OmpR, a Central Integrator of Several Cellular Responses in Yersinia enterocolitica

The implication of viability and pathogenicity by truncated lipopolysaccharide in Yersinia enterocolitica

The fast envelope stress responses play a key role in the transmission and pathogenesis of Yersinia enterocolitica, one of the most common foodborne pathogens. Our previous study showed that deletion of the waaF gene, essential for the biosynthesis of lipopolysaccharide (LPS) core polysaccharides, led to the formation of a truncated LPS structure and induced cell envelope stress. This envelope stress may disturb the intracellular signal transduction, thereby affecting the physiological functions of Y. enterocolitica. In this study, truncated LPS caused by waaF deletion was used as a model of envelope stress in Y. enterocolitica. We investigated the mechanisms of envelope stress responses and the cellular functions affected by truncated LPS. Transcriptome analysis and phenotypic validation showed that LPS truncation reduced flagellar assembly, bacterial chemotaxis, and inositol phosphate metabolism, presenting lower pathogenicity and viability both in vivo and in vitro environments. Further 4D label-free phosphorylation analysis confirmed that truncated LPS perturbed multiple intracellular signal transduction pathways. Specifically, a comprehensive discussion was conducted on the mechanisms by which chemotactic signal transduction and Rcs system contribute to the inhibition of chemotaxis. Finally, the pathogenicity of Y. enterocolitica with truncated LPS was evaluated in vitro using IPEC-J2 cells as models, and it was found that truncated LPS exhibited reduced adhesion, invasion, and toxicity of Y. enterocolitica to IPEC-J2 cells. Our research provides an understanding of LPS in the regulation of Y. enterocolitica viability and pathogenicity and, thus, opening new avenues to develop novel food safety strategies or drugs to prevent and control Y. enterocolitica infections. KEY POINTS: • Truncated LPS reduces flagellar assembly, chemotaxis, and inositol phosphate metabolism in Y. enterocolitica. • Truncated LPS reduces adhesion, invasion, and toxicity of Y. enterocolitica to IPEC-J2 cells. • Truncated LPS regulates intracellular signal transduction of Y. enterocolitica.

Interplay between the RNA Chaperone Hfq, Small RNAs and Transcriptional Regulator OmpR Modulates Iron Homeostasis in the Enteropathogen Yersinia enterocolitica

Iron is both essential for and potentially toxic to bacteria, so the precise maintenance of iron homeostasis is necessary for their survival. Our previous study indicated that in the human enteropathogen Yersinia enterocolitica, the regulator OmpR directly controls the transcription of the fur, fecA and fepA genes, encoding the ferric uptake repressor and two transporters of ferric siderophores, respectively. This study was undertaken to determine the significance of the RNA chaperone Hfq and the small RNAs OmrA and RyhB1 in the post-transcriptional control of the expression of these OmpR targets. We show that Hfq silences fur, fecA and fepA expression post-transcriptionally and negatively affects the production of FLAG-tagged Fur, FecA and FepA proteins. In addition, we found that the fur gene is under the negative control of the sRNA RyhB1, while fecA and fepA are negatively regulated by the sRNA OmrA. Finally, our data revealed that the role of OmrA results from a complex interplay of transcriptional and post-transcriptional effects in the feedback circuit between the regulator OmpR and the sRNA OmrA. Thus, the expression of fur, fecA and fepA is subject to complex transcriptional and post-transcriptional regulation in order to maintain iron homeostasis in Y. enterocolitica.

OmpR (TCS response regulator) of Aeromonas veronii plays a major role in drug resistance, stress resistance and virulence by regulating biofilm formation

Aeromonas veronii (A. veronii), a highly pathogenic bacteria with a wide range of hosts, widely exists in the environment of humans, animals and aquatic animals, and can cause a variety of diseases. In this study, the receptor regulator ompR in the envZ/ompR of two-component system was selected to construct a mutant strain (Δ ompR) and a complement strain (C-ompR) to explore the regulatory effect of ompR on the biological characteristics and virulence of TH0426. The results showed that the ability of biofilm formation and osmotic stress of TH0426 were significantly reduced (P < 0.001), the resistance to ceftriaxone and neomycin were slightly down-regulate when the ompR gene was deleted. At the same time, animal pathogenicity experiments showed that the virulence of TH0426 was significantly down-regulated (P < 0.001). These results indicated that ompR gene regulates the biofilm formation of TH0426, and regulates some biological characteristics of TH0426, including drug sensitivity, resistance to osmotic stress, and also affects its virulence.

