Isolation and investigation of probiotic effects of yeasts in rainbow trout (Oncorhynchus mykiss, Walbaum 1792)

In this thesis, isolation, identification, in vitro and in vivo probiotic properties of yeast strains from the intestines of rainbow trout were investigated. For this, first of all, yeast strains were isolated from the intestines of healthy rainbow trout. Then, antimicrobial effects (well diffusion agar assay) and their ability to adhere to bacterial fish pathogens (Lactococcus garvieae, Vagococcus salmoninarum, Vibrio anguillarum, Yersinia ruckeri and Aeromonas hydrophila), their tolerance to bile salts and low pH conditions, hydrophobicity and their pathogenicity in rainbow trout was investigated. Identification of yeast strains was performed by ITS gene region sequence analysis. Feeding trial was carried out by adding selected candidate probiotic yeasts to rainbow trout feeds for 35 days. After the feeding experiment, effects of yeast strains on serum immunological parameters, serum biochemistry and disease resistance against L. garvieae in fish were investigated. As a result of the study, it was determined that the two yeast isolates were hydrophobic, tolerant to bile salts and acidic conditions, not show in vitro antagonism to pathogens, showed adhesion to all pathogens and not pathogenic for rainbow trout. Yeast strains were identified as Candida zeylanoides (Y12-3) and Hyphopichia pseudoburtonii (Y12-1). As a result of feeding experiment with these yeast strains, serum myeloperoxidase and lysozyme activities were not affected (P>0.05). Serum biochemical parameters, glucose, total protein, globulin values were higher in fish fed with Hyphopichia pseudoburtonii compared to control group fish (P<0.05). Cholesterol value in fish fed with Candida zeylanoides (Y12-3) was lower compared to the control (P<0.05). As a result of experimental infection with L. garvieae, the mortality rates in fish fed with yeasts were lower than the control group (P<0.05). In conclusion, although Hyphopichia pseudoburtonii Y12-1 and Candida zeylanoides Y12-3 yeast strains have the potential to be used as probiotics in rainbow trout farming, it would be beneficial to investigate their probiotic effects with additional in vivo tests in future studies.