Characterization of the unusual non-thiosulfate-reducing Edwardsiella tarda isolated from eel ( Anguilla japonica ) farms

Unusual Edwardsiella tarda, that do not produce hydrogen sulfide (H2S) from thiosulfate, was isolated from diseased eels (Anguilla japonica) in varying farming sites. In this study, we examined the biochemical characteristics of H2S production from the bacterium E. tarda. Diseased eels, along with pond water, were sampled from eight eel farms. All isolates were used for biochemical characterization and 16S rRNA and fimbrial gene sequencing. The phs gene of some isolates were also sequenced. H2S production was analyzed from either thiosulfate (S2O32−)- or sulfite (SO3−)-containing media. DNA analysis identified all isolates as E. tarda. Of the 17 E. tarda isolates, 11 were unable to reduce thiosulfate to H2S, while all isolates produced H2S from sulfite. In addition, non-thiosulfate-reducing E. tarda were isolated from four of the eight sampled eel farms. In media containing sulfate (SO42−) and in media without thiosulfate or sulfite, no H2S production was observed. In addition, glucose and galactose appeared to repress thiosulfate reduction in all strains, whereas these different carbon sources did not affect sulfite reduction. This indicates that thiosulfate and sulfite reduction pathways do not overlap in E. tarda. For this reason, careful attention should be made when identifying or differentiating non-thiosulfate-reducing E. tarda using H2S production characteristics.