Salmonicida Strains Research Articles

Positive correlation between Aeromonas salmonicida vaccine antigen concentration and protection in vaccinated rainbow trout Oncorhynchus mykiss evaluated by a tail fin infection model.

Rainbow trout, Oncorhynchus mykiss (Walbaum), are able to raise a protective immune response against Aeromonas salmonicida subsp. salmonicida (AS) following injection vaccination with commercial vaccines containing formalin-killed bacteria, but the protection is often suboptimal under Danish mariculture conditions. We elucidated whether protection can be improved by increasing the concentration of antigen (formalin-killed bacteria) in the vaccine. Rainbow trout juveniles were vaccinated by intraperitoneal (i.p.) injection with a bacterin of Aeromonas salmonicida subsp. salmonicida strain 090710-1/23 in combination with Vibrio anguillarum serotypes O1 and O2a supplemented with an oil adjuvant. Three concentrations of AS antigens were applied. Fish were subsequently challenged with the homologous bacterial strain administered by perforation of the tail fin epidermis and 60-s contact with live A. salmonicida bacteria. The infection method proved to be efficient and could differentiate efficacies of different vaccines. It was shown that protection and antibody production in exposed fish were positively correlated to the AS antigen concentration in the vaccine.

Aeromonas salmonicida subsp. salmonicida strains isolated from Chinese freshwater fish contain a novel genomic island and possible regional-specific mobile genetic elements profiles.

Two strains of Aeromonas salmonicida, YK and BG, were isolated from largemouth bronze gudgeon and northern whitefish in China, and identified as A. salmonicida subsp. salmonicida based on phylogenetic analysis of vapA and 16S rRNA gene sequences. YK and BG originated from freshwater fish, one of which belonged to the cyprinid family, and the strains showed a difference in virulence. Subsequently, we performed whole genome sequencing of the strains, and comparison of their genomic sequences to the genome of the A449 reference strain revealed various genomic rearrangements, including a new variant of the genomic island AsaGEI in BG, designated as AsaGEI2c This is the first report on a GEI of A. salmonicida strain from China. Furthermore, both YK and BG strains contained a Tn7 transposon inserted at the same position in the chromosome. Finally, IS-dependent rearrangements on pAsa5 are deemed likely to have occurred, with omission of the resD gene in both strains as well as omission of genes related to the IncF conjugal transfer system in the YK isolate. This study demonstrates that A. salmonicida subsp. salmonicida can infect non-salmonids (cyprinids) in addition to salmonids, and that AsaGEI2c might be useful as a geographical indicator of Chinese A. salmonicida subsp. salmonicida isolates.

The Aeromonas salmonicida Lipopolysaccharide Core from Different Subspecies: The Unusual subsp. pectinolytica.

Initial hydridization tests using Aeromonas salmonicida typical and atypical strains showed the possibility of different lipopolysaccharide (LPS) outer cores among these strains. By chemical structural analysis, LPS-core SDS-PAGE gel migration, and functional and comparative genomics we demonstrated that typical A. salmonicida (subsp. salmonicida) strains and atypical subsp. masoucida and probably smithia strains showed the same LPS outer core. A. salmonicida subsp. achromogenes strains show a similar LPS outer core but lack one of the most external residues (a galactose linked α1-6 to heptose), not affecting the O-antigen LPS linkage. A. salmonicida subsp. pectinolytica strains show a rather changed LPS outer core, which is identical to the LPS outer core from the majority of the A. hydrophila strains studied by genomic analyses. The LPS inner core in all tested A. salmonicida strains, typical and atypical, is well-conserved. Furthermore, the LPS inner core seems to be conserved in all the Aeromonas (psychrophilic or mesophilic) strains studied by genomic analyses.

