Vibrio cholerae OmpR Represses the ToxR Regulon in Response to Membrane Intercalating Agents That Are Prevalent in the Human Gastrointestinal Tract

Emergence of the Dickeya genus involved duplication of the OmpF porin and the adaptation of the EnvZ-OmpR signaling network

Genome evolution, and more specifically gene duplication, is a key process shaping host-microorganism interaction. The conserved paralogs usually provide an advantage to the bacterium to thrive. If not, these genes become pseudogenes and disappear. Here, we show that during the emergence of the genus Dickeya, the gene encoding the porin OmpF was duplicated. Our results show that the ompF2 expression is deleterious to the virulence of Dickeya dadantii, the agent causing soft rot disease. Interestingly, ompF2 is regulated while ompF is constitutive but activated by the EnvZ-OmpR two-component system. In vitro, acidic pH triggers the system. The pH measured in four eudicotyledons increased from an initial pH of 5.5 to 7 within 8 h post-infection. Then, the pH decreased to 5.5 at 10 h post-infection and until full maceration of the plant tissue. Yet, the production of phenolic acids by the plant's defenses prevents the activation of the EnvZ-OmpR system to avoid the ompF2 expression even though environmental conditions should trigger this system. We highlight that gene duplication in a pathogen is not automatically an advantage for the infectious process and that, there was a need for our model organism to adapt its genetic regulatory networks to conserve these duplicated genes. IMPORTANCE Dickeya species cause various diseases in a wide range of crops and ornamental plants. Understanding the molecular program that allows the bacterium to colonize the plant is key to developing new pest control methods. Unlike other enterobacterial pathogens, Dickeya dadantii, the causal agent of soft rot disease, does not require the EnvZ-OmpR system for virulence. Here, we showed that during the emergence of the genus Dickeya, the gene encoding the porin OmpF was duplicated and that the expression of ompF2 was deleterious for virulence. We revealed that while the EnvZ-OmpR system was activated in vitro by acidic pH and even though the pH was acidic when the plant is colonized, this system was repressed by phenolic acid (generated by the plant's defenses). These results provide a unique- biologically relevant-perspective on the consequence of gene duplication and the adaptive nature of regulatory networks to retain the duplicated gene.

Vibrio cholerae RND efflux systems: mediators of stress responses, colonization and pathogenesis

Resistance Nodulation Division (RND) efflux systems are ubiquitous transporters in gram-negative bacteria that provide protection against antimicrobial agents and thereby enhance survival in virtually all environments these prokaryotes inhabit. Vibrio cholerae is a dual lifestyle enteric pathogen that spends much of its existence in aquatic environments. An unwitting encounter with a human host can lead to V. cholerae intestinal colonization by strains that encode cholera toxin and toxin co-regulated pilus virulence factors leading to potentially fatal cholera diarrhea and dissemination in the environment. Adaptive response mechanisms to host factors encountered by these pathogens are therefore critical both to engage survival mechanisms such as RND-mediated transporters and to induce timely expression of virulence factors. Sensing of cues encountered in the host may therefore activate more than protective responses such as efflux systems, but also be coordinated to initiate expression of virulence factors. This review summarizes recent advances that contribute towards the understanding of RND efflux physiological functions and how the transport systems interface with the regulation of virulence factor production in V. cholerae.

Bile Salts Promote ToxR Regulon Activation during Growth under Virulence-Inducing Conditions

Cholera is an epidemic disease caused by the Gram-negative bacterium Vibrio cholerae. V. cholerae is found in aquatic ecosystems and infects people through the consumption of V. cholerae-contaminated food or water. Following ingestion, V. cholerae responds to host cues to activate the expression of critical virulence genes that are under the control of a hierarchical regulatory system called the ToxR regulon. The ToxR regulon is tightly regulated and is expressed in vitro only under special growth conditions referred to as AKI conditions. AKI conditions have been instrumental in elucidating V. cholerae virulence regulation, but the chemical cues within AKI medium that activate virulence gene expression are unknown. In this study, we fractionated AKI medium on a reverse-phase chromatography column (RPCC) and showed that the virulence-activating molecules were retained on the RPCC column and recovered in the eluate. Liquid chromatography-high-resolution mass spectrometry (LC-HRMS) analysis of the eluate revealed the presence of a known ToxR regulon activator, taurocholate, and other bile salts. The RPCC eluate activated the ToxR regulon when added to noninducing medium and promoted TcpP dimerization in a two-hybrid system, consistent with taurocholate being responsible for the virulence-inducing activity of AKI medium. Additional experiments using purified bile salts showed that the ToxR regulon was preferentially activated in response to primary bile acids. The results of this study shed light on the chemical cues involved in V. cholerae virulence activation and suggested that V. cholerae virulence genes are modulated in response to regionally specific bile acid species in the intestine.