The lipoprotein NlpD in Cronobacter sakazakii responds to acid stress and regulates macrophage resistance and virulence by maintaining membrane integrity

Cronobacter sakazakii, an emerging opportunistic pathogen, is implicated in severe foodborne outbreak infections in premature and full-term infants. Generally, acid tolerance is vital for the pathogenesis of foodborne pathogens; however, its role in C. sakazakii virulence remains largely unknown. To screen out acid-tolerance determinants from transposon mutants, anovel counterselection method using gentamicin and acid was developed. Using the counterselection method and growth assay, we screened several acid-sensitive mutants and found that nlpD encodes an acid-resistance factor in C. sakazakii. 

Compared to the wild-type strain, the nlpD mutant exhibited attenuated virulence in a rat model. Using macrophage THP-1 cells and a pH probe, we verified that nlpD enables bacteria to resist macrophages by resisting acidification. Finally, we confirmed that nlpD maintains C. sakazakii membrane integrity in acid using propidium iodide permeabilization assays via flow cytometry. Our results confirm that nlpD is a novel virulence factor that permits C. sakazakii to survive under acid stress conditions. Considering that NlpD is a conserved lipoprotein located in the bacterial outer membrane, NlpD could be used as a target for drug development.

Rcs Phosphorelay Responses to Truncated Lipopolysaccharide-Induced Cell Envelope Stress in Yersinia enterocolitica

Lipopolysaccharide (LPS) is the major component of the outer membrane of Gram-negative bacteria, and its integrity is monitored by various stress response systems. Although the Rcs system is involved in the envelope stress response and regulates genes controlling numerous bacterial cell functions of Yersinia enterocolitica, whether it can sense the truncated LPS in Y. enterocolitica remains unclear. In this study, the deletion of the Y. enterocolitica waaF gene truncated the structure of LPS and produced a deep rough LPS. The truncated LPS increased the cell surface hydrophobicity and outer membrane permeability, generating cell envelope stress. The truncated LPS also directly exposed the smooth outer membrane to the external environment and attenuated the resistance to adverse conditions, such as impaired survival under polymyxin B and sodium dodecyl sulfate (SDS) exposure. Further phenotypic experiment and gene expression analysis indicated that the truncated LPS was correlated with the activation of the Rcs phosphorelay, thereby repressing cell motility and biofilm formation. Our findings highlight the importance of LPS integrity in maintaining membrane function and broaden the understanding of Rcs phosphorelay signaling in response to cell envelope stress, thus opening new avenues to develop effective antimicrobial agents for combating Y. enterocolitica infections.

Role of the RprY response regulator in P. gingivalis community development and virulence

Porphyromonas gingivalis expresses a limited number of two-component systems, including RprY, an orphan response regulator which lacks a cognate sensor kinase. In this study, we examined cross-phosphorylation of RprY on tyrosine residues and its importance for RprY function. We show that RprY reacts with phosphotyrosine antibodies, and found that the tyrosine (Y) residue at position 41 is predicted to be solvent accessible. Loss of RprY increased the level of heterotypic community development with Streptococcus gordonii, and the community-suppressive function of RprY required Y41. Expression of the Mfa1 fimbrial adhesin was increased in the rprY mutant and in the mutant complemented with rprY containing a Y41F mutation. In a microscale thermophoresis assay, recombinant RprY protein bound to the promoter region of mfa1, and binding was diminished with RprY containing the Y41F substitution. RprY was required for virulence of P. gingivalis in a murine model of alveolar bone loss. Transcriptional profiling indicated that RprY can control the expression of genes encoding the type IX secretion system (T9SS) machinery and virulence factors secreted through the T9SS, including the gingipain proteases and peptidylarginine deiminase (PPAD). Collectively, these results establish the RprY response regulator as a component of the tyrosine phosphorylation regulon in P. gingivalis, which can independently control heterotypic community development through the Mfa1 fimbriae and virulence through the T9SS.

