In the present study, a 126-day experiment was carried out under controlled conditions to compare the effects of prebiotic (galactooligosaccharide), probiotics (Enterococcus faecalis and Pediococcus acidilactici), and synbiotics (galactooligosaccharide + Enterococcus faecalis and galactooligosaccharide + Pediococcus acidilactici) on the growth performance, indices of nutritional efficiency, in vivo apparent digestibility coefficients of nutrients, digestive enzymes, hemolymph indices, and biological responses of juvenile crayfish Astacus leptodactylus leptodactylus (6.18±0.31 g) against 48-h Aeromonas hydrophila exposure challenge. The maximum specific growth rate (2.32% day-1), voluntary feed intake (2.68% body weight day-1), and survival rate (93.67%) as well as the minimum feed conversion ratio (2.07) were observed in the juvenile crayfish fed with the galactooligosaccharide + Enterococcus faecalis diet. The highest means of in vivo apparent digestibility coefficients of organic matter (85.33%), crude protein (91.67%), crude fat (82.33%), and gross energy (83.67%) were measured in the crayfish fed with the galactooligosaccharide + Enterococcus faecalis diet (p<0.05). Moreover, the highest ratios of presumptive autochthonous lactic acid bacteria to total viable aerobic heterotrophic bacteria were observed in the crayfish fed with the synbiotic diets (p<0.05). It was found that feeding crayfish with the galactooligosaccharide + Enterococcus faecalisdiet indicated the highest activities of phenoloxidase, superoxide dismutase, lysozyme, and nitric oxide synthase (p<0.05). The mean survival rate of Aeromonas hydrophila injected crayfish fed with the galactooligosaccharide + Enterococcus faecalis diet (77.67%) was higher than that of the crayfish fed with the control (8.33%) and other diets (58.33-72.33%) (p<0.05). Finally, considering all the tested levels, 7.53 log CFU E. faecalis g-1 + 10 g kg-1 galactooligosaccharide used in the diet was observed to be the optimum diet.

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