Construction and efficacy of Aeromonas veronii mutant Δhcp as a live attenuated vaccine for the largemouth bass (Micropterus salmoides)

The deficiency of virulence factor AerA in Aeromonas veronii alleviates intestinal injury and microbiota disorders in largemouth bass (Micropterus salmoides)

Aeromonas veronii is a zoonotic pathogen that causes considerable mortality in aquaculture, with typical manifestations of intestinal injury. Aerolysin (AerA) is a pivotal virulence factor that influences Aeromonas pathogenicity. This study aimed to investigate the functional mechanism of AerA in A. veronii-induced intestinal injury in largemouth bass (Micropterus salmoides). The results revealed that wild-type (WT) A. veronii infection provoked a noteworthy augmentation of abundance of A. veronii in the intestine, accompanied by inflammation, partial repression of immune response, intestinal injury and high mortality. Compared to WT A. veronii infection, significantly reduced levels of A. veronii in the intestine, inflammation, intestinal injury and mortality, as well as enhanced immune response were observed in AerA mutant (ΔAerA) A. veronii infection. In addition, an increase in abundance of A. veroniii in the intestine, inflammation, and higher mortality were once again perceived subsequent to the use of recombinant AerA protein to compensate for AerA depletion. These results demonstrated that AerA deficiency reduce the increased abundance of A. veronii in the intestine, inflammation and intestinal injury caused by A. veronii infection, thereby reducing the virulence of A. veronii. Our findings enhance the understanding of attributes and functional role of AerA in A. veronii infection, and provide guidance for the development of disease control strategies.

The molecular mechanism of gspD gene in regulating the biological characteristics, pathogenicity and virulence gene expression of Photobacterium damselae subsp. damselae

The pathogenic bacterium Photobacterium damselae subsp. damselae (PDD) has the capacity to infect various mariculture fish species. The Type II secretion system is a component of PDD, facilitating the secretion of extracellular products. The gspD gene encodes the outer membrane secretory channel protein of T2SS. To investigate the role of the gspD gene in T2SS during PDD infection, we generated a gspD gene deletion mutant of PDD (ΔgspD-PDD) using a suicide plasmid-mediated homologous recombination technique and compared the biological characteristics, virulence gene expression, and pathogenicity of ΔgspD-PDD with those of the wild-type strain (WT-PDD). Our results indicated that the hemolytic activity and phospholipase activity of ΔgspD-PDD were significantly diminished compared to WT-PDD, and the complementation strain was restored to levels similar to those of the WT-PDD. The expression levels of T2SS-related genes and the virulence genes were significantly down-regulated, while the outer membrane-related gene and flagella-related genes exhibited significant up-regulation. The LD50 values of ΔgspD-PDD and its ECP in Sebastes schlegelii were 62.70-fold and 18.76-fold higher than those of WT-PDD, respectively. In summary, the mutation of the gspD gene may lead to the down-regulation of T2SS-related genes, resulting in aberrant secretion of ECPs in PDD and subsequently diminishing its pathogenicity.

Pathogenic Aeromonas veronii with the cheZ-mshK-aerA triple-gene mutant is attenuated and exhibits a potential candidate as a live attenuated vaccine