Ecological diversification reveals routes of pathogen emergence in endemic Vibrio vulnificus populations

Pathogen emergence is a complex phenomenon that, despite its public health relevance, remains poorly understood. Vibrio vulnificus, an emergent human pathogen, can cause a deadly septicaemia with over 50% mortality rate. To date, the ecological drivers that lead to the emergence of clinical strains and the unique genetic traits that allow these clones to colonize the human host remain mostly unknown. We recently surveyed a large estuary in eastern Florida, where outbreaks of the disease frequently occur, and found endemic populations of the bacterium. We established two sampling sites and observed strong correlations between location and pathogenic potential. One site is significantly enriched with strains that belong to one phylogenomic cluster (C1) in which the majority of clinical strains belong. Interestingly, strains isolated from this site exhibit phenotypic traits associated with clinical outcomes, whereas strains from the second site belong to a cluster that rarely causes disease in humans (C2). Analyses of C1 genomes indicate unique genetic markers in the form of clinical-associated alleles with a potential role in virulence. Finally, metagenomic and physicochemical analyses of the sampling sites indicate that this marked cluster distribution and genetic traits are strongly associated with distinct biotic and abiotic factors (e.g., salinity, nutrients, or biodiversity), revealing how ecosystems generate selective pressures that facilitate the emergence of specific strains with pathogenic potential in a population. This knowledge can be applied to assess the risk of pathogen emergence from environmental sources and integrated toward the development of novel strategies for the prevention of future outbreaks.

The Role of OmpR in Bile Tolerance and Pathogenesis of Adherent-Invasive Escherichia coli

Gut microbiota dysbiosis toward adherent-invasive Escherichia coli (AIEC) plays an important role in Crohn’s disease (CD). The OmpR transcriptional regulator is required for the AIEC LF82 prototype strain to adhere and invade intestinal epithelial cells. In this study, we explored the role of OmpR in AIEC pathogenesis using a panel of eight Escherichia coli strains isolated from CD patients and identified as AIEC. The deletion of ompR together with the implementation of two cell-based assays revealed that the role of OmpR in adhesion in vitro was not conserved in AIEC clinical strains. Nevertheless, we showed that OmpR was required for robust gut colonization of transgenic mice expressing human CEACAM receptors, suggesting that OmpR is involved in alternative virulence mechanisms in AIEC strains. We found that deletion of ompR compromised the ability of AIEC strains to cope with the stress induced by bile salts, which may be key for AIEC pathogenesis. More specifically, we demonstrated that OmpR was involved in a tolerance mechanism toward sodium deoxycholate (DOC), one of bile salts main component. We showed that the misregulation of OmpF or the loss of outer membrane integrity are not the drivers of OmpR-mediated DOC tolerance, suggesting that OmpR regulates a specific mechanism enhancing AIEC survival in the presence of DOC. In conclusion, the newly discovered role of OmpR in AIEC bile tolerance suggests that OmpR inhibition would interfere with different aspects of AIEC virulence arsenal and could be an alternative strategy for CD-treatment.

ToxR Mediates the Antivirulence Activity of Phenyl-Arginine-β-Naphthylamide To Attenuate Vibrio cholerae Virulence

Multidrug efflux systems belonging to the resistance-nodulation-cell division (RND) family are ubiquitous in Gram-negative bacteria and are critical for antimicrobial resistance. This realization has led to efforts to develop efflux pump inhibitors (EPI) for use as adjuvants for antibiotic treatment of resistant organisms. However, the functions of RND transporters extend beyond antimicrobial resistance to include physiological functions that are critical for pathogenesis, suggesting that EPIs could also be used as antivirulence therapeutics. This was documented in the enteric pathogen Vibrio cholerae, in which EPIs were shown to attenuate the production of the critical virulence factors cholera toxin (CT) and the toxin-coregulated pilus (TCP). In this study, we investigated the antivirulence mechanism of action of the EPI phenyl-arginine-β-naphthylamide (PAβN) on V. cholerae. Using bioassays, we documented that PAβN inhibited virulence factor production in three epidemic V. cholerae isolates. Transcriptional reporter studies and mutant analysis indicated that PAβN initiated a ToxR-dependent regulatory circuit to activate leuO expression and that LeuO repressed the expression of the critical virulence activator aphA to attenuate CT and TCP production. The antivirulence activity of PAβN was found to be dependent on the ToxR periplasmic sensing domain (PPD), suggesting that a feedback mechanism was involved in its activity. Collectively, the data indicated that PAβN inhibited V. cholerae virulence factor production by activating a ToxR-dependent metabolic feedback mechanism to repress the expression of the ToxR virulence regulon. This suggests that efflux pump inhibitors could be used as antivirulence therapeutics for the treatment of cholera and perhaps that of other Gram-negative pathogens.