A New Furunculosis Challenge Method for Evaluation of Vaccine Efficacy in Rainbow Trout

Experimental infection of fish for vaccine efficacy studies is associated with several limitations. Administration of live bacteria with the purpose of causing disease in fish can be performed by co-habitation, immersion or injection. We have developed a new Aeromonas salmonicida challenge method for rainbow trout and have applied it for evaluation of furunculosis vaccine efficacy. The method reveals development of systemic immunity in fish as live bacteria are introduced in the tail fin epidermis distant from the vaccine injection site (peritoneal cavity). This method seeks to mimic natural infection in fish farms where tail biting and therefore bacterial exposure to tail fin ulcers is widespread. By use of a multi-needle device ten epidermal perforations were introduced in the dorsal part of the tail fin of anaesthetized rainbow trout (vaccinated or naive). Subsequently 100 μL (3.4 × 108 colony-forming units (CFU) mL-1) of a 48 hour culture of Aeromonas salmonicida subsp. salmonicida strain 090710-1/23 was placed at the perforation site for 60 sec whereafter fish were allowed to regain consciousness in clean freshwater. Immunohistochemistry and scanning electron microscopy illustrated the spread of bacteria from the injection site. Classical furunculosis symptoms associated with a high morbidity rate were observed in control fish whereas vaccinated fish exhibited a significantly higher survival.

Two Catechol Siderophores, Acinetobactin and Amonabactin, Are Simultaneously Produced by Aeromonas salmonicida subsp. salmonicida Sharing Part of the Biosynthetic Pathway.

The iron uptake mechanisms based on siderophore synthesis used by the fish pathogen Aeromonas salmonicida subsp. salmonicida are still not completely understood, and the precise structure of the siderophore(s) is unknown. The analysis of genome sequences revealed that this bacterium possesses two gene clusters putatively involved in the synthesis of siderophores. One cluster is a candidate to encode the synthesis of acinetobactin, the siderophore of the human pathogen Acinetobacter baumannii, while the second cluster shows high similarity to the genes encoding amonabactin synthesis in Aeromonas hydrophila. Using a combination of genomic analysis, mutagenesis, biological assays, chemical purification, and structural determination procedures, here we demonstrate that most A. salmonicida subsp. salmonicida strains produce simultaneously the two siderophores, acinetobactin and amonabactin. Interestingly, the synthesis of both siderophores relies on a single copy of the genes encoding the synthesis of the catechol moiety (2,3-dihydroxybenzoic acid) and on one encoding a phosphopantetheinyl transferase. These genes are present only in the amonabactin cluster, and a single mutation in any of them abolishes production of both siderophores. We could also demonstrate that some strains, isolated from fish raised in seawater, produce only acinetobactin since they present a deletion in the amonabactin biosynthesis gene amoG. Our study represents the first evidence of simultaneous production of acinetobactin and amonabactin by a bacterial pathogen and reveals the plasticity of bacterial genomes and biosynthetic pathways. The fact that the same siderophore is produced by unrelated pathogens highlights the importance of these systems and their interchangeability between different bacteria.

English

Thirty-seven (37) enterococcal isolates were recovered from the skin and intestines of the sea bream (Sparus aurata), the most economically important fish species of the Mediterranean sea from Tunisian fish farming sites, to investigate their antimicrobial potential. All isolates were identified to the species level using genotypic tools. An investigation employing 16S rDNA sequencing in combination with randomly amplified polymorphic DNA-polymerase chain reaction (RAPD-PCR) highlighted the predominance of the Enterococcus faecium (46%) and E. faecalis (19%) species. Other species, such as E. sanguinicola (3 strains), E. casseliflavus (3 strains), E. gallinarum (2 strains), Carnobacterium sp. (1 strain), Aerococcus viridans (2 strains) and Vagococcus carniphilus (2 strains) were also identified. The susceptibility to different antibiotics in addition to the antibacterial activities were investigated for all species identified. The isolates were sensitive to vancomycin but were resistant to several antibiotics relevant for therapy in human and animal medicine. Antibacterial profiles assayed against 39 bacterial indicators (including food-borne and fish pathogenic bacteria in aquaculture as well as other spoilage bacteria) showed that 46% of the isolates exhibited a large inhibition spectrum mainly towards Listeria monocytogenes, Staphylococcus aureus, Aeromonas hydrophila, Aeromonas salmonicida, Vibrio anguillarum and Carnobacterium strains. Therefore, highly inhibitory enterococcal strains could potentially be used as probiotics in sea bream and other farming fish fields. Key words: Enterococcus, lactic acid bacteria, aquaculture, probiotic, sea bream.

