The molecular biology of Aeromonas salmonicida

High immunogenicity of virus-like particles (VLPs) decorated with Aeromonas salmonicida VapA antigen in rainbow trout

The Gram-negative bacterium A. salmonicida is the causal agent of furunculosis and used to be one of the most loss-causing bacterial infections in the salmonid aquaculture industry with a mortality rate of about 90% until the 1990s, when an inactivated vaccine with mineral oil as adjuvant was successfully implemented to control the disease. However, the use of this vaccine is associated with inflammatory side effects in the peritoneal cavity as well as autoimmune reactions in Atlantic salmon, and incomplete protection has been reported in rainbow trout. We here aimed at developing and testing a recombinant alternative vaccine based on virus-like particles (VLPs) decorated with VapA, the key structural surface protein in the outer A-layer of A. salmonicida. The VLP carrier was based on either the capsid protein of a fish nodavirus, namely red grouper nervous necrotic virus (RGNNV) or the capsid protein of Acinetobacter phage AP205. The VapA and capsid proteins were expressed individually in E. coli and VapA was fused to auto-assembled VLPs using the SpyTag/SpyCatcher technology. Rainbow trout were vaccinated/immunized with the VapA-VLP vaccines by intraperitoneal injection and were challenged with A. salmonicida 7 weeks later. The VLP vaccines provided protection comparable to that of a bacterin-based vaccine and antibody response analysis demonstrated that vaccinated fish mounted a strong VapA-specific antibody response. To our knowledge, this is the first demonstration of the potential use of antigen-decorated VLPs for vaccination against a bacterial disease in salmonids.

Vaccination against atypical furunculosis and winter ulcer disease of fish

Atypical furunculosis is a problem in farming of salmonids and various other fish species caused by a heterogeneous group of atypical Aeromonas salmonicida strains. Winter ulcer is a disease of salmonids and cod caused by Moritella viscosa, but a number of fish species are susceptible to the infection. Vaccines are available against atypical furunculosis of salmonids, but their efficacy is dependent on the characteristics of the infective strain. Vaccines for non-salmonid fish are currently not commercially available. Furunculosis vaccines for salmon can induce cross protection against some atypical A. salmonicida infections and only in some fish species. Polyvalent injection vaccines based on inactivated bacterial cells are available against winter ulcer disease of salmonids. Outbreaks of winter ulcer disease in vaccinated salmon are, however, continuously reported.

Putative virulence factors of atypical Aeromonas salmonicida isolated from Arctic charr, Salvelinus alpinus (L.), and European grayling, Thymallus thymallus (L.)

Atypical Aeromonas salmonicida (AAS) causes generalized lethal infections in farmed Arctic charr, Salvelinus alpinus (L.), and European grayling, Thymallus thymallus (L.), and is thus a serious threat for culture of these fish species. Virulence factors were studied among isolates of AAS from Arctic charr (n = 20), European grayling (n = 19) and other fish species (n = 20), of which 48 were of Finnish and 11 of Swedish origin. All isolates produced an A-layer. Extracellular products (ECP) of the AAS isolates did not produce detectable gelatinase and caseinase activity in test assays. Analysis of the same ECP preparations with substrate sodium dodecyl sulphate-polyacrylamide gel electrophoresis showed weak proteolytic activity, indicating the different sensitivity of the detection methods used. The ECP from AAS isolates showed low cytotoxic activity against cultured cells. However, the ECP did not induce mortality in challenged Arctic charr. The results suggest that toxic components, like ECP, secreted by the bacterium may not be the major virulence factor in AAS-infection in Arctic charr and European grayling, and hence the pathogenesis also differs from the pathogenesis of AAS-infection in Atlantic salmon, Salmo salar L.

Distribution of Aeromonas spp. as identified by 16S rDNA restriction fragment length polymorphism analysis in a trout farm

AIMS

This study used restriction fragment length polymorphism (RFLP) with Aeromonas-specific primers to identify species of Aeromonas and to investigate their distribution in a trout farm and stream.

METHODS AND RESULTS

In January, May, August and November 2000, presumptive Aeromonas species were recovered from a farm and a sedimentation pond in a fish farm and stream, and identified by PCR-RFLP analysis with Aeromonas-specific primers. The specificity of Aeromonas-specific primers and the suitability of PCR-RFLP analysis for identifying Aeromonas spp. were confirmed with fatty acid methyl esters (FAMEs) and 16S rDNA sequencing analyses, respectively. Levels of Aeromonas spp. sampled in May and August were higher than in January and November at all sampling sites. Aeromonas salmonicida was the dominant species in January and November, and the proportion of pathogenic species (Aer. hydrophila, Aer. caviae and Aer. veronii) increased in May and August.

CONCLUSIONS

PCR-RFLP analysis with Aeromonas-specific primers is a rapid and reliable method for identifying widely distributed Aeromonas spp. from environmental samples.

SIGNIFICANCE AND IMPACT OF THE STUDY

To minimize human health risk, monitoring the levels and species composition of Aeromonas in fish farm is advisable.

Physical map of the chromosome of Aeromonas salmonicida and genomic comparisons between Aeromonas strains

I-Ceul and Pmel physical maps of the Aeromonas salmonicida A449 chromosome were constructed using PFGE. The circular chromosome of A. salmonicida A449 was estimated to be 4658 +/- 30 kb. The approximate location of several genes, including those encoding proteins implicated in virulence, were identified. The map showed that the known virulence-factor-encoding genes were not clustered. The I-Ceul genomic digestion fingerprints of several typical and atypical strains of A. salmonicida were compared. The results confirmed the homogeneity of typical strains, which provided further support for the clonality of the population structure of this group. Extensive diversity was observed in the I-Ceul digestion fingerprint of atypical strains, although a clonality was observed in the strains isolated from diseased goldfish. The results suggest that comparison of I-Ceul digestion fingerprints could be used as a powerful taxonomic tool to subdivide the atypical strains and also help clarify some of the current confusion associated with the taxonomy of the genus Aeromonas.