Assessment and Antibiotic Resistance Profiling in Vibrio Species Isolated from Wild Birds Captured in Danube Delta Biosphere Reserve, Romania

Antimicrobial and multidrug-resistant bacteria are a major problem worldwide and, consequently, the surveillance of antibiotic-resistant bacteria and assessment of the dissemination routes are essential. We hypothesized that migratory birds, coming from various environments, would carry more numerous Vibrio strains than sedentary species, with increased risk to be passed to their contacts or environment in habitats they transit or nest in. Similarly, we presumed that strains from migratory birds will show multidrug resistance. A total of 170 oral and rectal swabs were collected from wild birds captured in different locations of the Danube Delta (Malic, Sfantu-Gheorghe, Letea Forest) and processed using standardized selective media. V. cholerae strains were confirmed by serology and molecular methods and, subsequently, their susceptibility was evaluated. The prevalence of Vibrio species by host species, habitat type, and location was interpreted. The isolated Vibrio species were identified as Vibrio cholerae 14.33%, V. fluvialis 13.33%, V. alginolyticus 12%, V. mimicus 17.33%, V. vulnificus 10.88%, with V. parahaemolyticus and V. metschnikovii (16%) also being prevalent. Of the 76 Vibrio spp. isolates, 18.42% were resistant towards at least three antimicrobials, and 81.57% demonstrated a multidrug resistance phenotype, including mainly penicillins, aminoglycosides, and macrolides. The results of the present study indicate higher numbers of Vibrio strains in migratory (74.66%) than in sedentary birds (25.33%), confirming our hypothesis. Furthermore, the increased pathogenicity of Vibrio spp. strains, isolated from wild migratory and sedentary birds, was confirmed by their increased multiple antibiotic resistance (MAR) index (0.09–0.81).

Vibrio cholerae OmpR Contributes to Virulence Repression and Fitness at Alkaline pH

Vibrio cholerae is a Gram-negative human pathogen and the causative agent of the life-threatening disease cholera. V. cholerae is a natural inhabitant of marine environments and enters humans through the consumption of contaminated food or water. The ability to transition between aquatic ecosystems and the human host is paramount to the pathogenic success of V. cholerae The transition between these two disparate environments requires the expression of adaptive responses, and such responses are most often regulated by two-component regulatory systems such as the EnvZ/OmpR system, which responds to osmolarity and acidic pH in many Gram-negative bacteria. Previous work in our laboratory indicated that V. cholerae OmpR functioned as a virulence regulator through repression of the LysR-family transcriptional regulator aphB; however, the role of OmpR in V. cholerae biology outside virulence regulation remained unknown. In this work, we sought to further investigate the function of OmpR in V. cholerae biology by defining the OmpR regulon through RNA sequencing. This led to the discovery that V. cholerae ompR was induced at alkaline pH to repress genes involved in acid tolerance and virulence factor production. In addition, OmpR was required for V. cholerae fitness during growth under alkaline conditions. These findings indicate that V. cholerae OmpR has evolved the ability to respond to novel signals during pathogenesis, which may play a role in the regulation of adaptive responses to aid in the transition between the human gastrointestinal tract and the marine ecosystem.

Comparative transcriptome and phenotype analysis revealed the role and mechanism of ompR in the virulence of fish pathogenic Aeromonas hydrophila

Aeromonas hydrophila B11 strain was isolated from diseased Anguilla japonica, which had caused severe gill ulcers in farmed eel, causing huge economic losses. EnvZ‐OmpR is a model two‐component system in the bacteria and is widely used in the research of signal transduction and gene transcription regulation. In this study, the ompR of A. hydrophila B11 strain was first silenced by RNAi technology. The role of ompR in the pathogenicity of A. hydrophila B11 was investigated by analyzing both the bacterial comparative transcriptome and phenotype. The qRT‐PCR results showed that the expression of ompR in the ompR‐RNAi strain decreased by 97% compared with the wild‐type strain. The virulence test showed that after inhibition of the ompR expression, the LD₅₀ of A. hydrophila B11 decreased by an order of magnitude, suggesting that ompR is involved in the regulation of bacterial virulence. Comparative transcriptome analysis showed that the expression of ompR can directly regulate the expression of several important virulence‐related genes, such as the bacterial type II secretion system; moreover, ompR expression also regulates the expression of multiple genes related to bacterial chemotaxis, motility, adhesion, and biofilm formation. Further studies on the phenotype of A. hydrophila B11 and ompR‐RNAi also confirmed that the downregulation of ompR expression can decrease bacterial chemotaxis, adhesion, and biofilm formation.