Phenanthrene Metabolites from a New Polycyclic Aromatic Hydrocarbon-Degrading Bacterium Aeromonas salmonicida subsp. Achromogenes Strain NY4

A new polycyclic aromatic hydrocarbon (PAH)-degrading Gram-negative bacterium Aeromonas salmonicida subsp. achromogenes strain NY4 was isolated from coking wastewater sludge. Strain NY4 could not grow on1-hydroxy-2-naphthoic acid, hexadecane or hexanol as sole carbon source but phenanthrene. This strain was able to utilize several known microbial metabolites of PAHs, including catechol, protocatechuic acid, salicylic acid, and phenol as sole carbon source. The mixture of the acidic and neutral fraction, acidic fraction alone and neutral fraction alone of phenanthrene metabolites were prepared and analyzed by GC-MS. A total of at least 27 metabolites were identified as the degradation products of phenanthrene by strain NY4. Among them, 1-hydroxy-α-oxo-2-naphthaleneacetic acid, 2-hydroxy-α-oxo-1-naphthaleneacetic acid, 2-hydroxy-1-(1-hydroxy-2-naphthalenyl)-ethanone, 1,2-dihydro naphtha [2,1-b] furan-2-ol and 2,3-dihydro naphtha [1,2-b] furan-2-ol have not been reported as the metabolites of phenanthrenes so far. The accumulation patterns of phenanthrene metabolite accumulation by strain NY4 depended on the incubation time, inoculum size and concentration of phenanthrene. Based on the degradation metabolites of phenanthrene by strain NY4, degradation pathways are proposed. This is the first report of PAH degradation in Aeromonas salmonicida, which lay the foundation for recognition and further characterization of PAH degradation metabolites and pathways in Aeromonas salmonicida strains.

Draft genome sequence of the Chilean isolate Aeromonas salmonicida strain CBA100.

We report the draft genome sequence from Aeromonas salmonicida sp. strain CBA100, which was characterized as an antibiotic-resistant bacterium isolated from infected rainbow trout. The total size of the genome is 4,788,109 bp, with a G + C content of 60.55%. Comparison of its open reading frames shows that the closest homologue to one third of the genes of strain CBA100 are found in A. hydrophila. The strain contains several efflux pumps and putative genes that confer resistance to multiclass antibiotics, including macrolide, β-lactamics, florfenicol and quinolones. The antibiogram profile suggests that efflux pumps are the main mechanism of resistance to non-β-lactamic antibiotics. This is the first genome of a Chilean isolate of A. salmonicida, which should shed light on the design of strain-specific vaccines against this pathogen and reduce the use of antibiotics for preventive treatment in Chilean aquaculture.

Triclosan Resistance in a Bacterial Fish Pathogen, Aeromonas salmonicida subsp. salmonicida, is Mediated by an Enoyl Reductase, FabV.

