A mixed Bacillus gibsonii and Sphingomonas echinoides infection in cultured rainbow trout (Oncorhynchus mykiss)

Abstract

Rainbow trout (Oncorhynchus mykiss) is a fish species with a long history of cultivation and bacterial pathogens are limiting the success rate. The aim of this study is the biochemical and molecular identification of two opportunistic pathogens detected in the rainbow trout cultured in net cages in a dam lake located on the Kızılırmak river; revealing the pathological symptoms of them in the moribund fish samples; determination of their antimicrobial susceptibility profile and determination of the antagonistic effect of two probiotic-candidate strains against them. Depending on the results of the conventional bacteriologic and molecular identification studies, bacterial isolates recovered from the internal organs of the moribund fish samples, a mixed bacterial infection case of Bacillus gibsonii and Sphingomonas echinoides was identified in the moribund fish samples showing general bacterial hemorrhagic septicemia symptoms for the first time in rainbow trout. Despite it was not possible to identify these isolates at the species level using conventional bacteriological methods, our isolates separately showed similarities more than 99% with the above mentioned species in the 16s RNA sequence analysis. The results of this study showed that, long term water quality parameter determination and bacterial distribution monitoring studies which include molecular tools should be carried out in the aquaculture sites to increase the success in trout culture.