Atypical Aeromonas salmonicida infection in the black rockfish, Sebastes schlegeli Hilgendorf, in Korea

Abstract

Cultured black rockfish, Sebastes schlegeli, suffered mass mortalities during winter 2008 and spring 2009 in Korea, showing clinical signs of ulcer lesions and haemorrhages over their body surface. The aetiological agent was identified as Aeromonas salmonicida (strains RFAS-1, -2 and -3), which is a non-pigmented, slow-growing bacterium. Phenotypes of RFAS strains showed variation, while 16S rRNA, gyrB, rpoD, dnaJ and recA gene sequences of all the strains were affiliated to A. salmonicida. In particular, vapA gene sequences of the strains were most closely related to one of the five subspecies of A. salmonicida subsp. masoucida (=KCCM 40239(T) ). LD(50) values of RFAS-1 for intraperitoneal and intramuscular injection were 1.5 × 10(5.25) and 1.5 × 10(6.4) cfu/rockfish, respectively. However, A. salmonicida strains KCCM 40239(T) and SAS-1, which originate from masou and chum salmon, respectively, were not pathogenic to black rockfish. RFAS strains, possessing A-layer protein on their surface, exhibited β-haemolytic activity against rockfish erythrocytes and capability to survive in rockfish serum, which seem to be associated with virulence.

Molecular Detection and Characterization of Furunculosis and Other Aeromonas Fish Infections

“…pectinolytica), which are practically impossible to differentiate using phenotypic or molecular methods. All of them, except the subspecies pectinolytica have been implicated in fish pathology (Austin & Austin, 2007;Goldschmidt-Clermont et al, 2009;Han et al, 2011;Noga, 2010;Wiklund & Dalsgaard, 1998). Being unable to assign strains implicated in fish disease to any of the subspecies masoucida, smithia, achromogenes or pectinolytica using biochemical characteristics, they have been termed "atypical A. salmonicida" to differentiate them from the "typical A. salmonicida", a term restricted to the subspecies salmonicida (Beaz-Hidalgo et al, 2008;Goodwin & Merry, 2009;Wiklund & Daalsgard 1998).…”

“…The mentioned systems normally tend to identify all Aeromonas strains as A. hydrophila (Figueras, 2005;Soler et al, 2003b). Some other authors have also reported poor results from the API systems and discrepancies with traditional tube testing when identifying Aeromonas strains isolated from fish (Godoy et al, 2010;Han et al, 2011;Joseph & Carnahan, 1994;and references therein). Some methods may even misidentify Aeromonas strains as Vibrio species (Park et al, 2003;Soler et al, 2003b).…”

“…In the genus Aeromonas the sequences of the 16S rRNA gene are not very useful tools for identification because only a few distant species can be well discriminated, but it is useful for confirming whether or not the strains under analysis belong to the genus (Figueras et al, 2011b;Han et al, 2011).…”

“…A. encheleia (9.3%), A. salmonicida (3.7%) and A. bestiarum (3%). Using several housekeeping genes (gyrB, rpoD, dnaJ, recA) also enabled Han et al (2011) to identify A. salmonicida from ulcer lessions and haemorrhages in the black rockfish (Sebastes schlegeli) in Korea. These authors also sequenced the vapA gene (encoding the A-protein, a subunit of the A-layer or S-layer) and found that the sequences from the isolated A. salmonicida strains clustered with A. salmonicida subsp.…”

“…These authors also sequenced the vapA gene (encoding the A-protein, a subunit of the A-layer or S-layer) and found that the sequences from the isolated A. salmonicida strains clustered with A. salmonicida subsp. masoucida in a neighbour-joining phylogenetic tree that included the sequences of other atypical A. salmonicida strains isolated from fish and of the subspecies salmonicida, masoucida and smithia (Han et al, 2011). However, these and other authors (Lund & Mikkelsen, 2004) stated that many atypical strains do not cluster with any of the subspsepecies of A. salmonicida when using the vapA sequences, and consider this gene not to be useful in the delineation of A. salmonicida to the subspecies level.…”