Triclosan, the widely used biocide, specifically targets enoyl-acyl carrier protein reductase (ENR) in the bacterial fatty acid synthesis system. Although the fish pathogen Aeromonas salmonicida subsp. salmonicida exhibits triclosan resistance, the nature of this resistance has not been elucidated. Here, we aimed to characterize the triclosan resistance of A. salmonicida subsp. salmonicida causing furunculosis. The fosmid library of triclosan-resistant A. salmonicida subsp. salmonicida was constructed to select a fosmid clone showing triclosan resistance. With the fosmid clone showing triclosan resistance, a subsequent secondary library search resulted in the selection of subclone pTSR-1. DNA sequence analysis of pTSR-1 revealed the presence of a chromosomal-borne fabV-encoding ENR homolog. The ENR of A. salmonicida (FabVas) exhibited significant homology with previously known FabV, including the catalytic domain YX(8)K. fabVas introduction into E. coli dramatically increased its resistance to triclosan. Heterologous expression of FabVas might functionally replace the triclosan-sensitive FabI in vivo to confer E. coli with triclosan resistance. A genome-wide search for fabVas homologs revealed the presence of an additional fabV gene (fabVas2) paralog in A. salmonicida strains and the fabVas orthologs from other gram-negative fish pathogens. Both of the potential FabV ENRs expressed similarly with or without triclosan supplement. This is the first report about the presence of two potential FabV ENRs in a single pathogenic bacterium. Our result suggests that triclosan-resistant ENRs are widely distributed in various bacteria in nature, and the wide use of this biocide can spread these triclosan-tolerant ENRs among fish pathogens and other pathogenic bacteria.

Association of Aeromonas salmonicida subsp. achromogenes in the haemorrhagic blister of cultured carp Cyprinus carpio in West Bengal, India

ObjectiveTo study the association of Aeromonas salmonicida subsp. achromogenes in the haemorrhagic blister of cultured exotic carp Cyprinus carpio in West Bengal, India, its cellular fatty acid composition and antibiotic sensitivity. MethodsThe phenotypic characterization and antibiotic sensitivity of bacterial flora of diseased fish were performed. The whole cellular fatty acid composition of Aeromonas salmonicida (A. salmonicida) was determined by MIS Sherlock automatic identification system. ResultsAeromonas hydrophila, A. salmonicida and Pseudomonas alcaligenes were isolated from the haemorrhagic blister. The fatty acid methyl esters pattern confirmed the A. salmonicida strain as A. salmonicida subsp. achromogenes. Unsaturated fatty acid 16:1 w7c/16:1 w6c (39.09%), saturated fatty acid 16:0 (26.84%) and the mono-unsaturated fatty acids 18:1 w7c (8.89%) and 16:1 iso I/14:0 3OH (8.49%) were the most common fatty acids, which accounted for 83.31% of the total fatty acids. A. salmonicida subsp. achromogenes was highly susceptible to broad spectrum antibiotics such as chloramphenicol, ciprofloxacin, co-trimoxazole, gentamycin, nitrofurantoin and oxytetracycline. ConclusionsThe finding on the presence of A. salmonicida in carps necessitates molecular level study on establishing the prevalence of this bacterium in Indian aquaculture systems especially on its free-living viable but non-culturable state.

Antigens of the type-three secretion system of Aeromonas salmonicida subsp. salmonicida prevent protective immunity in rainbow trout

Aeromonas salmonicida subsp. salmonicida is the etiologic agent of furunculosis, a frequent and significant disease of fisheries worldwide. The disease is largely controlled by commercial oil adjuvanted vaccines containing bacterins. However, the mechanisms leading to a protective immune response remain poorly understood. The type-three secretion system (T3SS) plays a central role in virulence of A. salmonicida subsp. salmonicida and thus may have an influence on the immune response of the host. The aim of this study was to evaluate the role of the T3SS antigens in mounting a protective immune response against furunculosis.Rainbow trout were intraperitoneally vaccinated in two independent experiments with bacterins prepared from a wild-type A. salmonicida strain and an isogenic strain carrying a deletion in the T3SS (ΔascV). Fish were challenged with the wt strain eight weeks after vaccination. In both trials, the survival rate of trout vaccinated with the ΔascV strain was significantly higher (23–28%) in comparison to the group vaccinated with the wt strain. High-throughput proteomics analysis of whole bacteria showed the ascV deletion in the mutant strain resulted in lower expression of all the components of the T3SS, several of which have a potential immunosuppressive activity. In a third experiment, fish were vaccinated with recombinant AcrV (homologous to the protective antigen LcrV of Yersinia) or S-layer protein VapA (control). AcrV vaccinated fish were not protected against a challenge while fish vaccinated with VapA were partially protected.The presence of T3SS proteins in the vaccine preparations decreased the level of protection against A. salmonicida infection and that AcrV was not a protective antigen. These results challenge the hypothesis that mounting specific antibodies against T3SS proteins should bring better protection to fish and demonstrate that further investigations are needed to better understand the mechanisms underlying effective immune responses against A. salmonicida infection.

