Seasonal recovery of Edwardsiella piscicida from wild European eels and natural waters: Isolation methods, virulence and reservoirs

Identification of a novel transcriptional regulator, CorR, for copper stress response in Edwardsiella piscicida

Copper plays a vital role in the host-pathogen interface, potentially making components of the bacterial copper response suitable targets for the development of innovative antimicrobial strategies. The anti-copper arsenal of intracellular pathogens has expanded as an adaptation to survive copper toxicity in order to escape intracellular killing by the host immune system. Herein, we employed transposon insertion sequencing to investigate the genetic mechanisms underlying the survival of Edwardsiella piscicida under copper stress. A novel transcriptional regulator, ETAE_2324 (named CorR), was identified to participate in the response to copper ions by controlling the expression of copA, the core component of cytoplasmic copper homeostasis. Furthermore, CorR regulated the expression of virulent determinant eseB, influencing the in vivo colonization of E. piscicida. Collectively, our results contribute to the comprehension of the underlying mechanism of the adaption of intracellular pathogens to copper stress during bacterial infections.IMPORTANCECopper ions play a pivotal role in the interaction between bacteria and the host during infection. The host's innate immune system employs copper ions for their bactericidal properties, thereby making bacterial copper tolerance a crucial determinant of virulence. Edwardsiella piscicida, a significant marine pathogen, has caused substantial losses in the global aquaculture industry. To comprehensively investigate how E. piscicida responds to copper stress, we utilized transposon insertion sequencing to explore genes associated with copper tolerance in culture media containing different concentrations of copper ions. A novel transcriptional regulator, CorR, was identified to respond to copper ions and regulates the expression of crucial components of copper homeostasis CopA, along with the essential virulence factor EseB. These findings offer valuable insights into the underlying mechanisms that govern bacterial copper tolerance and present novel perspectives for the development of vaccines and therapeutic strategies targeting E. piscicida.

Monitoring the Starvation–Survival Response of Edwardsiella piscicida and E. tarda in Freshwater Microcosms, at Various Temperatures

Edwardsiella piscicida is an important fish pathogen responsible for economic losses in global aquaculture, and E. tarda is also a human zoonotic pathogen. In this study, the survival of E. piscicida and E. tarda strains kept in filtered and sterilized lake water microcosms was investigated during a 20-week period at 7 °C, 15 °C and 25 °C, as well as its pathogenicity retention during a starvation period. E. tarda V43.2 stayed culturable for 6 weeks at 7 °C, 9 weeks at 25 °C and 12 weeks at 15 °C. Both E. piscicida strains (V12.1 and V57.2) stayed culturable even longer, for at least 12 weeks at 7 °C, 15 °C and 25 °C under the same starvation conditions. After Edwardsiella cells entered into the VBNC state, some became shorter and ”rounded up,” but others aggregated and retained a short rod shape. Aggregates of Edwardsiella cells were common throughout the VBNC period, and a well-formed biofilm was observed for all tested strains at the end of the experiment. The growth capacity of VBNC cells was restored by cultivating microcosm water samples in LB broth at 28 °C. Resuscitated E. piscicida cells were as virulent for the European eel as the controls. Natural waters can be a reservoir for Edwardsiella, and its underestimation in environmental samples poses a risk to public health and aquaculture.

Interplay between ferric uptake regulator Fur and horizontally acquired virulence regulator EsrB coordinates virulence gene expression in Edwardsiella piscicida

Edwardsiella piscicida mediates hemorrhagic septicemia and is a leading pathogen of fish. E. piscicida invades and colonizes macrophages using type III and VI secretion systems (T3/T6SS) that are controlled by a two-component system (TCS) EsrA-EsrB. Iron acquisition is essential for E. piscicida pathogenesis and coordination between iron and TCS signaling in modulating bacterial virulence is not well understood. Here, we show that iron uptake systems are co-regulated by ferric uptake regulator (Fur) in E. piscicida. Fur bound to 98 genes that harbored conserved Fur-box to globally control the expression of ∼755 genes, including those encoding iron uptake systems, T3/T6SS, and Icc, cAMP phosphodiesterase that represses biofilm formation. Additionally, Fur, in complex with iron, bound to the esrB promoter to repress expression and ultimately attenuated virulence. Conversely, EsrB activated the expression of T3/T6SS and iron uptake systems to mitigate a shortage of intracellular iron during iron scarcity. Furthermore, EsrB directly bound to and activated the fur promoter, leading to Fur-ferrous ion-dependent esrB repression in the presence of iron. Finally, Fur-EsrB interplay was essential for bacterial fitness during in vivo infection and survival in seawater environments. Collectively, we highlight the mechanisms that underlie the reciprocal regulatory networks of iron homeostasis and virulence systems in E. piscicida.