Aeromonas veronii is a zoonotic pathogen that is commonly found in various aquatic environments and causes serious damage to the aquaculture industry. Anti-virulence strategies based on mutating the virulence factors are important antibiotic alternative methods against A. veronii infection. The type Ⅳ pili, polar flagella, and aerolysin are considered to be the major virulence factors. In this study, we constructed a triple-gene deletion mutant strain (cheZ/mshK/aerA, ΔCMA-AV) and evaluated its potential as a live attenuated vaccine (LAV) candidate. Pathogenicity test showed that the LD50 of ΔCMA-AV in crucian carp (Carassius auratus) was 5.34 times higher than that of WT-AV. Further research found that the decline of pathogenicity was associated with the decrease of hemolysis, biofilm formation ability and expression levels of virulence-related genes (type IV pili and flagella). Our preliminary results on ΔCMA-AV as a LAV showed that crucian carp immunized with ΔCMA-AV at a concentration of 3 × 106 CFU/mL did not show any evident pathological alterations or clinical symptoms. Non-specific immune indicators, including serum immune-related enzyme activities and the expression of immune-related genes in immune organs, showed an up-regulation trend in the ΔCMA-AV group. A. veronii specific antibody levels increased significantly from 2 to 4 weeks. The relative percent survival in ΔCMA-AV groups was 73.97 % ± 3.57 %, significantly higher than that of the inactivated A. veronii groups (53.73 % ± 10.87 %). These results indicate that the three genes (cheZ/mshK/aerA) play an important role in the pathogenicity of A. veronii, in additionΔCMA-AV could induce an effective immune response and provide strong protection against A. veronii in crucian carp without adverse effects.

Genome-wide association analysis study on host resistance against the Aeromonas veronii of largemouth bass Micropterus salmoides

Largemouth bass (Micropterus salmoides) has become one of the most important freshwater economic fish farmed almost all over China in recent years. At the same time, the increasing outbreaks of diseases in its aquaculture process have caused substantial economic losses to this industry. However, at present, the genetic basis of disease resistance, including resistance against Aeromonas veronii infection, in largemouth bass is very limited. Therefore, a genome-wide association study (GWAS) on host resistance against the A. veronii of largemouth bass was conducted in the present study. A total of 627 largemouth bass were artificially challenged by A. veronii, among which 160 of the earliest deaths and 173 of the final survivals were genotyped. A total of 3076 high-quality SNPs were used for further analysis employing two analysis models, of which six shared SNPs were finally identified as significant molecular markers with the explaining phenotypic variance ranging from 2.28 % to 8.95 %. Furthermore, seven candidate genes were identified, including one gene, T-cell surface antigen CD2, which is directly involved in T cell activation and the cellular immune response. Additionally, the other identified genes play roles in critical processes such as cell survival, inflammatory responses, and signal transduction. This study lays a genetic foundation for research on largemouth bass disease resistance and studies related to A. veronii. It also contributes significantly to the future development of the commercial production of largemouth bass.

Emodin Improves Juvenile Largemouth Bass (Micropterus salmoides) Liver Health Through Nrf2/NF-κB Pathway and Fat Metabolism: Growth Performance, Immune Response and Resistance Against Aeromonas veronii Infection

The experiment was aimed at examining the influence of adding emodin to feeds on the growth performance, liver immunity, and resistance against Aeromonas veronii infection among juvenile largemouth basses and other potential mechanisms. A total of 540 fish (45 ± 0.3 g) were randomly divided into 6 diets, including EM-0, EM-250, EM-500, EM-1000, EM-2000, and EM-4000 diets, in which 0, 250, 500, 1000, 2000, and 4000 mg kg-1 emodin was added. Following a 60-day feeding test, it demonstrated that the feed conversion ratio (FCR) of juveniles within the EM-500 and EM-1000 groups remarkably exceeded that of the EM-0 group. Subsequently, unlike those in EM-0 group, the fish in the EM-1000 group showed heightened hepatocyte count, induced hepatic lipolysis-associated expression of peroxisome proliferators-activated receptor α (PPARα) and acyl-coenzyme an oxidase (ACO), and reduced the hepatic triglyceride (TG) levels. Additionally, EM-1000 could up-regulate the expressions of nuclear factor erythroid 2-associated factor 2 (Nrf2) and heme oxygenase-1 (HO-1) in livers compared with controls and boost antioxidant enzymes activities of superoxide dismutase (SOD), glutathione peroxidase (GSH-Px), and catalase (CAT), along with a lower content of reactive oxygen species (ROS) and malondialdehyde (MDA). Meanwhile, the EM-1000 group increased anti-inflammatory cytokines of interleukin-10 (IL-10) while suppressing the interleukin-8 (IL-8) expression of pro-inflammatory cytokines in livers by contrast to controls. In the end, juvenile largemouth bass in the EM-1000 group showed a comparatively highest survival rate, whereas fish in the EM-2000 and EM-4000 groups exhibited a little higher mortality than that of the EM-0 group. To sum up, our study exposed that supplementing emodin with 1000 mg kg-1 in diet could enhance the hepatic antioxidant status and unspecific immunity to reinforce the protective effect on disease resistance, resulting in improving the growth performance in juvenile largemouth bass.

