The aim of this study was to determine the best synbiotic combination (based on growth and short chain fatty acids production) between Enterococcus faecalis and eight prebiotics. Based on the results of in vitro studies, E. faecalis + galactooligosaccharide (EGOS) and E. faecalis + mannanoligosaccharide (EMOS) were selected as synbiotics. A 126-day feeding trial was conducted to compare the effects of prebiotics, probiotic and synbiotics on the growth indices, In vivo ADC of nutrients, digestive enzymes, hemolymph indices and finally, biological responses against 48-hr Aeromonas hydrophila exposure challenges of juvenile (4.13±0.12 g) crayfish. The highest values of SGR (2.19% day-1), VFI (2.75% body weight day-1), survival rate (96.67%) and the lowest FCR (2.33) were observed in the juvenile crayfish fed the EGOS- diet. The significantly (p<0.05) highest means of in vivo ADCOM, in vivo ADCCP, in vivo ADCCF and in vivo ADCGE were measured in crayfish fed the EGOS-diet. The mean survival rate of Aeromonas hydrophila injected crayfish fed the EGOS- diet (56%) was significantly (p<0.05) higher than those of fed the control (8.67%) and other diets (22.67-35.32%). At the levels tested, 7.86 log CFU E. faecalis g-1 + 10 g kg-1 GOS in the diet was considered optimum.

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