Biogeography of the fish pathogen Aeromonas salmonicida inferred by vapA genotyping

Abstract

A recently described typing system based on sequence variation in the virulence array protein ( vapA ) gene, encoding the A-layer surface protein array, allows unambiguous subtyping of Aeromonas salmonicida . In the present study, we compile A-layer typing results from a total of 675 A. salmonicida isolates, recovered over a 59-year period from 50 different fish species in 26 countries. Nine novel A-layer types (15–23) are identified, several of which display a strong predilection towards certain fish hosts, including e.g. Cyprinidae and Pleuronectidae species. Moreover, we find indications that anthropogenic transport of live fish may have aided the near global dissemination of two cyprinid-associated A-layer types. Comparison of whole genome phylogeny and A-layer typing for a subset of strains further resulted in compatible tree topologies, indicating the utility of vapA as a phylogenetic as well as an epizootiological marker in A. salmonicida . A Microreact project ( microreact.org/project/r1pcOAx9m ) has been created, allowing public access to the vapA analyses and relevant metadata. In sum, the results generated provide valuable insights into the global population structure of A. salmonicida , particularly in relation to its piscine host spectrum and the geographic distribution of these hosts.

Pinpointing the role of Aeromonas salmonicida in the development of skin ulcerations in common dab (Limanda limanda)

“…Sequence 1 was identical to that of the challenge strain ( A. salmonicida strain 2CK). The partial vapA sequence of the A. salmonicida strain from common dab in Denmark in 1999, identified previously as A‐layer type 15 (Gulla et al, ), received an intermediate position in the maximum‐likelihood tree, between sequences 1 and 2 (File S6).…”

“…A‐layer typing based on sequence heterogeneity in the vapA virulence gene was performed on 21 (out of 61) A. salmonicida isolates (Gulla et al, ) from both control fish and challenged fish that were killed at 21 DPI. DNA extraction, PCR (primers vapA F2 and R3), Sanger sequencing and subsequent processing were performed as previously described by Gulla et al (), Gulla et al (). The partial vapA sequence from the challenge strain 2CK (accession no.…”

“…and (accession no. ), respectively, recovered from common dab in Denmark in 1999 and Atlantic halibut ( Hippoglossus hippoglossus ) in Norway in 2015 (Gulla et al, ), was acquired from NCBI GenBank and included for comparison and tree rooting. Sequence alignments were conducted in ClustalX v2.1 (Larkin, Blackshields, & Brown, ), and a maximum‐likelihood (ML) tree was constructed using PhyML v3.0 (Guindon et al, ) with default settings.…”

“…Various subtypes of the pathogen have been described and may be discerned based on the sequence of the virulence array protein gene ( vapA ), producing the crystalline, outer membrane protein (A layer; Gulla, Lund, Kristoffersen, Sørum, & Colquhoun, ). The isolates recovered from common dab in the BNS could all be assigned to the distinct A‐layer type 15 (Vercauteren et al, ), a type which has subsequently been identified as exclusively associated with northern European specimens of this fish species (Gulla et al, ).…”

Development of diagnostic assays for differentiation of atypical Aeromonas salmonicidavapA type V and type VI in ballan wrasse (Labrus bergylta, Ascanius)

“…A virulence factor, the paracrystalline surface array protein (known as A‐layer protein), has successfully been used for the differentiation of typical and atypical As strains (Gulla, Lund, et al., 2016; Gulla et al., 2019). This additional outer membrane protein plays an important role in host infection and providing protection for the bacterium by resisting host immune responses (Daly et al., 1996; Kay & Trust, 1991; Munn et al., 1982; Udey & Fryer, 1978).…”