Expression and Quorum Sensing Regulation of Type III Secretion System Genes of Vibrio harveyi during Infection of Gnotobiotic Brine Shrimp

Abstract

Type III secretion systems enable pathogens to inject their virulence factors directly into the cytoplasm of the host cells. The type III secretion system of Vibrio harveyi, a major pathogen of aquatic organisms and a model species in quorum sensing studies, is repressed by the quorum sensing master regulator LuxR. In this study, we found that during infection of gnotobiotic brine shrimp larvae, the expression levels of three type III secretion operons in V. harveyi increased within the first 12h after challenge and decreased again thereafter. The in vivo expression levels were highest in a mutant with a quorum sensing system that is locked in low cell density configuration (minimal LuxR levels) and lowest in a mutant with a quorum sensing system that is locked in the high cell density configuration (maximal LuxR levels), which is consistent with repression of type III secretion by LuxR. Remarkably, in vivo expression levels of the type III secretion system genes were much (> 1000 fold) higher than the in vitro expression levels, indicating that (currently unknown) host factors significantly induce the type III secretion system. Given the fact that type III secretion is energy-consuming, repression by the quorum sensing master regulators might be a mechanism to save energy under conditions where it does not provide an advantage to the cells.

QStatin, a Selective Inhibitor of Quorum Sensing in Vibrio Species

“…5b ). This apparently conflicting result is not unprecedented, because a V. harveyi QS mutant also exhibits reduced virulence to brine shrimp despite expressing more type 3 secretion system (T3SS) components ( 17 , 49 ). QStatin decreased the expression of many other virulence factors ( Fig.…”

Integration host factor and LuxR synergistically bind DNA to coactivate quorum‐sensing genes in Vibrio harveyi

“…IHF represses a subset of type III secretion genes that are indirectly repressed by LuxR (van Kessel et al ., ). The type III secretion system genes are highly expressed during infection in the brine shrimp model (Ruwandeepika et al ., ) and, thus, likely play a role in virulence. Interestingly, LuxR and IHF both activate a set of prophage genes, although LuxR does not directly regulate any of these.…”

Quorum Sensing Gene Regulation by LuxR/HapR Master Regulators in Vibrios

“…In V. harveyi, AphA controls 170 genes at LCD, and among these are Ͼ40 type III secretion system (T3SS) genes that are repressed by AphA (47,48). Repression of T3SS genes at LCD by AphA and at HCD by LuxR results in the expression of T3SS genes at mid-cell density, and these genes are upregulated Ͼ1,000-fold during infection (48,71). Thus, AphA plays a critical role in virulence gene regulation at LCD in vibrios.…”

“…The type III and type VI secretion systems (T3SS and T6SS, respectively) are intimately linked to quorum sensing in several Vibrio species (Fig. 3) (41,49,57,71,90,133). T3SS and T6SS are complex syringe-like structures that are capable of penetrating proximal cellular membranes to deliver effector proteins that interfere with various cellular processes to cause cell death (134).…”

“…In V. harveyi and V. parahaemolyticus, LuxR and OpaR repress expression of the T3SS operons, including those encoding the structural proteins, effector proteins, and the transcription factors in the system (41,49,135). Not only is the expression of T3SS vastly different between LCD and HCD in culture, but these operons are also highly upregulated during infection in a quorum sensing-dependent manner (71). While other vibrios, such as V. cholerae, contain T3SS, the regulatory connection to quorum sensing remains to be explored (136).…”