Occurrence and virulence of Pseudoalteromonas spp. in cultured gilthead sea bream (Sparus aurata L.) and European sea bass (Dicentrarchus labrax L.). Molecular and phenotypic characterisation of P. undina strain U58

During two long-term bacteriological surveys in European sea bass (Dicentrarchus labrax L.) and gilthead sea bream (Sparus aurata L.) culture facilities on the Spanish Mediterranean coast, four different species of Pseudoalteromonas were found in the internal organs of animals. Pseudoalteromonas undina, Pseudoalteromonas haloplanktis, P. espejiana and Pseudoalteromonas sp. were isolated from 9% out of 177 European sea bass and 11.7% out of 547 gilthead sea bream and they were recovered as pure cultures in 50% of isolations. Among fish with positive bacterial growth, Pseudoalteromonas was more prevalent in sea bass showing clinical signs (27.7%) and in gilthead sea bream without clinical signs (22.5%). The possible pathogenicity of seven strains of three Pseudoalteromonas spp., selected by their higher frequency in fish with clinical signs, was tested in both sea bass and sea bream. None of them was virulent for gilthead sea bream at any of the assayed doses. Only one strain, – initially isolated from a diseased sea bass and named U58 – was weakly virulent for this fish species, as it killed 100% of experimentally inoculated fish at a dose of 107 cfu/fish (LD50 = 1.3 × 107 cfu/fish). This U58 strain was further characterized as Gram-negative, motile, non-fermenting and non-pigmented rod that requires Na+ ion for growth. A further phenotypical (including Biolog GN) and phylogenetical (16S rDNA) characterization identified it as P. undina. This is the first report on P. undina as a pathogen for marine fish.