The GacA-GacS Type Two-Component System Modulates the Pathogenicity of Dickeya oryzae EC1 Mainly by Regulating the Production of Zeamines.

The GacS-GacA type two-component system (TCS) positively regulates pathogenicity-related phenotypes in many plant pathogens. In addition, Dickeya oryzae EC1, the causative agent of soft rot disease, produces antibiotic-like toxins called zeamines as one of the major virulence factors that inhibit the germination of rice seeds. The present study identified a GacS-GacA type TCS, named TzpS-TzpA, that positively controls the virulence of EC1, mainly by regulating production of the toxin zeamines. RNA-seq analysis of strain EC1 and its tzpA mutant showed that the TCS regulated a wide range of virulence genes, especially those encoding zeamines. Protein-protein interaction was detected between TzpS and TzpA through the bacterial two-hybrid system and pull-down assay. In trans expression of tzpA failed to rescue the defective phenotypes in both the ΔtzpS and ΔtzpSΔtzpA mutants. Furthermore, TzpA controls target gene expression by direct binding to DNA promoters that contain a Gac-box motif, including a regulatory RNA rsmB and the vfm quorum-sensing system regulator vfmE. These findings therefore suggested that the EC1 TzpS-TzpA TCS system mediates the pathogenicity of Dickeya oryzae EC1 mainly by regulating the production of zeamines.[Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

RasI/R Quorum Sensing System Controls the Virulence of Ralstonia solanacearum Strain EP1.

Quorum sensing (QS) is a widely conserved bacterial regulatory mechanism that relies on production and perception of autoinducing chemical signals to coordinate diverse cooperative activities, such as virulence, exoenzyme secretion, and biofilm formation. In Ralstonia solanacearum, a phytopathogen causing severe bacterial wilt diseases in many plant species, previous studies identified the PhcBSR QS system, which plays a key role in regulation of its physiology and virulence. In this study, we found that R. solanacearum strain EP1 contains the genes encoding uncharacterized LuxI/LuxR (LuxI/R) QS homologues (RasI/RasR [designated RasI/R here]). To determine the roles of the RasI/R system in strain EP1, we constructed a specific reporter for the signals catalyzed by RasI. Chromatography separation and structural analysis showed that RasI synthesized primarily N-(3-hydroxydodecanoyl)-homoserine lactone (3-OH-C12-HSL). In addition, we showed that the transcriptional expression of rasI is regulated by RasR in response to 3-OH-C12-HSL. Phenotype analysis unveiled that the RasI/R system plays a critical role in modulation of cellulase production, motility, biofilm formation, oxidative stress response, and virulence of R. solanacearum EP1. We then further characterized this system by determining the RasI/R regulon using transcriptome sequencing (RNA-seq) analysis, which showed that this newly identified QS system regulates the transcriptional expression of over 154 genes associated with bacterial physiology and pathogenic properties. Taken together, the findings from this study present an essential new QS system in regulation of R. solanacearum physiology and virulence and provide new insight into the complicated regulatory mechanisms and networks in this important plant pathogen. IMPORTANCE Quorum sensing (QS) is a key regulator of virulence factors in many plant-pathogenic bacteria. Previous studies unveiled two QS systems (i.e., PhcBSR and SolI/R) in several R. solanacearum strains. The PhcBSR QS system is known for its key roles in regulation of bacterial virulence, and the LuxI/LuxR (SolI/R) QS system appears dispensable for pathogenicity in a number of R. solanacearum strains. In this study, a new functional QS system (i.e., RasI/R) was identified and characterized in R. solanacearum strain EP1 isolated from infected eggplants. Phenotype analyses showed that the RasI/R system plays an important role in regulation of a range of biological activities associated with bacterial virulence. This QS system produces and responds to the QS signal 3-OH-C12-HSL and hence regulates critical bacterial abilities in survival and infection. To date, multiple QS signaling circuits in R. solanacearum strains are still not well understood. Our findings from this study provide new insight into the complicated QS regulatory networks that govern the physiology and virulence of R. solanacearum and present a valid target and clues for the control and prevention of bacterial wilt diseases.

Nutrient Availability and Phage Exposure Alter the Quorum-Sensing and CRISPR-Cas-Controlled Population Dynamics of Pseudomonas aeruginosa.

Quorum sensing (QS) coordinates bacterial communication and cooperation essential for virulence and dominance in polymicrobial settings. QS also regulates the CRISPR-Cas system for targeted defense against parasitic genomes from phages and horizontal gene transfer. Although the QS and CRISPR-Cas systems are vital for bacterial survival, they undergo frequent selection in response to biotic and abiotic factors. Using the opportunistic Pseudomonas aeruginosa with well-established QS and CRISPR-Cas systems, we show how the social interactions between the acyl-homoserine lactone (AHL)-QS signal-blind mutants (ΔlasRrhlR) and the CRISPR-Cas mutants are affected by phage exposure and nutrient availability. We demonstrate that media conditions and phage exposure alter the resistance and relative fitness of ΔlasRrhlR and CRISPR-Cas mutants while tipping the fitness advantage in favor of the QS signal-blind mutants under nutrient-limiting conditions. We also show that the AHL signal-blind mutants are less selected by phages under QS-inducing conditions than the CRISPR-Cas mutants, whereas the mixed population of the CRISPR-Cas and AHL signal-blind mutants reduce phage infectivity, which can improve survival during phage exposure. Our data reveal that phage exposure and nutrient availability reshape the population dynamics between the ΔlasRrhlR QS mutants and CRISPR-Cas mutants, with key indications for cooperation and conflict between the strains. IMPORTANCE The increase in antimicrobial resistance has created the need for alternative interventions such as phage therapy. However, as previously observed with antimicrobial resistance, phage therapy will not be effective if bacteria evolve resistance and persist in the presence of the phages. The QS is commonly known as an arsenal for bacteria communication, virulence, and regulation of the phage defense mechanism, the CRISPR-Cas system. The QS and CRISPR-Cas systems are widespread in bacteria. However, they are known to evolve rapidly under the influence of biotic and abiotic factors in the bacterial environment, resulting in alteration in bacterial genotypes, which enhance phage resistance and fitness. We believe that adequate knowledge of the influence of environmental factors on the bacterial community lifestyle and phage defense mechanisms driven by the QS and CRISPR-Cas system is necessary for developing effective phage therapy.