Postreplication Roles of the Brucella VirB Type IV Secretion System Uncovered via Conditional Expression of the...

Brucella abortus, the bacterial agent of the worldwide zoonosis brucellosis, primarily infects host phagocytes, where it undergoes an intracellular cycle within a dedicated membrane-bound vacuole, the Brucella-containing vacuole (BCV). Initially of endosomal origin (eBCV), BCVs are remodeled into replication-permissive organelles (rBCV) derived from the host endoplasmic reticulum, a process that requires modulation of host secretory functions via delivery of effector proteins by the Brucella VirB type IV secretion system (T4SS). Following replication, rBCVs are converted into autophagic vacuoles (aBCVs) that facilitate bacterial egress and subsequent infections, arguing that the bacterium sequentially manipulates multiple cellular pathways to complete its cycle. The VirB T4SS is essential for rBCV biogenesis, as VirB-deficient mutants are stalled in eBCVs and cannot mediate rBCV biogenesis. This has precluded analysis of whether the VirB apparatus also drives subsequent stages of the Brucella intracellular cycle. To address this issue, we have generated a B. abortus strain in which VirB T4SS function is conditionally controlled via anhydrotetracycline (ATc)-dependent complementation of a deletion of the virB11 gene encoding the VirB11 ATPase. We show in murine bone marrow-derived macrophages (BMMs) that early VirB production is essential for optimal rBCV biogenesis and bacterial replication. Transient expression of virB11 prior to infection was sufficient to mediate normal rBCV biogenesis and bacterial replication but led to T4SS inactivation and decreased aBCV formation and bacterial release, indicating that these postreplication stages are also T4SS dependent. Hence, our findings support the hypothesis of additional, postreplication roles of type IV secretion in the Brucella intracellular cycle.

Many intracellular bacterial pathogens encode specialized secretion systems that deliver effector proteins into host cells to mediate the multiple stages of their intracellular cycles. Because these intracellular events occur sequentially, classical genetic approaches cannot address the late roles that these apparatuses play, as secretion-deficient mutants cannot proceed past their initial defect. Here we have designed a functionally controllable VirB type IV secretion system (T4SS) in the bacterial pathogen Brucella abortus to decipher its temporal requirements during the bacterium's intracellular cycle in macrophages. By controlling production of the VirB11 ATPase, which energizes the T4SS, we show not only that this apparatus is required early to generate the Brucella replicative organelle but also that it contributes to completion of the bacterium's cycle and bacterial egress. Our findings expand upon the pathogenic functions of the Brucella VirB T4SS and illustrate targeting of secretion ATPases as a useful strategy to manipulate the activity of bacterial secretion systems.