Salmonella T3SSs: successful mission of the secret(ion) agents

Bacteria of the genus Salmonella express nanosyringe-like organelles called type three secretion systems (T3SSs). These systems promote the secretion of bacterial compounds and their translocation into host cells. Pathogenic Salmonella use two distinct T3SSs, with specialized functions, having the purpose to modify the biology of the host organism and to ensure a successful infection. The bacterial proteins translocated through the first T3SS (T3SS-1) facilitate the entry of Salmonella into host cells, whereas T3SS-2 is an important factor for shaping the intracellular replication niche. In addition both T3SSs have a strong impact on the host inflammation. For a long time the two T3SSs were thought to act separately. However, there is increasing evidence that their regulation depends not only on separate but also shared regulatory mechanisms and that their time of action during infection overlaps. Here, we review the current understanding of the structure and of the regulation of expression and activity of both T3SSs. The output image is multifaceted, as recent studies show that subpopulations of Salmonella present diverging patterns of expression and activity of T3SSs during important steps of infection. These diversities may advance the chances of Salmonella to outpace competitors and to well establish itself in its niche in the host.