The putative Type 4 secretion system effector BspD is involved in maintaining envelope integrity of the pathogen Brucella

Brucellosis is a debilitating disease caused by the Gram-negative, facultative intracellular zoonotic pathogen Brucella . En route to its intracellular replicative niche, Brucella encounters various stressful environments that may compromise envelop integrity. Here we show, that the proposed Type 4 secretion system (T4SS) effector BspD is a conserved protein of the Rhizobiales, which does not show signs of co-evolution with the presence of a T4SS or a certain lifestyle. We further present data indicating that BspD is critical for Brucella abortus envelope integrity in the stationary phase and in the presence of EDTA, a compound known to destabilize the outer membrane. Deletion of bspD resulted in abnormal bacterial morphologies, indicating its involvement in maintaining envelope integrity. Additionally, the absence of BspD led to the formation of fewer and smaller intracellular microcolonies in a macrophage infection model. From our observations, we propose that BspD of B. abortus is critical for preserving the integrity of the bacterial envelope, particularly under stressful conditions, which may enhance Brucella ’s ability to survive within host cells.