A comprehensive understanding of the influence and molecular mechanism of exeA on the pathogenicity in Aeromonas hydrophila

Aeromonas hydrophila is a serious human and animal co-pathogenic bacterium. The type II secretion system (T2SS), a key virulence factor, is vital for the secretion of exotoxins from the bacterium. exeA gene is important for the assembly of the T2SS. However, the role of exeA in the pathogenesis of A. hydrophila is not yet clear. In this study, we constructed a stable A. hydrophila strain with exeA mutation (∆exeA-AH) using homologous recombination. Compared to wild type A. hydrophila (WT-AH), the median lethal doses (LD50) significantly increased in ∆exeA-AH. Biological properties of ∆exeA-AH were analyzed to explain the reasons for changes in virulence. The results showed that there was a significant decline in biofilm formation capacity, no significant differences were found in growth ability, hemolytic activity, motility and external structure. In order to further investigate the molecular mechanism of decreased virulence, WT-AH and ∆exeA-AH were subjected to transcriptomic analysis and validated by realtime fluorescence quantitative polymerase chain reaction and western blot. The results showed that the mutation of exeA affected the assembly of T2SS and biofilm formation capacity by decreasing the uptake capacity of iron ions. However, the abilities of T6SS, Sec system, Tat system, signaling peptidase and Lol system were enhanced, hindering further reduction in virulence. In summary, exeA mutation led to a reduction in virulence by impairing the function of T2SS and the ability of biofilm formation but impeded further decline by enhancing T6SS, Sec system, Tat system, signaling peptidase and Lol system.

Updates on infectious diseases of largemouth bass: A major review

The largemouth bass (Micropterus salmoides) is native to North America and has now become a crucial economic species in aquaculture. With the rapid development of high-density intensive farming models, the continuous emergence and spread of diseases pose significant challenges to the sustainable development of largemouth bass aquaculture, including Micropterus salmoides rhabdovirus (MSRV), largemouth bass virus (LMBV), Nocardia spp. and Aeromonas spp. Here, we provide a comprehensive overview of the latest research progress on common diseases of largemouth bass, including pathogen isolation and identification, pathological characteristics, morphological features, epidemiological characteristics, pathogen-host interactions, detection and diagnosis, vaccines, and other control technologies. This information will enhance a more comprehensive understanding of the occurrence of diseases in largemouth bass, and provide insights into future research directions, facilitating more effective disease prevention and control. The collaborative progress among rapid detection technology, the interaction mechanism between pathogen and host, and prevention and control techniques will be the curial to achieving green prevention and control of largemouth bass disease and healthy aquaculture in future.

The role of aroA and ppk1 in Aeromonas veronii pathogenicity and the efficacy evaluation of mutant strain AV-ΔaroA/ppk1 as a live attenuated vaccine

Aeromonas veronii is an opportunistic pathogen that poses great threat to aquaculture and human health, so there is an urgent need for green and efficient methods to deal with its infection. In this study, single and double gene deletion strains (AV-ΔaroA, AV-Δppk1 and AV-ΔaroA/ppk1) that can be stably inherited were constructed. Pathogenicity test showed that the toxicity of AV-ΔaroA and AV-ΔaroA/ppk1 was significantly lower compared to wild-type A. veronii. Biological characterization analysis revealed that the decrease in pathogenicity might be due to the declined growth, motility, biofilm formation abilities and the expression of virulence-related genes in mutants. Subsequently, we evaluated the efficacy of AV-ΔaroA/ppk1 as a live attenuated vaccine (LAV). Safety assessment experiments showed that AV-ΔaroA/ppk1 injected at a concentration of 3 × 107 CFU/mL was safe for C. carassius. The relative percentage survival of AV-ΔaroA/ppk1 was 67.85 %, significantly higher than that of the inactivated A. veronii, which had an RPS of 54.84 %. This improved protective effect was mainly attributed to the increased levels of A. veronii specific IgM antibody, enhanced alkaline phosphatase, lysozyme and superoxide dismutase activities, as well as higher expression levels of several immune related genes. Together, these findings deepen our understanding of the functional roles of aroA and ppk1 in A. veronii pathogenicity, provide a good candidate of LAV for A. veronii.

