Description of Shewanella glacialipiscicola sp nov and Shewanella algidipiscicola sp nov., isolated from marine fish of the Danish Baltic Sea, and proposal that Shewanella affinis is a later heterotypic synonym of Shewanella colwelliana

Two novel species belonging to the genus Shewanella are described on the basis of a polyphasic taxonomic approach. A total of 40 strains of Gram-negative, psychrotolerant, H2S-producing bacteria were isolated from marine fish (cod and plaice) caught in the Baltic Sea off Denmark. Strains belonging to group 1 (seven strains) were a lactate-assimilating variant of Shewanella morhuae with a G + C content of 44 mol%. The strains of group 2 (33 strains) utilized lactate, N-acetylglucosamine and malate but did not produce DNase or ornithine decarboxylase. Their G + C content was 47 mol%. Phylogenetic analysis of the 16S rRNA gene sequence data placed the two novel species within the genus Shewanella. Group 1 showed greatest sequence similarity with S. morhuae ATCC BAA-1205(T) (99.9%). However, gyrB gene sequence analysis and DNA-DNA hybridization differentiated these isolates from S. morhuae, with 95.6% sequence similarity and less than 57% DNA relatedness, respectively. Group 2 strains shared more than 99% 16S rRNA gene sequence similarity with the type strains of Shewanella colwelliana and Shewanella affinis, but gyrB sequence similarity (similar to 85%) and the results of DNA hybridization (similar to 28%) indicated that the new isolates represented a novel species. Furthermore, when compared to each other, the type strains of S. colwelliana and S. affinis had almost identical gyrB sequences and significantly high DNA reassociation values (76-83%), indicating that they belonged to the same species. Based on the conclusions of this study, we propose the novel species Shewanella glacialipiscicola sp. nov. (type strain T147(T) = LMG 23744(T) = NBRC 102030(T)) for group 1 strains and Shewanella algidipiscicola sp. nov. (type strain S13(T) = LMG 23746(T) = NBRC 102032(T)) for group 2 strains, and we propose that Shewanella affinis as a later heterotypic synonym of Shewanella colwelliana.