Attenuation of a virulent Aeromonas hydrophila with novobiocin and pathogenic characterization of the novobiocin-resistant strain

Cloning and characterization of an aerolysin gene from a marine pathogen Vibrio splendidus

Vibrio splendidus is one of the main pathogens caused diseases with a diversity of marine cultured animals, especially the skin ulcer syndrome in Apostichopus japonicus. However, limited virulence factors have been identified in V. splendidus. In this study, one aerAVs gene coding an aerolysin of V. splendidus was cloned and conditionally expressed in Escherichia coli. The haemolytic activity of the recombinant AerAVs was analyzed. Western blotting was used to study of the secretion pathway of proaerolysin, and it showed that the proaerolysin was secreted via both outer membrane vehicles and classical secretion pathways. Since no active protein of aerolysin was obtained, one aerolysin surface displayed bacterium DH5α/pAT-aerA was constructed, and its haemolytic activity and virulence were determined. The results showed that the AerAVs displayed on the surface showed obvious haemolytic activity and cytotoxic to the coelomocyte of A. japonicus. Artificial immerse infection separately using the DH5α/pAT or DH5α/pAT-aerA was conducted. The result showed that the mortality percent of sea cucumber A. japonicus challenged with DH5α/pAT-aerA was 38.89 % higher than that challenged with the control strain DH5α/pAT, and earlier death occurred. Combined all the results indicates that aerolysin with the haemolytic activity and cytotoxic activity is a virulence factor of V. splendidus.

The Attenuation Mechanism and Live Vaccine Potential of a Low-Virulence Edwardsiella ictaluri Strain Obtained by Rifampicin Passaging Culture

The rifampicin-resistant strain E9-302 of Edwardsiella ictaluri strain 669 (WT) was generated by continuous passage on BHI agar plates containing increasing concentrations of rifampicin. E9-302 was attenuated significantly by 119 times to zebrafish Danio rerio compared to WT in terms of the 50% lethal dose (LD₅₀). Zebrafish vaccinated with E9-302 via intraperitoneal (IP) injection at a dose of 1 × 10³ CFU/fish had relative percentage survival (RPS) rates of 85.7% when challenged with wild-type E. ictaluri via IP 14 days post-vaccination (dpv). After 14 days of primary vaccination with E9-302 via immersion (IM) at a dose of 4 × 10⁷ CFU/ml, a booster IM vaccination with E9-302 at a dose of 2 × 10⁷ CFU/ml exhibited 65.2% RPS against challenge with wild-type E. ictaluri via IP 7 days later. These results indicated that the rifampicin-resistant attenuated strain E9-302 had potential as a live vaccine against E. ictaluri infection. A previously unreported amino acid site change at position 142 of the RNA polymerase (RNAP) β subunit encoded by the gene rpoB associated with rifampicin resistance was identified. Analysis of the whole-genome sequencing results revealed multiple missense mutations in the virulence-related genes esrB and sspH2 in E9-302 compared with WT, and a 189 bp mismatch in one gene, whose coding product was highly homologous to glycosyltransferase family 39 protein. This study preliminarily explored the molecular mechanism underlying the virulence attenuation of rifampicin-resistant strain E9-302 and provided a new target for the subsequent study of the pathogenic mechanism of E. ictaluri.

Construction and Characterization of an Aeromonas hydrophila Multi-Gene Deletion Strain and Evaluation of Its Potential as a Live-Attenuated Vaccine in Grass Carp