Differential regulation of physiological activities by RcsB and OmpR in Yersinia enterocolitica

A thorough understanding of the mechanisms of Rcs and EnvZ/OmpR phosphorelay systems that allow Yersinia enterocolitica to thrive in various environments is crucial to prevent and control Y. enterocolitica infections. In this study, we showed that RcsB and OmpR have the ability to function differently in modulating a diverse array of physiological processes in Y. enterocolitica. The rcsB mutant stimulated flagella biosynthesis and increased motility, biofilm formation and c-di-GMP production by upregulating flhDC, hmsHFRS and hmsT. However, mutation in ompR exhibited a non-motile phenotype due to the lack of flagella. Biofilm formation was reduced and less c-di-GMP was produced through the downregulation of flhDC, hmsHFRS and hmsT expression when Y. enterocolitica was exposed to low osmolarity conditions. Furthermore, OmpR was identified to be important for Y. enterocolitica to grow in extreme temperature conditions. Importantly, ompR mutations in Y. enterocolitica were more sensitive to polymyxin B and sodium dodecyl sulfate than rcsB mutations. Since motility, biofilm formation and environmental tolerance are critical for bacterial colonization of the host, these findings indicated that OmpR is more critical than RcsB in shaping the pathogenic phenotype of Y. enterocolitica.

Transcriptomic analysis reveals the role of RcsB in suppressing bacterial chemotaxis, flagellar assembly and infection in Yersinia enterocolitica

Defining the Rcs (Regulator of Capsule Synthesis) regulon in Enterobacteriaceae has been the major focus of several recent studies. The overall role of the Rcs system in Yersinia enterocolitica is largely unknown. Our previous study showed that RcsB inhibits motility, biofilm formation and c-di-GMP production by negatively regulating flhDC, hmsHFRS and hmsT expression. To identify other cellular functions regulated by the RcsB, gene expression profiles of the wild type and ΔrcsB mutant were compared by RNA-Seq in this study. A total of 132 differentially expressed genes regulated by the RcsB have been identified, of which 114 were upregulated and 18 were downregulated. Further, the results of RNA sequencing were discussed with a focus on the predictive roles of RcsB in the inhibition of bacterial chemotaxis, flagellar assembly and infection. To confirm these predictions, we experimentally verified that the ΔrcsB mutant activated chemotactic behavior and flagella biosynthesis, and exhibited enhanced adhesion and invasion of Y. enterocolitica to Caco-2 cells. Although RcsB largely inhibits these physiological activities, the presence of RcsB is still of great significance for optimizing the survival of Y. enterocolitica as evidenced by our previous report that RcsB confers some level of resistance to the cationic antimicrobial peptide polymyxin B in Y. enterocolitica. Overall, the information provided in this study complements our understanding of Rcs phosphorelay in the regulation of Y. enterocolitica pathogenicity, and, simultaneously, provides clues to additional roles of the Rcs system in other members of family Enterobacteriaceae.

Urease Expression in Pathogenic Yersinia enterocolitica Strains of Bio-Serotypes 2/O:9 and 1B/O:8 Is Differentially Regulated by the OmpR Regulator

Yersinia enterocolitica exhibits a dual lifestyle, existing as both a saprophyte and a pathogen colonizing different niches within a host organism. OmpR has been recognized as a regulator that controls the expression of genes involved in many different cellular processes and the virulence of pathogenic bacteria. Here, we have examined the influence of OmpR and varying temperature (26°C vs. 37°C) on the cytoplasmic proteome of Y. enterocolitica Ye9N (bio-serotype 2/O:9, low pathogenicity). Differential label-free quantitative proteomic analysis indicated that OmpR affects the cellular abundance of a number of proteins including subunits of urease, an enzyme that plays a significant role in acid tolerance and the pathogenicity of Y. enterocolitica. The impact of OmpR on the expression of urease under different growth conditions was studied in more detail by comparing urease activity and the transcription of ure genes in Y. enterocolitica strains Ye9N and Ye8N (highly pathogenic bio-serotype 1B/O:8). Urease expression was higher in strain Ye9N than in Ye8N and in cells grown at 26°C compared to 37°C. However, low pH, high osmolarity and the presence of urea did not have a clear effect on urease expression in either strain. Further analysis showed that OmpR participates in the positive regulation of three transcriptional units encoding the multi-subunit urease (ureABC, ureEF, and ureGD) in strain Ye9N, but this was not the case in strain Ye8N. Binding of OmpR to the ureABC and ureEF promoter regions was confirmed using an electrophoretic mobility shift assay, suggesting that this factor plays a direct role in regulating the transcription of these operons. In addition, we determined that OmpR modulates the expression of a ureR-like gene encoding a putative regulator of the ure gene cluster, but in the opposite manner, i.e., positively in Ye9N and negatively in Ye8N. These findings provide some novel insights into the function of OmpR in adaptation strategies of Y. enterocolitica.

