Essential role of the VirB machinery in the maturation of the Brucella abortus-containing vacuole

In epithelial cells, the intracellular pathogen Brucella abortus escapes fr om the endocytic pathway, exploits the autophagic machinery of the host cel l and establishes a unique replication niche in the endoplasmic reticulum. The molecular mechanisms underlying these processes are still poorly unders tood. Recently, a B. abortus type IV-related secretion system encoded by th e virB operon has been described as being involved in the intracellular tra fficking of the bacteria. In this study, we have analysed the intracellular pathway of B, abortus virB10 mutant strains by confocal microscopy, We dem onstrate that a functional virB operon is essential for the biogenesis of t he Brucella-containing vacuole, Polar mutation preventing the transcription of virB10 and downstream sequences did not allow Brucella to bypass the en docytic pathway. Consequently, polar mutant-containing vacuoles fused with lysosomes in which bacteria underwent a degradation process. In contrast, v irB10 non-polar mutants were capable of avoiding interactions with the endo cytic pathway but, diverging to wild-type Brucella, were unable to reach th e endoplasmic reticulum to establish their intracellular replication niche and seemed to be recycled to the cell surface. Based on the two particular phenotypes described in this work, a model of maturation of the Brucella-co ntaining vacuole is proposed.