Phenotypic differences between Edwardsiella piscicida and Edwardsiella anguillarum isolates in Japan.

Abstract

Most studies refer to Edwardsiella tarda as the causative agent of edwardsiellosis in the cultured marine and freshwater fish species in Japan; however, our previous study was the first to genetically classify an E. tarda-like isolate from diseased olive flounder as E. piscicida and that from diseased red sea bream as E. anguillarum. This study aimed to understand the phenotypical differences between the two species. The API 50 CH test showed that L-arabinose and D-mannitol were fermented by 7 E. anguillarum isolates, but not by 14 E. piscicida isolates; the difference is likely to be useful for the recognition of E. piscicida, although it was not common among previously-reported E. anguillarum isolates. The use of the defined medium consisting of M9 minimal medium supplemented with 43 components revealed that the growth of E. piscicida and E. anguillarum was reduced in the defined medium without methionine and iron sulfate. Individually, the growth of E. piscicida was reduced in the defined medium without phenylalanine, tyrosine, alanine, or nicotinic acid, whereas the growth of E. anguillarum was reduced without serine, cysteine, leucine, threonine, or isoleucine. Tyrosine and alanine were present in higher concentrations in the olive flounder serum and threonine and isoleucine were present in higher concentrations in the red sea bream serum, compared with each other, suggesting that E. piscicida and E. anguillarum may have the advantage of growth opportunity in their hosts. Finally, the minimal defined medium for the two species was determined to enable safe vaccine development.