to analyse Salmonella survival and replication within the cytosol of several cell lines. It was found that Salmonella replicates within the cytosol of epithelial cells at a higher rate than that achieved when replicating within the vacuole, but is defective for replication in the cytosol of fibroblasts or macrophages. Using an aroC purD double mutant strain which does not replicate within host cells, it was shown that Salmonella encounters a killing activity within the cytosol of macrophages. Furthermore, in vitro experiments using cytosol extracted from either infected or uninfected macrophages suggested that this activity is activated upon Salmonella infection.

Intracellular replication of the bacterial pathogen Salmonella enterica occurs in membrane‐bound compartments called Salmonella ‐containing vacuoles (SCVs). Maturation of the SCV has been shown to occur by selective interactions with the endocytic pathway. We show here that after invasion of epithelial cells and migration to a perinuclear location, the majority of SCVs become surrounded by membranes of the Golgi network. This process is dependent on the Salmonella pathogenicity island 2 type III secretion system effector SseG. In infected cells, SseG was associated with the SCV and peripheral punctate structures. Only bacterial cells closely associated with the Golgi network were able to multiply; furthermore, mutation of sseG or disruption of the Golgi network inhibited intracellular bacterial growth. When expressed in epithelial cells, SseG co‐localized extensively with markers of the trans ‐Golgi network. We identify a Golgi‐targeting domain within SseG, and other regions of the protein that are required for localization of bacteria to the Golgi network. Therefore, replication of Salmonella in epithelial cells is dependent on simultaneous and selective interactions with both endocytic and secretory pathways.