Induction of the Yersinia Type 3 Secretion System as an All-or-None Phenomenon

Pathogenic Yersinia spp. possess a protein secretion system, designated as type 3, that plays a clear role in promoting their survival vis-à-vis the macrophage. Inductive expression of the Yersinia type 3 secretion system (T3SS), triggered either by host cell contact, or, in the absence of host cells, by a reduction in extracellular calcium ion levels, is accompanied by a withdrawal from the bacterial division cycle. Here, we analyzed Ca²⁺-dependent induction of the T3SS at the single-cell level to understand how Yersinia coordinates pro-survival and growth-related activities. We utilized a novel high-throughput quantitative microscopy approach as well as flow cytometry to determine how Ca²⁺ levels, T3SS expression, and cellular division are interrelated. Our analysis showed that there is a high degree of homogeneity in terms of T3SS expression levels among a population of Y. pseudotuberculosis cells following the removal of Ca²⁺, and that T3SS expression appears to be independent of the cellular division cycle. Unexpectedly, our analysis showed that Ca²⁺ levels are inversely related to the initiation of inductive T3SS expression, and not to the intensity of activation once initiated, thus providing a basis for the seemingly graded response of T3SS activation observed in bulk-level analyses. The properties of the system described here display both similarities to and differences from that of the lac operon first described 50 years ago by Novick and Weiner.