Effector mechanisms of pyroptosis.

<p>(<b>A</b>) Caspase-1 activation in response to microbial infection (black ovals) initiates numerous pathways that promote death or recovery of the cell, directly combat pathogen infection, or signal to other cells. (<b>B</b>) The proinflammatory cytokines IL-1β and IL-18 are cleaved and secreted, and HMGB1, IL-1α, FGF2, and numerous damage-associated molecular patterns are also released. (<b>C</b>) Caspase-1 can also initiate programmed cell death, eliminating a niche for intracellular pathogens while releasing both pathogen and proinflammatory signals. (<b>D</b>) Intracellular pathogens and antimicrobial factors that kill extracellular bacteria can be released by lysosomal exocytosis, also promoting adaptive immune responses through cross-priming. (<b>E</b>) Caspase-1 promotes cellular integrity by removing microbes or damaged organelles by stimulating autophagy, enhanced lysosome activity, induction of lipid metabolism, and exocytosis of damaged or infected organelles. (<b>F</b>) Proinflammatory signals released by lysis, exocytosis, and other secretory pathways recruit and activate immune cells (blue; clockwise from top: neutrophils, lymphocytes, macrophages/dendritic cells). The specific responses of a cell vary depending on the kinetics and magnitude of caspase-1 stimulation, the activating stimulus, and cell type.</p

AHL-based QS regulates secondary metabolism.

<p>RNA was isolated from the indicated strains of <i>Y. pestis</i>, and levels of mRNA were measured by qRT-PCR as outlined in Material and Methods. Data represents the mean of triplicate measurements of the transcript differences between each mutant and that of strain R88 normalized to the 16 S rRNA of each sample, and are representative of at least three independent experiments. AHL^– = strain R114; QS^– = strain R115; AI-2^– = strain ISM1980.</p

Selected genes significantly up-regulated by QS.

*<p>FC = fold change.</p

AHL quorum sensing upregulates glyoxylate bypass and enhances growth on acetate.

<p>(A) Growth of R88 (black), and an R115 AHL null mutant strain (dashed) of <i>Y. pestis</i> in minimal acetate medium (light grey is medium only control) was monitored in a Bioscreen C microplate reader incubating at 28°C with agitation. (B) Complementation of AHL mutant bacteria with p<i>ypeIR</i> (black, dashed) or p<i>yspIR</i> (grey, dashed) restores growth on acetate, whereas control plasmid (black, solid) does not.</p

Transcriptome Analysis of Acetyl-Homoserine Lactone-Based Quorum Sensing Regulation in <i>Yersinia pestis</i>

<div><p>The etiologic agent of bubonic plague, <i>Yersinia pestis,</i> senses self-produced, secreted chemical signals in a process named quorum sensing. Though the closely related enteric pathogen <i>Y. pseudotuberculosis</i> uses quorum sensing system to regulate motility, the role of quorum sensing in <i>Y. pestis</i> has been unclear. In this study we performed transcriptional profiling experiments to identify <i>Y. pestis</i> quorum sensing regulated functions. Our analysis revealed that acyl-homoserine lactone-based quorum sensing controls the expression of several metabolic functions. Maltose fermentation and the glyoxylate bypass are induced by acyl-homoserine lactone signaling. This effect was observed at 30°C, indicating a potential role for quorum sensing regulation of metabolism at temperatures below the normal mammalian temperature. It is proposed that utilization of alternative carbon sources may enhance growth and/or survival during prolonged periods in natural habitats with limited nutrient sources, contributing to maintenance of plague in nature.</p></div

AHLs upregulate the maltose operon and enhance growth on maltose.

<p>(A) Growth of wild-type (WT) (black), and AHL mutant (dashed) <i>Y. pestis</i> in minimal maltose medium (light grey) was monitored in Bioscreen C microplate reader incubating at 28°C with agitation. (B) Complementation of AHL mutant bacteria with p<i>ypeIR</i> (black, dashed) or p<i>yspIR</i> (grey, dashed) restores growth on maltose, whereas control plasmid (black, solid) does not.</p

Identification of the <i>Y. pestis</i> quorum-sensing molecules.

<p>(A) HPLC fractionation profiles of ¹⁴C-labeled AHL produced by R88 <i>Y. pestis</i> (solid circles) compared to R115 QS^-<i>Y. pestis</i> (empty circles). The peaks absent from organic extracts of R115 <i>Y. pestis</i> supernatants correspond to C8-, C6-, and oxo-C6-AHL. (B) AI-2 production during the growth of R88 <i>Y. pestis</i> (solid circles) and R115 (empty circles) was monitored by adding the cell-free supernatants at the indicated time points to a <i>V. harveyii</i> reporter strain that is bioluminescent (RLU) in response to AI-2. Data are representative of at least three independent studies. (C and D) The production of AI-2 (C) and AHL (D) signals as a function of growth.</p

Quorum-sensing regulates fermentation of sugars.

<p>(A) Colony phenotype after 96 hrs growth on LB plates containing Congo red and 0.2% maltose and incubated at the temperature indicated. <i>Y. pestis</i> ferments maltose to acid, converting the Congo red to black, whereas a <i>yspIR ypeIR</i> mutant does not. This fermentation does not occur at temperatures above 30°C, or on other sugars tested (data not shown.) (B) Duplicate growth stabs of <i>Y. pestis</i> grown on solid medium under anaerobic conditions for 96 h at 28°C. An indicator dye, Congo Red, turns dark upon production of fermentative end products. Deletion of <i>yspIR</i> and <i>ypeIR</i> in R114 and R115 results in a lag in fermentation of maltose under anaerobic conditions. 1 = <i>Y. pseudotuberculosis</i>; 2 = R88; 3 = R114; 4 = R115.</p