Insertion sequenceAS5(ISAS5) is involved in the genomic plasticity ofAeromonas salmonicida

The genome of the fish pathogen Aeromonas salmonicida subsp salmonicida harbors a large number of insertion sequences (ISs), many of which are located on plasmids. In the present study, we analyzed the small plasmid profile of A. salmonicida strains to identify evidences of plasmid alterations. Ten out of 78 strains analyzed displayed an unconventional plasmid profile. However the HER1104 strain was unique, having a positive PCR signal for pAsal1 plasmid despite not carrying this plasmid. Instead, HER1104 was bearing a plasmid at higher molecular weight than pAsal1. We characterized this new larger plasmid, which we called pAsal1B since it is a derivative of pAsal1 containing one more complete IS (ISAS5) than the parental plasmid. An additional 96 bp relic of ISAS5 was also present in pAsal1B. These results propose that ISAS5 is another active mobile genetic element in A. salmonicida subsp salmonicida and provided further proof of the genomic plasticity of this bacterium.

Biological Control ofAeromonas salmonicidasubsp.salmonicidaInfection in Rainbow Trout (Oncorhynchus mykiss) UsingAeromonasPhage PAS-1

The potential control efficacy of Aeromonas phage PAS-1 was evaluated against Aeromonas salmonicida subsp. salmonicida infection in rainbow trout (Oncorhynchus mykiss) model in this study. The phage was co-cultured with the virulent A.salmonicida subsp. salmonicida strain AS05 that possesses the type III secretion system (TTSS) ascV gene, and efficient bacteriolytic activity was observed against the bacteria. The administration of PAS-1 in rainbow trout demonstrated that the phage was cleared from the fish within 200h post-administration, and a temporal neutralizing activity against the phage was detected in the sera of phage-administrated fish. The administration of PAS-1 (multiplicity of infection: 10000) in A.salmonicida subsp. salmonicida infected rainbow trout model showed notable protective effects, with increased survival rates and mean times to death. These results demonstrated that Aeromonas phage PAS-1 could be considered as an alternative biological control agent against A.salmonicida subsp. salmonicida infections in rainbow trout culture.

Clustering subspecies of Aeromonas salmonicida using IS630typing

BackgroundThe insertion element IS630 found in Aeromonas salmonicida belongs to the IS630-Tc1-mariner superfamily of transposons. It is present in multiple copies and represents approximately half of the IS present in the genome of A. salmonicida subsp. salmonicida A449.ResultsBy using High Copy Number IS630 Restriction Fragment Length Polymorphism (HCN-IS630-RFLP), strains of various subspecies of Aeromonas salmonicida showed conserved or clustering patterns, thus allowing their differentiation from each other. Fingerprints of A. salmonicida subsp. salmonicida showed the highest homogeneity while ‘atypical’ A. salmonicida strains were more heterogeneous. IS630 typing also differentiated A. salmonicida from other Aeromonas species. The copy number of IS630 in Aeromonas salmonicida ranges from 8 to 35 and is much lower in other Aeromonas species.ConclusionsHCN-IS630-RFLP is a powerful tool for subtyping of A. salmonicida. The high stability of IS630 insertions in A. salmonicida subsp. salmonicida indicates that it might have played a role in pathoadaptation of A. salmonicida which has reached an optimal configuration in the highly virulent and specific fish pathogen A. salmonicida subsp. salmonicida.