Interleukin-1 beta (IL-1β) as adjuvant enhances the immune effects of Aeromonas veronii inactivated vaccine in largemouth bass (Micropterus salmoides)

Largemouth bass (Micropterus salmoides) has emerged as a significant economic fish species, with a rise in Aeromonas veronii infections in farming. However, research on adjuvants for vaccines against A. veronii in largemouth bass remains scarce. In present study, recombinant largemouth bass IL-1β (LbIL-1β) was expressed to explore its adjuvant effect on the A. veronii inactivated vaccine. Following vaccination with recombinant LbIL-1β (rLbIL-1β) and the inactivated A. veronii, higher serum SOD levels and lysozyme activities were observed in largemouth bass from inactivated A. veronii + rLbIL-1β vaccinated group. Furthermore, it was discovered that rLbIL-1β was able to boost the serum-specific antibody levels induced by the inactivated A. veronii. The qRT-PCR analysis revealed that rLbIL-1β also enhanced the expression of IgM, CD4, and MHC II in largemouth bass triggered by the inactivated A. veronii. After challenged with live A. veronii, the outcomes demonstrated that the relative percentage survival (RPS) for largemouth bass resulting from the inactivated A. veronii in combination with rLbIL-1β was 76.67 %, surpassing the RPS of 60 % in the inactivated A. veronii group. Collectively, these findings indicate that rLbIL-1β enhances the protective effect of the A. veronii inactivated vaccine on largemouth bass, showcasing potential as an adjuvant for further development.

Diet with optimal glutathione supplement improves growth, nonspecific immunity, intestinal microbiota, and antioxidant ability in Micropterus salmoides

In this study, Micropterus salmoides were fed with dietary glutathione (GSH, 0, 100, 300, and 500 mg/kg) for 56 days to investigate its effects on growth performance, serum nonspecific immunity, liver antioxidant capacity, tissue morphology, and intestinal microbiota. The results showed that the survival rate, weight gain rate, and specific growth rate and condition factor increased, whereas the feed conversion ratio, hepato-somatic index, and viscerosomatic index decreased in the GSH groups. Compared with the control group, the serum total protein content significantly increased, whereas the triglyceride and total cholesterol significantly decreased in the 300-mg/kg dietary GSH group. The activities of lysozyme, alkaline phosphatase, and acid phosphatase were significantly higher in GSH-supplemented groups, peaking at 300-mg/kg GSH. GSH supplementation significantly increased total antioxidant capacity and decreased malondialdehyde content, with the most pronounced effects at 300-mg/kg GSH. Further antioxidant indicators showed that a dietary supplement of 300-mg/kg GSH significantly increased the activities of superoxide dismutase, glutathione transferase, endogenous glutathione, glutathione reductase, and catalase. At 300-mg/kg GSH, the liver exhibited improved characteristics with alleviated vacuolation and hepatocyte nuclear shift, and intestine showed enhanced structure with increased villus height and intestinal wall thickness. Additionally, a 300-mg/kg GSH supplementation improved the diversity of intestinal microbiota, increased the abundance of probiotics such as Bacillus, and inhibited the development of pathogenic bacteria such as Plesiomonas. Overall, the results suggest that the effect of GSH addition on improving growth performance, nonspecific immunity, antioxidant capacity, and intestinal microbiota of M. salmoides is best in the 300-mg/kg addition group. Based on second-degree polynomial regression analysis of weight gain, the optimum requirement of dietary GSH in M. salmoides is a 336.84-mg/kg diet.