Osmoregulated Periplasmic Glucans Transmit External Signals Through Rcs Phosphorelay Pathway in Yersinia enterocolitica

Fast response to environmental changes plays a key role in the transmission and pathogenesis of Yersinia enterocolitica. Osmoregulated periplasmic glucans (OPGs) are known to be involved in environmental perception of several Enterobacteriaceae pathogens; however, the biological function of OPGs in Y. enterocolitica is still unclear. In this study, we investigated the role of OPGs in Y. enterocolitica by deleting the opgGH operon encoding enzymes responsible for OPGs biosynthesis. Complete loss of OPGs in the ΔopgGH mutant resulted in decreased motility, c-di-GMP production, biofilm formation and smaller cell size, whereas the overproduction of OPGs through restoration of opgGH expression promoted c-di-GMP/biofilm production and increased antibiotic resistance of Y. enterocolitica. Gene expression analysis revealed that opgGH deletion reduced transcription of flhDC, ftsAZ, hmsT and hmsHFRS genes regulated by the Rcs phosphorelay system, whereas additional deletion of rcs family genes (rcsF, rcsC, or rcsB) reversed this effect and restored motility and c-di-GMP/biofilm production but further reduced cell size. Furthermore, disruption of the Rcs phosphorelay increased the motility and promoted the induction of biofilm and c-di-GMP production regulated by OPGs through upregulating the expression of flhDC, hmsHFRS, and hmsT. However, deletion of genes encoding the EnvZ/OmpR phosphorelay downregulated the flhDC, hmsHFRS and hmsT expression, leading to the decreased motility and prevented the induction of biofilm and c-di-GMP production regulated by OPGs. These results indicated that Rcs phosphorelay had the effect on OPGs-mediated functional responses in Y. enterocolitica. Our findings disclose part of the biological role of OPGs and the underlying molecular mechanisms associated with Rcs system in the regulation of the pathogenic phenotype in Y. enterocolitica.

OmpR-Mediated Transcriptional Regulation and Function of Two Heme Receptor Proteins of Yersinia enterocolitica Bio-Serotype 2/O:9

We show that Yersinia enterocolitica strain Ye9 (bio-serotype 2/O:9) utilizes heme-containing molecules as an iron source. The Ye9 genome contains two multigenic clusters, hemPRSTUV-1 and hemPRST-2, encoding putative heme receptors HemR1 and HemR2, that share 62% amino acid identity. Expression of these proteins in an Escherichia coli mutant defective in heme biosynthesis allowed this strain to use hemin and hemoglobin as a source of porphyrin. The hemPRSTUV-1 and hemPRST-2 clusters are organized as operons, expressed from the p_hem−1 and weaker p_hem−2 promoters, respectively. Expression of both operons is negatively regulated by iron and the iron-responsive transcriptional repressor Fur. In addition, OmpR, the response regulator of two component system (TCSs) EnvZ/OmpR, represses transcription of both operons through interaction with binding sequences overlapping the −35 region of their promoters. Western blot analysis of the level of HemR1 in ompR, fur, and ompRfur mutants, showed an additive effect of these mutations, indicating that OmpR may regulate HemR expression independently of Fur. However, the effect of OmpR on the activity of the p_hem−1 promoter and on HemR1 production was observed in both iron-depleted and iron-replete conditions, i.e., when Fur represses the iron-regulated promoter. In addition, a hairpin RNA thermometer, composed of four uracil residues (FourU) that pair with the ribosome-binding site in the 5′-untranslated region (5′-UTR) of hemR1 was predicted by in silico analysis. However, thermoregulated expression of HemR1 could not be demonstrated. Taken together, these data suggest that Fur and OmpR control iron/heme acquisition via a complex mechanism based on negative regulation of hemR1 and hemR2 at the transcriptional level. This interplay could fine-tune the level of heme receptor proteins to allow Y. enterocolitica to fulfill its iron/heme requirements without over-accumulation, which might be important for pathogenic growth within human hosts.