Transcriptome RNA-seq revealed lncRNAs activated by Edwardsiella anguillarum post the immunization of OmpA protecting European eel (Anguilla anguilla) from being infected

Long noncoding RNAs (lncRNAs) play important roles in various biological activities as vital regulators. However, no study has focused on the lncRNA regulation of Outer membrane protein (OMP) immunization against aquatic bacterial infection. In this study, we examined the genome-wide expression of lncRNAs in the liver of European eel (Anguilla anguilla, Aa) administrated by a recombinant OmpA (rOmpA) from Edwardsiella anguillarum (Ea) to elucidate the functions of lncRNAs in the process of Ea infection and Aa anti-Ea infection using strand specific RNA-seq. Eels were challenged by Ea at 28 d post the immunization (dpi) of OmpA, and the result showed, compared to uninfected livers in the PBS group (Con group), the infected livers in the PBS group (Con_inf group) showed severe bleeding, hepatocyte atrophy and thrombi formed in the hepatic vessels; livers in the OmpA group (OmpA_inf) also formed slight thrombi in the hepatic vessels. The relative percent survival of eels in OmpA_inf vs Con_inf was 78.6%. Using high-throughput transcriptomics, we found 13405 lncRNAs in 3 compares of Con_inf vs Con, OmpA_inf vs Con and OmpA_inf vs Con_inf, of which 111, 129 and 158 DE-lncRNAs were ascertained. GO analysis of the DE-lncRNAs revealed the targeting DEGs were mainly involved in single-organism process, signaling, biological process and response to stimulus in BP, component of membrane in CC and binding in MF; KEGG pathways showed that the targeting DEGs in co-expression and co-location enriched in cell adhesion molecules. Finally, 54 DE-lncRNAs targeting 1675 DEGs were involved in an interaction network of 21692 co-expression and 483 co-location related links, of which 18 DE-lncRNAs appear to play crucial roles in anti-Ea infection. Thus, the interaction networks revealed crucial DE-lncRNAs underlying the process of Ea infection and Aa anti-Ea infection pre and post the immunization of OmpA.

Transcriptome of Edwardsiella anguillarum in vivo and in vitro revealed two-component system, ABC transporter and flagellar assembly are three pathways pathogenic to European eel (Anguilla anguilla)

Edwardsiella anguillarum is one of the common bacterial pathogens for the cultivated eels in China. The aim of this study was to reveal the cause of E. anguillarum pathogenic to European eel (Anguilla anguilla) from the perspective of the transcriptome. In this study, we first prepared E. anguillarum cultured in vitro and analysed the whole transcriptome after extracting the total RNA. Then, eels were i.p injected with E. anguillarum, and total RNA were extracted from the liver of European eels 48 h after the infection. After sequencing the transcriptome, we obtained average 1.97 × 10⁸ clean reads cultured in vitro and 1.36 × 10⁵ clean reads located in vivo after annotating all reads into the genome of E. anguillarum. The whole transcriptome showed, compared to the E. anguillarum cultured in vitro, 503 significantly up and 657 significantly down-regulated different expressed genes (DEGs) were observed. KEGG analysis showed that 38 DEGs of Two-Component System, 41 DEGs of ABC transporter, and 10 DEGs flagellar assembly pathways were highly upregulated in E. anguillarum located in vivo. Then, we designed primers to analyse the up-regulated DEGs through qRT-PCR and confirmed some up-regulated DEGs. The results of this study provide important reference for the further study of pathogen-host interaction between E. anguillarum and European eel.

Characterization of a novel live attenuated Edwardsiella piscicida vaccine based on the overexpressed type III secretion system and systematic deletion of the associated effectors