IS-mediated loss of virulence byAeromonas salmonicida

Insertion sequences (IS) are abundant in the bacterial fish pathogen Aeromonas salmonicida genome. IS are involved in rearrangement events that lead to the loss of virulence. In previous work, we studied a plasmid rearrangement that causes the deletion of the type three secretion system in A. salmonicida, resulting in a loss of virulence. We showed that the rearrangement is caused by the recombination of two IS (ISAS11) on an unstable plasmid (pAsa5). However, many rearrangements cannot be explained by our experimental approach and are thought to be the result of more complex or incomplete rearrangement events, as suggested by other plasmid loss profiles observed in various A. salmonicida strains. In this commentary, we examine the genetic instability of A. salmonicida indicating that its genome is rapidly evolving.

Virulence factors of Aeromonas salmonicida subsp. salmonicida strains associated with infections in turbot Psetta maxima

Virulence factors for Aeromonas salmonicida subsp. salmonicida (ASS) strains isolated from cultured turbot Psetta maxima L. are unknown with regard to this host. The presence of virulence genes associated with different stages of ASS infection in salmonids (vapA, tapA, fla, ascV, ascC, aexT, satA and aspA) was analysed using a polymerase chain reaction (PCR) technique in ASS strains isolated from turbot. Other ASS strains isolated from salmonids and environmental A. salmonicida (AS) strains were included for comparison. The presence of the genes was evaluated with respect to ASS virulence in turbot based on intraperitoneal and bath challenges. The genetic profile, including all of the genes studied, that was linked to virulent behaviour after intraperitoneal challenge was significantly more frequent in strains isolated from turbot than in those from salmonids or the environment. The data prove that it is not possible to predict the virulence of ASS in turbot based only on the presence of all genes tested. Moreover, the combined PCR results of vapA, aexT, ascV and ascC were useful for separating most of the ASS from environmental A. salmonicida strains. An association between virulence or genetic profile and the geographical or facility origin of the strains was not found.

Isolation and Characterization of a Lytic Myoviridae Bacteriophage PAS-1 with Broad Infectivity in Aeromonas salmonicida

To search for candidate control agents against Aeromonas salmonicida subsp. salmonicida infections in aquaculture, one bacteriophage (phage), designated as PAS-1, was isolated from the sediment samples of the rainbow trout (Oncorhynchus mykiss) culture farm in Korea. The PAS-1 was morphologically classified as Myoviridae and possessed approximately 48 kb of double-strand genomic DNA. The phage showed broad host ranges to other subspecies of A. salmonicida as well as A. salmonicida subsp. salmonicida including antibiotic-resistant strains. Its latent period and burst size were estimated to be approximately 40 min and 116.7 PFU/cell, respectively. Furthermore, genomic and structural proteomic analysis of PAS-1 revealed that the phage was closely related to other Myoviridae phages infecting enterobacteria or Aeromonas species. The bacteriolytic activity of phage PAS-1 was evaluated using three subspecies of A. salmonicida strain at different doses of multiplicity of infection, and the results proved to be efficient for the reduction of bacterial growth. Based on these results, PAS-1 could be considered as a novel Aeromonas phage and might have potentiality to reduce the impacts of A. salmonicida infections in aquaculture.

