The delivery of effectors into host cells involves secretion, the crossing of both bacterial membranes via the basal body, and translocation, the passage through the eukaryotic cell membrane. Following assembly of the external needle, the proteins secreted via the T3SS fall into two main categories: translocators and effectors. Upon host cell contact, translocators assemble into the host cell membrane, forming a pore complex, or translocon, that triggers the subsequent export of effectors.4 Since translocators must be secreted before effectors, so that effectors will be exported directly into host cells instead of the extracellular milieu, pathogens require mechanisms to ensure hierarchical and temporal control over their secretion. Although the exact mechanisms underlying these processes are not clearly established, several cytoplasmic and inner-membrane proteins have been identified that recognize secretion substrates and respond to specific signals to ensure that structural and sensing components (needle subunits and pore proteins) are secreted first, and that virulence proteins are not secreted before contact with a host cell.