Edwardsiella piscicida causes edwardsiellosis in a variety of fish species and leads to tremendous economic losses in the global aquaculture industries. Thus, effective and safe prevention and control of this bacterium are urgently needed to combat the related infections. Live attenuated vaccines (LAVs) effectively prevent infectious diseases. However, most of the existing E. piscicida LAVs are based on the deletion of genes encoding the translocon components of the type III secretion system (T3SS), the core virulence system, which is the most prominent protective bacterial antigen with the strongest immunogenicity. In this study, we systematically deleted all of the 9 established T3SS effectors in E. piscicida (aka 9Δ) and the rpoS gene encoding the alternative sigma factor, the esrB repressor (10Δ), then we overexpressed esrB and T3SS in E. piscicida to obtain the recombinant strain 10Δ/esrBOE. The modified strains 10Δ and 10Δ/esrBOE exhibited severe attenuation and in vivo colonization defects. Additionally, vaccination by intraperitoneal injection with 10Δ and 10Δ/esrBOE could significantly upregulate the expression of the antigen recognition related gene (TLR5) and the adaptive immune response-related gene (MHC II) in the spleen/kidney of turbot fish, and it also enhanced the hosts' serum bactericidal capacity. Finally, vaccination with 10Δ/esrBOE led to increased immune protection against the challenge of wild type E. piscicida EIB202 in turbot fish. Collectively, these findings demonstrated that 10Δ/esrBOE was a novel LAV strain and therefore a potential novel strategy for the construction of LAVs against bacterial pathogens.

MviN mediates the regulation of environmental osmotic pressure on esrB to control the virulence in the marine fish pathogen Edwardsiella piscicida

Edwardsiella piscicida is a notorious pathogen infecting diverse kinds of fish and causes substantial economic losses in the global aquaculture industries. The EsrA-EsrB two-component system plays a critical role in the regulation of virulence genes expression, including type III and type VI secretion systems (T3/T6SSs). In this study, the putative regulators of esrB were screened by the transposon insertion sequencing (TIS) technology. As a result, MviN, a lipid II flippase, was identified as a modulator to upregulate esrB and downstream T3/T6SS gene expression in the earlier growth phases while downregulate esrB at the later stages. Complement or overexpression of the mviN restored the esrB as well as T3/T6SS expression in the ΔmviN mutant strain. Moreover, MviN also mediated the regulation of environmental osmotic pressure on the expression of esrB. MviN was also found to significantly influence the in vivo colonization of E. piscicida in turbot. Collectively, this study enhanced our understanding of pathogenesis and virulence regulatory network of E. piscicida.

Hematological and immune genes responses in yellow catfish (Pelteobagrus fulvidraco) with septicemia induced by Edwardsiella ictaluri

Yellow catfish (Pelteobagrus fulvidraco) has been an economically important freshwater species in China because of its good meat quality. In present, the high-density breeding industry has suffered great damage from bacterial infections, in especial, the rapid illness and death of fish caused by bacterial septicemia leads to huge economic losses. Therefore, it is urgent and important to identify pathogenic bacteria and study its pathogenicity. In this study, we isolated a bacterial strain from the yellow catfish with typical septicemia and named it E. 719, then, by morphological observations, regression infection, biochemical identification, 16S rDNA sequence analysis and triple PCR identification, E. 719 was determined to be Edwardsiella ictaluri. Further, we infected yellow catfish with E. ictaluri to study its effects on mortality rate, hematological, histopathological disturbances and expression of immune genes. The mortality results showed that E. ictaluri was highly pathogenic, all infected fish died after 14 days post injection, and the distribution of bacteria in body kidney, spleen, liver, head kidney and brain of fish was continuously detected by measuring the amount of bacteria in the tissues. In addition, the number of red blood cells decreased significantly with the time of infection, while the number of white blood cells and thrombocytes increased. In particular, the number of monocytes and neutrophils increased significantly in the differential leucocyte count (DLC). Histopathologic changes observed by HE staining showed similar results, gill, intestine, spleen and head kidney showed obvious inflammation, bleeding and necrosis. Besides, checking by real time quantitative RT-PCR assays, in both spleen and head kidney tissues which were the major immune organs, mRNA expressions of immune gene IL-1β, TNF-α, and MR significantly increased in the early and middle stages of infection, which suggested that the infection of E. ictaluri caused a strong immune response in yellow catfish. This study provides a preliminary basis for the diagnosis and treatment of pathophysiology septicemia in yellow catfish induced by E. ictaluri.

Universal stress proteins contribute Edwardsiella piscicida adversity resistance and pathogenicity and promote blocking host immune response