Prevalence and Characterization of Typical Aeromonas salmonicida Chum Salmon Isolates in Korea

Aeromonas salmonicida is an important fish pathogen commonly associated with furunculosis in salmonids. Typical A. salmonicida strains have the surface virulence A-layer protein, a major virulence determinant encoded by the vapA gene. In this study, 880 chum salmon Oncorhynchus keta were collected from the east coast of Korea during 2006-2011, including 560 wild adults and 320 artificially hatched fry pools, and the presence of typical A. salmonicida was examined by PCR using the typical A. salmonicida-specific vapA gene primers. The results demonstrated that 34.5% of the samples (304/880 samples) were PCR positive, implying that a typical A. salmonicida infection is highly prevalent among chum salmon in Korea. Twenty typical A. salmonicida isolates were recovered based on their brown pigmentation on Trypticase Soy Agar (TSA) plates, which indicates the existence of the A-layer protein. Further biochemical analyses with the four randomly selected typical A. salmonicida isolates revealed some variations in their amino acid decarboxylation and carbohydrate fermentation activity. A phylogenetic analysis based on the entire vapA gene sequence suggested that the A. salmonicida isolates from chum salmon were clustered with those isolated from Atlantic salmon in Europe. Further study is needed to resolve such an interesting relationship in detail.

Complete genome sequence and characterization of a broad-host range T4-like bacteriophage phiAS5 infecting Aeromonas salmonicida subsp. salmonicida

In this study, we report one lytic Myoviridae bacteriophage (phage) infecting Aeromonas salmonicida subsp. salmonicida. The phage (named as phiAS5) was isolated from environmental river waters in Korea, and showed broad infectivity to other bacterial species in the family Aeromonadaceae as well as antibiotic-resistant A. salmonicida subsp. salmonicida strains. The biological properties and complete genome of phiAS5 were simultaneously investigated. The complete genome of phiAS5 composed of linear double-stranded DNA of 225,268bp with G+C content of 43.0%, and encoded 343 putative ORFs, 69 putative promoters, 33 transcriptional terminator regions and 24 tRNA-encoding genes. A high degree of similarity to other T4-like Aeromonas phage was found in most ORFs of phiAS5. Therefore, the genome of phiAS5 was further compared with T4 phage and the closest relative, Aeromonas phage Aeh1, and the result demonstrated that it could be classified as a new member of the T4-like group. The bacteriolytic activity of phiAS5 against A. salmonicida subsp. salmonicida was evaluated at different doses of multiplicity of infection using one each of virulent strain that possesses the ascV gene and multi-drug resistant strain, and the results proved to be efficient for the reduction of bacterial growth. Based on these results, phiAS5 may have the potential for reducing the impacts of virulent or antibiotic-resistant A. salmonicida subsp. salmonicida in aquaculture and may also advance our understanding of the biodiversity of T4-like Aeromonas phages.

Vibrio salmonicida pathogenesis analyzed by experimental challenge of Atlantic salmon (Salmo salar)

Cold-water vibriosis (CV) is a bacterial septicemia of farmed salmonid fish and cod caused by the Gram-negative bacterium Vibrio (Aliivibrio) salmonicida. To study the pathogenesis of this marine pathogen, Atlantic salmon was experimentally infected by immersion challenge with wild type V. salmonicida and the bacterial distribution in different organs was investigated at different time points. V. salmonicida was identified in the blood as early as 2 h after challenge demonstrating a rapid establishment of bacteremia without an initial period of colonization of the host. Two days after immersion challenge, only a few V. salmonicida were identified in the intestines, but the amount increased with time. In prolonged CV cases, V. salmonicida was the dominating bacterium of the gut microbiota causing a release of the pathogen to the water. We hypothesize that V. salmonicida uses the blood volume for proliferation during the infection of the fish and the salmonid intestine as a reservoir that favors survival and transmission. In addition, a motility-deficient V. salmonicida strain led us to investigate the impact of motility in the CV pathogenesis by comparing the virulence properties of the mutant with the wild type LFI1238 strain in both i.p. and immersion challenge experiments. V. salmonicida was shown to be highly dependent on motility to gain access to the fish host. After invasion, motility was no longer required for virulence, but the absence of normal flagellation delayed the disease development.