Regulatory principles governing Salmonella and Yersinia virulence

Enteric pathogens such as Salmonella and Yersinia evolved numerous strategies to survive and proliferate in different environmental reservoirs and mammalian hosts. Deciphering common and pathogen-specific principles for how these bacteria adjust and coordinate spatiotemporal expression of virulence determinants, stress adaptation, and metabolic functions is fundamental to understand microbial pathogenesis. In order to manage sudden environmental changes, attacks by the host immune systems and microbial competition, the pathogens employ a plethora of transcriptional and post-transcriptional control elements, including transcription factors, sensory and regulatory RNAs, RNAses, and proteases, to fine-tune and control complex gene regulatory networks. Many of the contributing global regulators and the molecular mechanisms of regulation are frequently conserved between Yersinia and Salmonella. However, the interplay, arrangement, and composition of the control elements vary between these closely related enteric pathogens, which generate phenotypic differences leading to distinct pathogenic properties. In this overview we present common and different regulatory networks used by Salmonella and Yersinia to coordinate the expression of crucial motility, cell adhesion and invasion determinants, immune defense strategies, and metabolic adaptation processes. We highlight evolutionary changes of the gene regulatory circuits that result in different properties of the regulatory elements and how this influences the overall outcome of the infection process.

Deletion analysis of RcsC reveals a novel signalling pathway controlling poly-N-acetylglucosamine synthesis and biofilm formation in Escherichia coli

RcsC is a hybrid histidine kinase that forms part of a phospho-relay signal transduction pathway with RcsD and RcsB. Besides the typical domains of a sensor kinase, i.e. the periplasmic (P), linker (L), dimerization and H-containing (A), and ATP-binding (B) domains, RcsC possesses a receiver domain (D) at the carboxy-terminal domain. To study the role played by each of the RcsC domains, four plasmids containing several of these domains were constructed (PLAB, LAB, AB and ABD) and transformed into Escherichia coli K-12 strain BW25113. Different amounts of biofilm were produced, depending on the RcsC domains expressed: the plasmid expressing the ABD subdomains produced the highest amount of biofilm. This phenotype was also observed when the plasmids were transformed in a ΔrcsCDB strain. Biofilm formation was abolished in the pgaABCD and nhaR backgrounds. The results indicate the existence of a novel signalling pathway that depends on RcsC, yet independent of RcsD and RcsB, that activates the pgaABCD operon and, as a consequence, biofilm formation. This signalling pathway involves the secondary metabolite acetyl phosphate and the response regulator OmpR.

Insights into Vibrio parahaemolyticus CHN25 response to artificial gastric fluid stress by transcriptomic analysis

Vibrio parahaemolyticus is the causative agent of food-borne gastroenteritis disease. Once consumed, human acid gastric fluid is perhaps one of the most important environmental stresses imposed on the bacterium. Herein, for the first time, we investigated Vibrio parahaemolyticus CHN25 response to artificial gastric fluid (AGF) stress by transcriptomic analysis. The bacterium at logarithmic growth phase (LGP) displayed lower survival rates than that at stationary growth phase (SGP) under a sub-lethal acid condition (pH 4.9). Transcriptome data revealed that 11.6% of the expressed genes in Vibrio parahaemolyticus CHN25 was up-regulated in LGP cells after exposed to AGF (pH 4.9) for 30 min, including those involved in sugar transport, nitrogen metabolism, energy production and protein biosynthesis, whereas 14.0% of the genes was down-regulated, such as ATP-binding cassette (ABC) transporter and flagellar biosynthesis genes. In contrast, the AGF stress only elicited 3.4% of the genes from SGP cells, the majority of which were attenuated in expression. Moreover, the number of expressed regulator genes was also substantially reduced in SGP cells. Comparison of transcriptome profiles further revealed forty-one growth-phase independent genes in the AGF stress, however, half of which displayed distinct expression features between the two growth phases. Vibrio parahaemolyticus seemed to have evolved a number of molecular strategies for coping with the acid stress. The data here will facilitate future studies for environmental stresses and pathogenicity of the leading seafood-borne pathogen worldwide.

A Moraxella catarrhalis two-component signal transduction system necessary for growth in liquid media affects production of two lysozyme inhibitors

