Identification of Differentially Abundant Proteins of Edwardsiella ictaluri during Iron Restriction

Versatile lifestyles of Edwardsiella: Free-living, pathogen, and core bacterium of the aquatic resistome

Edwardsiella species in aquatic environments exist either as individual planktonic cells or in communal biofilms. These organisms encounter multiple stresses, include changes in salinity, pH, temperature, and nutrients. Pathogenic species such as E. piscicida, can multiply within the fish hosts. Additionally, Edwardsiella species (E. tarda), can carry antibiotic resistance genes (ARGs) on chromosomes and/or plasmids, that can be transmitted to the microbiome via horizontal gene transfer. E. tarda serves as a core in the aquatic resistome. Edwardsiela uses molecular switches (RpoS and EsrB) to control gene expression for survival in different environments. We speculate that free-living Edwardsiella can transition to host-living and vice versa, using similar molecular switches. Understanding such transitions can help us understand how other similar aquatic bacteria switch from free-living to become pathogens. This knowledge can be used to devise ways to slow down the spread of ARGs and prevent disease outbreaks in aquaculture and clinical settings.

Role of aspartate ammonia-lyase in Pasteurella multocida

BACKGROUND

Pasteurella multocida is responsible for a highly infectious and contagious disease in birds, leading to heavy economic losses in the chicken industry. However, the pathogenesis of this disease is poorly understood. We recently identified an aspartate ammonia-lyase (aspA) in P. multocida that was significantly upregulated under iron-restricted conditions, the protein of which could effectively protect chicken flocks against P. multocida. However, the functions of this gene remain unclear. In the present study, we constructed aspA mutant strain △aspA::kan and complementary strain C△aspA::kan to investigate the function of aspA in detail.

RESULT

Deletion of the aspA gene in P. multocida resulted in a significant reduction in bacterial growth in LB (Luria-Bertani) and MH (Mueller-Hinton) media, which was rescued by supplementation with 20 mM fumarate. The mutant strain △aspA::kan showed significantly growth defects in anaerobic conditions and acid medium, compared with the wild-type strain. Moreover, growth of △aspA::kan was more seriously impaired than that of the wild-type strain under iron-restricted conditions, and this growth recovered after supplementation with iron ions. AspA transcription was negatively regulated by iron conditions, as demonstrated by quantitative reverse transcription-polymerase chain reaction. Although competitive index assay showed the wild-type strain outcompetes the aspA mutant strain and △aspA::kan was significantly more efficient at producing biofilms than the wild-type strain, there was no significant difference in virulence between the mutant and the wild-type strains.

CONCLUSION

These results demonstrate that aspA is required for bacterial growth in complex medium, and under anaerobic, acid, and iron-limited conditions.

Cathelicidins enhance protection of channel catfish, Ictalurus punctatus, and channel catfish ♀ × blue catfish, Ictalurus furcatus ♂ hybrid catfish against Edwarsiella ictaluri infection

Cathelicidins are a class of antimicrobial peptides (AMPs) known to possess rapid and direct antimicrobial activities against a variety of microorganisms. Recently identified cathelicidins derived from alligator and sea snake were found to be more effective in inhibiting microbial growth than other AMPs previously characterized. The ability of these two cathelicidins along with the peptides, cecropin and pleurocidin, to protect channel catfish (Ictalurus punctatus, Rafinesque) and hybrid catfish (I. punctatus ♀ × blue catfish, Ictalurus furcatus, Valenciennes ♂) against Edwardsiella ictaluri, one of the most prevalent pathogens affecting commercial catfish industry, was investigated. Cathelicidin-injected fish (50 µg ml^-1  fish^-1 ) that were simultaneously challenged with E. ictaluri through bath immersion at a concentration of ~1 × 10⁶ CFU/ml had increased survival rates compared with other peptide treatments and the infected control. Bacterial numbers were also reduced in the liver and kidney of channel catfish and hybrid catfish in the cathelicidin treatments 24 hr post-infection. After 8 days of challenge, serum was collected to determine immune-related parameters such as bactericidal activity, lysozyme, serum protein, albumin and globulin. These immune-related parameters were significantly elevated in fish injected with the two cathelicidins as compared to other peptide treatments. These results indicate that cathelicidins derived from alligator and sea snake can stimulate immunity and enhance the resistance to E. ictaluri infection in channel catfish and hybrid catfish.

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.