Universal stress proteins (Usps) exist ubiquitously in bacteria and other organisms. Usps play an important role in adaptation of bacteria to a variety of environmental stresses. There is increasing evidence that Usps facilitate pathogens to adapt host environment and are involved in pathogenicity. Edwardsiella piscicida (formerly included in E. tarda) is a severe fish pathogen and infects various important economic fish including tilapia (Oreochromis niloticus). In E. piscicida, a number of systems and factors that are involved in stress resistance and pathogenesis were identified. However, the function of Usps in E. piscicida is totally unknown. In this study, we examined the expressions of 13 usp genes in E. piscicida and found that most of these usp genes were up-regulated expression under high temperature, oxidative stress, acid stress, and host serum stress. Particularly, among these usp genes, usp13, exhibited dramatically high expression level upon several stress conditions. To investigate the biological role of usp13, a markerless usp13 in-frame mutant strain, TX01Δusp13, was constructed. Compared to the wild type TX01, TX01Δusp13 exhibited markedly compromised tolerance to high temperature, hydrogen peroxide, and low pH. Deletion of usp13 significantly retarded bacterial biofilm growth and decreased resistance against serum killing. Pathogenicity analysis showed that the inactivation of usp13 significantly impaired the ability of E. piscicida to invade into host cell and infect host tissue. Introduction of a trans-expressed usp13 gene restored the lost virulence of TX01Δusp13. In support of these results, host immune response induced by TX01 and TX01Δusp13 was examined, and the results showed reactive oxygen species (ROS) levels in TX01Δusp13-infected macrophages were significantly higher than those in TX01-infected cells. The expression level of several cytokines (IL-6, IL-8, IL-10, TNF-α, and CC2) in TX01Δusp13-infected fish was significantly higher than that in TX01-infected fish. These results suggested that the deletion of usp13 attenuated the ability of bacteria to overcome the host immune response to pathogen infection. Taken together, our study indicated Usp13 of E. piscicida was not only important participant in adversity resistance, but also was essential for E. piscicida pathogenicity and contributed to block host immune response to pathogen infection.

PepA binds to and negatively regulates esrB to control virulence in the fish pathogen Edwardsiella piscicida

As an important marine fish pathogen, Edwardsiella piscicida infects a broad range of fish species and causes substantial economic losses. The EsrA-EsrB two-component system is essential for the expression of type III and type VI secretion systems (T3/T6SSs), the key virulence determinants in the bacterium. In this study, a pull-down assay with the esrB promoter as bait was performed to identify the upstream regulators of esrB. As a result, PepA, a leucyl aminopeptidase, was identified as a repressor of EsrB and T3/T6SS expression. PepA bound to the esrB promoter region and negatively regulated the production of T3/T6SS proteins in early stages. Moreover, PepA was found to affect the in vivo colonization of E. piscicida in turbot livers through the regulation of EsrB expression. Collectively, our results enhance the understanding of the virulence regulatory network and in vivo colonization mechanism of E. piscicida. One sentence summary: PepA regulates EsrB expression in Edwardsiella piscicida.

Genome-Wide Identification of Fitness Factors in Seawater for Edwardsiella piscicida

Marine pathogens are transmitted from one host to another through seawater. Therefore, it is important for marine pathogens to maintain survival or growth in seawater. However, little is known about how marine pathogens adapt to living in seawater environments. Here, transposon insertion sequencing was performed to explore the genetic determinants of Edwardsiella piscicida survival in seawater at 16 and 28°C. Seventy-one mutants with mutations mainly in metabolism-, transportation-, and type III secretion system (T3SS)-related genes showed significantly increased or impaired fitness in 16°C water. In 28°C seawater, 63 genes associated with transcription and translation, as well as energy production and conversion, were essential for optimal survival of the bacterium. In particular, 11 T3SS-linked mutants displayed enhanced fitness in 16°C seawater but not in 28°C seawater. In addition, 13 genes associated with oxidative phosphorylation and 4 genes related to ubiquinone synthesis were identified for survival in 28°C seawater but not in 16°C seawater, which suggests that electron transmission and energy-producing aerobic respiration chain factors are indispensable for E. piscicida to maintain survival in higher-temperature seawater. In conclusion, we defined genes and processes related to metabolism and virulence that operate in E. piscicida to facilitate survival in low- and high-temperature seawater, which may underlie the infection outbreak mechanisms of E. piscicida and facilitate the development of improved vaccines against marine pathogens.IMPORTANCE Edwardsiella piscicida is one of the most important marine pathogens and causes serious edwardsiellosis in farmed fish during the summer-autumn seasonal changes, resulting in enormous losses to aquaculture industries worldwide. Survival and transmission of the pathogen in seawater are critical steps that increase the risk of outbreaks. To investigate the mechanism of survival in seawater for this marine pathogen, we used transposon insertion sequencing analysis to explore the fitness determinants in summer and autumn seawater. Approximately 127 genes linked to metabolism and virulence, as well as other processes, were revealed in E. piscicida to contribute to better adaptations to the seasonal alternations of seawater environments; these genes provide important insights into the infection outbreak mechanisms of E. piscicida and potential improved treatments or vaccines against marine pathogens.