Comparison of the efficacy of Aeromonas veronii ΔhisJ vaccine in Carassius auratus via different immunization routes

Introdction

Aeromonas veronii is a significant pathogen to various aquatic life. Infections in fish can lead to high mortality rates, causing substantial economic losses in aquaculture. Vaccination is proposed as a substitute for antibiotics in aquaculture to decrease disease-related mortality and morbidity. Our study previously constructed a hisJ-deleted strain of A. veronii, which provided protective effect to Loach.

Methods

To further assess the vaccine's applicability, this study evaluated its genetic stability and safety, and the immune protective effects in Carassius auratus through four distinct administration routes: intraperitoneal injection, intramuscular injection, oral administration, and immersion, to determine the efficacy of these administration routes.

Results

The results showed that the vaccine remained genetically stable after 45 generations. Immunization via these administration routes was safe for Carassius auratus, with intraperitoneal and intramuscular injections causing stronger adverse reactions. Immersion immunization resulted in mild adverse reactions, and no significant adverse reactions were observed following oral immunization. Immunizing Carassius auratus at safe concentrations via these routes enhanced the phagocytic activity in serum, increased the levels of non-specific immune-related enzymes (ACP, AKP, C3, C4, LZM, SOD, and IgM), and improved specific serum antibody levels. It also elevated levels of cytokines related to inflammatory responses (IL-1β, IL-10, TNF-α, TGF-β) in organ tissues (liver, spleen, kidney, mid-post intestine, and gills). The survival rates of Carassius auratus were measured after challenging with the virulent strain A. veronii TH0426, resulting in the relative survival rates of 64% for Intraperitoneal vaccine group, 56% for Intramuscular vaccine group, 52% for oral vaccine group, and 48% for immersion vaccine group. Analysis of bacterial load in the liver, spleen, and kidney post-challenge showed a decreasing trend in the control group, indicating that the vaccine strain ΔhisJ could gradually restrict the rapid proliferation of bacteria in these tissues, thereby providing a certain level of immune protection against A. veronii.

Discussion

In brief, the vaccine strain ΔhisJ can serve as a safe live attenuated vaccine for Carassius auratus, and this study lays the foundation for the development of live attenuated vaccines against Aeromonas veronii.

The role and molecular mechanism of flgK gene in biological properties, pathogenicity and virulence genes expression of Aeromonas hydrophila

Aeromonas hydrophila is a highly pathogenic aquatic resident bacterium that can cause co-morbidity in aquatic animals, waterfowl, poultry, and humans. Flagellum is the motility organ of bacteria important for bacterium tissue colonization and invasion. The flgK gene encodes a flagellar hook protein essential for normal flagellar formation. In order to explore the role of flgK in A. hydrophila, a flgK gene mutant strain of A. hydrophila (∆flgK-AH) was constructed using an efficient suicide plasmid-mediated homologous recombination method, and gene sequencing confirmed successful mutation of the flgK gene. The biological properties, pathogenicity and virulence genes expression were compared. The results showed that there was no significant difference in the growth, hemolytic, and swarming abilities, but the swimming and biofilm formation abilities of ∆flgK-AH were significantly reduced and the transmission electron microscope (TEM) results showed that the ∆flgK-AH strain did not have a flagellar structure. The median lethal dose (LD50) value of the ∆flgK-AH in Carassius auratus was 1.47-fold higher than that of the wild-type strain (WT-AH). The quantitative real-time PCR results showed that only the expression level of the lapA gene was up-regulated by 1.47 times compared with the WT-AH, while the expression levels of other genes were significantly down-regulated. In conclusion, flgK gene mutant led to a decline in the pathogenicity possibly by reducing swimming and biofilm formation abilities, these biological properties might result from the down-regulated expression of flagellate and pilus-related genes.