Development and Characterization of a Novel Live Attenuated Vaccine Against Enteric Septicemia of Catfish

Edwardsiella ictaluri is a Gram-negative intracellular pathogen causing enteric septicemia of channel catfish (ESC). Type six secretion system (T6SS) is a sophisticated nanomachine that delivers effector proteins into eukaryotic host cells as well as other bacteria. In the current work, we in-frame deleted the E. ictalurievpB gene located in the T6SS operon by allelic exchange. The safety and efficacy of EiΔevpB as well as Aquavac-ESC, a commercial vaccine manufactured by Intervet/Merck Animal Health, were evaluated in channel catfish (Ictalurus punctatus) fingerlings and fry by immersion exposure. Our results showed that the EiΔevpB strain was avirulent and fully protective in catfish fingerlings. The EiΔevpB strain was also safe in catfish fry, and immersion vaccination with EiΔevpB at doses 10⁶ and 10⁷ CFU/ml in water resulted in 34.24 and 80.34% survival after wild-type immersion challenge compared to sham-vaccinated fry (1.79% survival). Catfish fry vaccinated with EiΔevpB at doses 10⁶, 10⁷, and 10⁸ CFU/ml in water exhibited dose-dependent protection. When compared with Aquavac-ESC, EiΔevpB provided significantly higher protection in catfish fingerlings and fry (p < 0.05). Results indicate that the EiΔevpB strain is safe and can be used to protect catfish fingerlings and fry against E. ictaluri.

Phagocytic and Bactericidal Properties of Channel Catfish Peritoneal Macrophages Exposed to Edwardsiella ictaluri Live Attenuated Vaccine and Wild-Type Strains

Edwardsiella ictaluri (E. ictaluri), a Gram-negative, intracellular, facultative bacterium, is the causative agent of enteric septicemia of catfish (ESC), which is one of the most significant diseases of farmed channel catfish. Macrophages have a critical role in major defense mechanisms against bacterial infections by migrating to the site of infection, engulfing and killing pathogens, and priming adaptive immune responses. Vaccination of catfish with E. ictaluri live attenuated vaccine (LAV) strains increased the efficiency of phagocytosis and bacterial killing in catfish peritoneal macrophages compared in vitro with macrophages from non-vaccinated fish. Recently, our group developed several protective LAV strains from E. ictaluri. However, their effects on the antigen uptake and bacterial killing in catfish macrophages have not been evaluated. In this study, we assessed the phagocytic and bactericidal activity of peritoneal macrophages in the uptake of E. ictaluri wild-type (WT) and two LAV strains. We found that phagocytosis of LAV strains was significantly higher compared to their WT counterpart in peritoneal macrophages. Moreover, the uptake of E. ictaluri opsonized with sera from vaccinated catfish was more efficient than when opsonized with sera from sham-vaccinated fish. Notably, catfish macrophages did not lose their phagocytic properties at 4°C, as described previously in mammalian and zebrafish models. Also, opsonization of E. ictaluri with inactivated sera from vaccinated and sham-vaccinated catfish decreased significantly phagocytic uptake of bacteria at 32°C, and virtually suppressed endocytosis at 4°C, suggesting the important role of complement-dependent mechanisms in catfish macrophage phagocytosis. In conclusion, our data on enhanced phagocytic capacity and effective killing ability in macrophages of vaccine strains suggested the LAVs’ advantage if processed and presented in the form of peptides to specific lymphocytes of an adaptive immune system and emphasize the importance of macrophage-mediated immunity against ESC. Furthermore, we showed the role of complement-dependent mechanisms in the phagocytic uptakes of E. ictaluri in catfish peritoneal macrophages at 4 and 32°C. Finally, LAV vaccine-induced bacterial phagocytosis and killing properties of peritoneal macrophages emphasized the importance of the innate immune responses in ESC.

The virulence and immune protection of Edwardsiella ictaluri HemR mutants in catfish

Edwardsiella ictaluri is a Gram-negative facultative intracellular rod, causing enteric septicemia of catfish (ESC). Several heme uptake systems have been described in bacterial pathogens, most of which involve outer membrane proteins (OMPs). We have shown recently that heme/hemoglobin receptor family protein (HemR) is significantly up-regulated in E. ictaluri under iron-restricted conditions. In this work, our goal was to construct E. ictaluri HemR mutants and assess their virulence and immune protection potentials in catfish. To accomplish this, an in-frame deletion mutant (EiΔhemR) was constructed, and its virulence and immune protection were determined in catfish fingerlings and fry. The results indicated that the EiΔhemR was attenuated completely in catfish fingerlings, but it was virulent in 14 day-old catfish fry. To increase the attenuation of EiΔhemR in fry, we introduced frdA and sdhC gene deletions to the mutant, yielding two double (EiΔhemRΔfrdA and EiΔhemRΔsdhC) and one triple (EiΔhemRΔfrdAΔsdhC) mutants. Results indicated that two double HemR mutants did not exhibit increased attenuation, but the triple HemR mutant showed significantly less virulence and high protection in fry (p < 0.05). Histological examination of fry tissues vaccinated with the triple mutant displayed similar inflammation to that of wild-type infected fry, but much less necrosis and far fewer bacteria were observed. Immunohistochemistry (IHC) result indicated fewer numbers of bacteria around blood vessel and in the hematopoietic tissue in fry infected with triple mutant compared to control group infected with E. ictaluri wild-type. Our data indicated that EiΔhemR was safe and protective in catfish fingerlings, while EiΔhemRΔfrdAΔsdhC was much safer in catfish fry.