Aeromonas hydrophila is an important pathogen that causes motile Aeromonas septicemia (MAS) in the aquaculture industry. Aerolysin, hemolysin, serine protease and enterotoxins are considered to be the major virulence factors of A. hydrophila. In this study, we constructed a five-gene (aerA, hly, ahp, alt and ast) deletion mutant strain (named Aeromonas hydrophila five-gene deletion strain, AHFGDS) to observe the biological characteristics and detect its potential as a live-attenuated vaccine candidate. AHFGDS displayed highly attenuated and showed increased susceptibility to fish blood and skin mucus killing, while the wild-type strain ZYAH72 was highly virulent. In zebrafish (Danio rerio), AHFGDS showed a 240-fold higher 50% lethal dose (LD50) than that of the wild-type strain. Immunization with AHFGDS by intracelomic injection or immersion routes both provided grass carp (Ctenopharyngodon idella) significant protection against the challenge of the strain ZYAH72 or J-1 and protected the fish organs from serious injury. Further agglutinating antibody titer test supported that AHFGDS could elicit a host-adaptive immune response. These results suggested the potential of AHFGDS to serve as a live-attenuated vaccine to control A. hydrophila infection in aquaculture.

Advancements in Characterizing Tenacibaculum Infections in Canada

Tenacibaculum is a genus of gram negative, marine, filamentous bacteria, associated with the presence of disease (tenacibaculosis) at aquaculture sites worldwide; however, infections induced by this genus are poorly characterized. Documents regarding the genus Tenacibaculum and close relatives were compiled for a literature review, concentrating on ecology, identification, and impacts of potentially pathogenic species, with a focus on Atlantic salmon in Canada. Tenacibaculum species likely have a cosmopolitan distribution, but local distributions around aquaculture sites are unknown. Eight species of Tenacibaculum are currently believed to be related to numerous mortality events of fishes and few mortality events in bivalves. The clinical signs in fishes often include epidermal ulcers, atypical behaviors, and mortality. Clinical signs in bivalves often include gross ulcers and discoloration of tissues. The observed disease may differ based on the host, isolate, transmission route, and local environmental conditions. Species-specific identification techniques are limited; high sequence similarities using conventional genes (16S rDNA) indicate that new genes should be investigated. Annotating full genomes, next-generation sequencing, multilocus sequence analysis/typing (MLSA/MLST), matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF), and fatty acid methylesters (FAME) profiles could be further explored for identification purposes. However, each aforementioned technique has disadvantages. Since tenacibaculosis has been observed world-wide in fishes and other eukaryotes, and the disease has substantial economic impacts, continued research is needed.

Immune responses induced by inactivated vaccine against Aeromonas hydrophila in pacu, Piaractus mesopotamicus

Aeromonas hydrophila is responsible for outbreaks of a severe infectious disease in fish farms around the world and is one of the major causes of economic losses to the neotropical fish farmers. This study assessed the induction of immune responses and protection against A. hydrophila in pacu, Piaractus mesopotamicus, vaccinated through intraperitoneal and immersion route with inactivated virulent strain. Fish were randomly distributed in three vaccinated groups: intraperitoneal (i.p.) route; immersion; and immersion + booster; and control group (unvaccinated). All vaccination protocols used the concentration of 1.7 × 10⁸ CFU mL^-1 of inactivated A. hydrophila., and an oil adjuvant was used for vaccine prepararion for i.p. route vaccination. Blood and skin mucus from 9 fishes per treatment were collected at 14, 28, 42 and 84 days post-vaccination (DPV) for determination of lysozyme concentration in skin mucus, as well as antibodies anti-A. hydrophila in blood serum and skin mucus. Fish were challenged at 84 DPV with homologous and virulent strain of A. hydrophila for evaluation of resistance against bacterial infection. The results demonstrated that vaccination with inactivated A. hydrophila suspension by i.p. or immersion resulted in significant increase of skin mucus lysozyme and specific antibody levels in serum and skin mucus, at 28 and 42 DPV, and this increase in innate and adaptive immunity remained significant in pacu vaccinated through i.p. route up to 84 DPV. Although no significant differences were observed in the survival study, pacu vaccinated through i.p. route presented 31,33% of relative percentage survival (RPS) in LD_50-96h when compared unvaccinated fish challenged at 84 DPV. The results observed in this study indicate that vaccination programs with inactivated A. hydrophila, including booster doses by i.p. or immersion routes, could result in more effective protection in pacu against this bacteriosis, by increasing innate and adaptive mucosal and systemic immune responses.

