The viability of probiotic bacteria as a factor influencing the immune response in rainbow trout Oncorhynchus mykiss

The objective of the present study was to understand the immune response induced by different forms of the probiont Lactobacillus rhamnosus JCM 1136 in rainbow trout Oncorhynchus mykiss and to clarify the carry over effect after withdrawal of the probiotic diet. A prebiotic-based diet was formulated, which served as the control diet, and to it the selected lactic acid bacteria (LAB) was incorporated in one of the three forms: (1) heat-killed, (2) live-sprayed or (3) freeze-dried. A rearing trial was conducted in triplicate with juvenile rainbow trout (average 126 g), which were fed on the control and three other diets containing the respective forms of the probionts, two times daily until the satiation level for a period of 30 days, following which the test diets were withdrawn and control diet was fed up to 45 days. Sampling was done at 10, 20, 30 and 45 days for gut microflora composition and the non-specific humoral and cellular immune responses of the fish. The viable forms (live spray or freeze-dried) were found to induce better phagocytic activity and complement activity compared to that of the non-viable heat-killed form. The plasma immunoglobulin level showed an increasing trend in the fish groups that received the viable probiont, but the trend did not exist towards the end of the study. Upon withdrawal of the probiotic diets, the LAB disappeared from the intestine and the elevated immune parameters returned to the prefed level. This study elucidates that probiont viability could probably influence the immune responses they induce.