The Role of TonB Gene in Edwardsiella ictaluri Virulence

Edwardsiella ictaluri is a Gram-negative facultative intracellular pathogen that causes enteric septicemia in catfish (ESC). Stress factors including poor water quality, poor diet, rough handling, overcrowding, and water temperature fluctuations increase fish susceptibility to ESC. The TonB energy transducing system (TonB-ExbB-ExbD) and TonB-dependent transporters of Gram-negative bacteria support active transport of scarce resources including iron, an essential micronutrient for bacterial virulence. Deletion of the tonB gene attenuates virulence in several pathogenic bacteria. In the current study, the role of TonB (NT01EI_RS07425) in iron acquisition and E. ictaluri virulence were investigated. To accomplish this, the E. ictaluri tonB gene was in-frame deleted. Growth kinetics, iron utilization, and virulence of the EiΔtonB mutant were determined. Loss of TonB caused a significant reduction in bacterial growth in iron-depleted medium (p > 0.05). The EiΔtonB mutant grew similarly to wild-type E. ictaluri when ferric iron was added to the iron-depleted medium. The EiΔtonB mutant was significantly attenuated in catfish compared with the parent strain (21.69 vs. 46.91% mortality). Catfish surviving infection with EiΔtonB had significant protection against ESC compared with naïve fish (100 vs. 40.47% survival). These findings indicate that TonB participates in pathogenesis of ESC and is an important E. ictaluri virulence factor.

Shotgun proteomic analysis of Yersinia ruckeri strains under normal and iron-limited conditions

Yersinia ruckeri is the causative agent of enteric redmouth disease of fish that causes significant economic losses, particularly in salmonids. Bacterial pathogens differentially express proteins in the host during the infection process, and under certain environmental conditions. Iron is an essential nutrient for many cellular processes and is involved in host sensing and virulence regulation in many bacteria. Little is known about proteomics expression of Y. ruckeri in response to iron-limited conditions. Here, we present whole cell protein identification and quantification for two motile and two non-motile strains of Y. ruckeri cultured in vitro under iron-sufficient and iron-limited conditions, using a shotgun proteomic approach. Label-free, gel-free quantification was performed using a nanoLC-ESI and high resolution mass spectrometry. SWATH technology was used to distinguish between different strains and their responses to iron limitation. Sixty-one differentially expressed proteins were identified in four Y. ruckeri strains. These proteins were involved in processes including iron ion capture and transport, and enzymatic metabolism. The proteins were confirmed to be differentially expressed at the transcriptional level using quantitative real time PCR. Our study provides the first detailed proteome analysis of Y. ruckeri strains, which contributes to our understanding of virulence mechanisms of Y. ruckeri, and informs development of novel control methods for enteric redmouth disease.

Quantitative proteomic analysis of cell envelope preparations under iron starvation stress in Aeromonas hydrophila

Background

Iron homeostasis is an essential process over the entire lives of both hosts and bacterial pathogens, and also plays roles in many other metabolic functions. Currently, knowledge is limited on the iron scavenging mechanism of the cell envelope in the aquatic pathogen, Aeromonas hydrophila. To understand the iron homeostasis mechanism in A. hydrophila, a dimethyl labelling based quantitative proteomics method was used to compare the differential expression of cell envelope proteins under iron starvation.

Results

A total of 542 cell envelope proteins were identified by LC-MS/MS, with 66 down-regulated and 104 up-regulated proteins. Bioinformatics analysis showed that outer membrane siderophores, heme and iron receptors, periplasmic iron binding proteins, inner membrane ABC transporters and H⁺-ATP synthase subunits increased in abundance while iron-cluster proteins, electron transport chain and redox proteins were down-regulated. Further q-PCR validation, in vivo addition of exogenous metabolites, and an enzyme inhibition assay revealed that redox, the energy generation process, and ATP synthase elevated the susceptibility of A. hydrophila to iron starvation.

Conclusions

Our study demonstrates that the redox and energy generation process, and ATP synthase in A. hydrophila may play critical roles in iron acquisition under conditions of iron-stress. An understanding of the iron scavenging mechanism may be helpful for the development of strategies for preventing and treating A. hydrophila infection.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-016-0769-5) contains supplementary material, which is available to authorized users.