There are a paucity of data concerning gene products that could contribute to the ability of Moraxella catarrhalis to colonize the human nasopharynx. Inactivation of a gene (mesR) encoding a predicted response regulator of a two-component signal transduction system in M. catarrhalis yielded a mutant unable to grow in liquid media. This mesR mutant also exhibited increased sensitivity to certain stressors, including polymyxin B, SDS, and hydrogen peroxide. Inactivation of the gene (mesS) encoding the predicted cognate sensor (histidine) kinase yielded a mutant with the same inability to grow in liquid media as the mesR mutant. DNA microarray and real-time reverse transcriptase PCR analyses indicated that several genes previously shown to be involved in the ability of M. catarrhalis to persist in the chinchilla nasopharynx were upregulated in the mesR mutant. Two other open reading frames upregulated in the mesR mutant were shown to encode small proteins (LipA and LipB) that had amino acid sequence homology to bacterial adhesins and structural homology to bacterial lysozyme inhibitors. Inactivation of both lipA and lipB did not affect the ability of M. catarrhalis O35E to attach to a human bronchial epithelial cell line in vitro. Purified recombinant LipA and LipB fusion proteins were each shown to inhibit human lysozyme activity in vitro and in saliva. A lipA lipB deletion mutant was more sensitive than the wild-type parent strain to killing by human lysozyme in the presence of human apolactoferrin. This is the first report of the production of lysozyme inhibitors by M. catarrhalis.

Regulatory protein OmpR influences the serum resistance of Yersinia enterocolitica O:9 by modifying the structure of the outer membrane

The EnvZ/OmpR two-component system constitutes a regulatory pathway involved in bacterial adaptive responses to environmental cues. Our previous findings indicated that the OmpR regulator in Yersinia enterocolitica O:9 positively regulates the expression of FlhDC, the master flagellar activator, which influences adhesion/invasion properties and biofilm formation. Here we show that a strain lacking OmpR grown at 37°C exhibits extremely high resistance to the bactericidal activity of normal human serum (NHS) compared with the wild-type strain. Analysis of OMP expression in the ompR mutant revealed that OmpR reciprocally regulates Ail and OmpX, two homologous OMPs of Y. enterocolitica, without causing significant changes in the level of YadA, the major serum resistance factor. Analysis of mutants in individual genes belonging to the OmpR regulon (ail, ompX, ompC and flhDC) and strains lacking plasmid pYV, expressing YadA, demonstrated the contribution of the respective proteins to serum resistance. We show that Ail and OmpC act in an opposite way to the OmpX protein to confer serum resistance to the wild-type strain, but are not responsible for the high resistance of the ompR mutant. The serum resistance phenotype of ompR seems to be multifactorial and mainly attributable to alterations that potentiate the function of YadA. Our results indicate that a decreased level of FlhDC in the ompR mutant cells is partly responsible for the serum resistance and this effect can be suppressed by overexpression of flhDC in trans. The observation that the loss of FlhDC enhances the survival of wild-type cells in NHS supports the involvement of FlhDC regulator in this phenotype. In addition, the ompR mutant exhibited a lower level of LPS, but this was not correlated with changes in the level of FlhDC. We propose that OmpR might alter the susceptibility of Y. enterocolitica O:9 to complement-mediated killing through remodeling of the outer membrane.

Co-regulation of polysaccharide production, motility, and expression of type III secretion genes by EnvZ/OmpR and GrrS/GrrA systems in Erwinia amylovora

The EnvZ/OmpR and GrrS/GrrA systems, two widely distributed two-component systems in gamma-Proteobacteria, negatively control amylovoran biosynthesis in Erwinia amylovora, and the two systems regulate motility in an opposing manner. In this study, we examined the interplay of EnvZ/OmpR and GrrS/GrrA systems in controlling various virulence traits in E. amylovora. Results showed that amylovoran production was significantly higher when both systems were inactivated, indicating that the two systems act as negative regulators and their combined effect on amylovoran production appears to be enhanced. In contrast, reduced motility was observed when both systems were deleted as compared to that of grrA/grrS mutants and WT strain, indicating that the two systems antagonistically regulate motility in E. amylovora. In addition, glycogen accumulation was much higher in envZ/ompR and two triple mutants than that of grrS/grrA mutants and WT strain, suggesting that EnvZ/OmpR plays a dominant role in regulating glycogen accumulation, whereas levan production was significantly lower in the grrS/grrA and two triple mutants as compared with that of WT and envZ/ompR mutants, indicating that GrrS/GrrA system dominantly controls levan production. Furthermore, both systems negatively regulated expression of three type III secretion (T3SS) genes and their combined negative effect on hrp-T3SS gene expression increased when both systems were deleted. These results demonstrated that EnvZ/OmpR and GrrS/GrrA systems co-regulate various virulence factors in E. amylovora by still unknown mechanisms or through different target genes, sRNAs, or proteins, indicating that a complex regulatory network may be involved, which needs to be further explored.