Efficacy of live attenuated vaccine derived from the Streptococcus agalactiae on the immune responses of Oreochromis niloticus

Streptococcus agalactiae species have been recognized as the main pathogen causing high mortality in fish leading to significant worldwide economical losses to the aquaculture industries. Vaccine development has become a priority in combating multidrug resistance in bacteria; however, there is a lack of commercial live attenuated vaccine (LAV) against S. agalactiae in Malaysia. The aim of this study is to compare two methods using attenuated bacteria as live vaccine and to evaluate the efficacy of selected LAV on the immune responses and resistance of Oreochromis niloticus (tilapia) against S. agalactiae. The LAV derived from S. agalactiae had been weakened using the chemical agent Acriflavine dye (LAV1), whereas the second vaccine was weakened using serial passages of bacteria on broth media (LAV2). Initial immunization was carried out only on day one, given twice-in the morning and evening, for the 42 day period. Serum samples were collected to determine the systemic antibody (IgM) responses and lysozymal (LSZ) activity using ELISA. On day 43 after immunization, the fish were injected intraperitoneally (i.p) with 0.1 mL of S. agalactiae at LD₅₀ = 1.5 × 10⁵ (CFU)/fish. Fish were monitored daily for 10 days. Clinical signs, mortality and the relative percent of survival (RPS) were recorded. Trial 1 results showed a significant increased (P < 0.05) in serum IgM titers and LSZ activity as compared to LAV2 and the control group (unvaccinated fish). The efficacy of LAV1 was proven effective as determined by the RPS values, LAV1 at 81.58% as compared to LAV2 at 65.79%. Trial 2 of LAV1 and control group were further determined by administering primary and booster doses revealed a RPS value for LAV1 of 82.05%, with the significant enhancement on the immune responses of tilapia as compared to control group. In conclusion, LAV revealed to elevate antibody IgM levels, LSZ activity and provide long-term protection when added to feed. LAV is a low-cost vaccine shown to rapidly increase the immune response of fish and increase survival rates of fish against S. agalactiae infection.

More than just antibodies: Protective mechanisms of a mucosal vaccine against fish pathogen Flavobacterium columnare

A recently developed attenuated vaccine for Flavobacterium columnare has been demonstrated to provide superior protection for channel catfish, Ictalurus punctatus, against genetically diverse columnaris isolates. We were interested in examining the mechanisms of this protection by comparing transcriptional responses to F. columnare challenge in vaccinated and unvaccinated juvenile catfish. Accordingly, 58 day old fingerling catfish (28 days post-vaccination or unvaccinated control) were challenged with a highly virulent F. columnare isolate (BGSF-27) and gill tissues collected pre-challenge (0 h), and 1 h and 2 h post infection, time points previously demonstrated to be critical in early host-pathogen interactions. Following RNA-sequencing and transcriptome assembly, differential expression (DE) analysis within and between treatments revealed several patterns and pathways potentially underlying improved survival of vaccinated fish. Most striking was a pattern of dramatically higher basal expression of an array of neuropeptides (e.g. somatostatin), hormones, complement factors, and proteases at 0 h in vaccinated fish. Previous studies indicate these are likely the preformed mediators of neuroendocrine cells and/or eosinophilic granular (mast-like) cells within the fish gill. Following challenge, these elements fell to almost undetectable levels (>100-fold downregulated) by 1 h in vaccinated fish, suggesting their rapid release and/or cessation of synthesis following degranulation. Concomitantly, levels of pro-inflammatory cytokines (IL-1b, IL-8, IL-17) were induced in unvaccinated fish. In contrast, in vaccinated catfish, we observed widespread induction of genes needed for collagen deposition and tissue remodeling. Taken together, our results indicate an important component of vaccine protection in fish mucosal tissues may be the sensitization, proliferation and arming of resident secretory cells in the period between primary and secondary challenge.

Vaccination of channel catfish with extracellular products of Aeromonas hydrophila provides protection against infection by the pathogen

Aeromonas hydrophila, a Gram-negative bacterium, is one of the economically-important pathogens in modern aquaculture. Among various traits, extracellular products (ECP) secreted by the bacterium are considered to be essential factors for virulence. Whether vaccination with the ECP could produce immune protection in catfish against the pathogen was determined in this study. The results showed that fish vaccinated with ECP had 100% of relative percent survival (RPS) when challenged with the pathogen two weeks post vaccination. The anti-ECP serum from vaccinated fish could aggregate cells of homogeneous bacteria as well as other virulent strains (isolates) of A. hydrophila but not an A. veronii isolate and a low virulent field isolate. The agglutination titers increased from two weeks to four weeks post immunization and sustained a high level at week seven when the RPS remained at 100%. The anti-ECP serum could also provide naïve fish with immediate protection against A. hydrophila as evidenced by passive immunization. Immunoblotting analysis showed that the anti-ECP serum contained antibodies that bound to specific targets, including protein and lipopolysaccharide-like molecules, in the ECP. Mass spectrometric analysis identified following putative proteins that may serve as important immunogens: chitinase, chitodextrinase, outer membrane protein85, putative metalloprotease, extracellular lipase, hemolysin and elastase. Findings revealed in this study suggest that, while ECP prepared in a conventional and convenient way could be a vaccine candidate, further characterization of antibody-mediated targets in the ECP would uncover quintessential antigens for the future development of highly efficacious vaccines.

Biochemical and molecular characterization of the novobiocin and rifampicin resistant Aeromonas hydrophila vaccine strain AL09-71N+R compared to its virulent parent strain AL09-71

To understand the fitness cost of novobiocin- and rifampicin-resistance in an attenuated Aeromonas hydrophiila vaccine strain AL09-71 N+R compared to its virulent parent strain AL09-71, colony size, cell size, cell proliferation rate, chemotactic response, and the ability to invade catfish gill cells of the two strains were compared. Our results revealed that: (1) the cell size and the colony size of AL09-71 N+R was significantly (P<0.05) smaller than that of AL09-71; (2) the proliferation rate of AL09-71 N+R was significantly (P<0.05) slower than that of AL09-71; (3) AL09-71 N+R had a significantly (P<0.05) lower chemotactic response to catfish mucus than that of AL09-71; 4) the ability of AL09-71 N+R to invade catfish gill cells was significantly (P<0.05) lower than that of AL09-71. To understand whether target site mutation might play a role in antibiotic resistance, novobiocin's target site DNA gyrase subunit B gyrB and rifampicin's target site RNA polymerase subunit B rpoB were sequenced from the two strains. Our results revealed the following five mutations: (1) two missense mutations (CGC to ATC resulting in arginine/R to serine/S; TAC to TGC resulting in tyrosine/Y to cysteine/C) between AL09-71 gyrB and AL09-71 N+R gyrB; (2) three missense mutations (GAC to AAC resulting in aspartic acid/D to asparagine/N; CTG to CCG resulting in leucine/L to proline/P; CTG to CCG resulting in leucine/L to proline/P) between AL09-71 rpoB and AL09-71 N+R rpoB. To determine whether any unique DNA sequences were present in AL09-71 but absent in AL09-71 N+R, PCR-select bacterial genome subtractive hybridization was performed. Of 96 clones selected from the subtractive genomic DNA library, 32 sequences were found. None of the 32 sequences was confirmed to be present in AL09-71 but absent in AL09-71 N+R. At the transcription level, 29 of the 32 genes were found to be expressed greater than 10-fold in AL09-71 N+R compared to that in AL09-71.