In recent years, probiotics, especially lactic acid bacteria, have been used as dietary supplements to protect fish from various
infections. Here, we examined the protective effects of Lactobacillus rhamnosus against experimental Edwardsiella tarda infection
in tilapia (Oreochromis niloticus). Cumulative mortality was significantly lower in probiotic-supplemented fish than in control fish.
In a histopathological survey, pyogranulomatous responses were observed at an earlier stage and to a greater extent in the probioticsupplemented
fish than in the control fish. Immunohistochemistry using an anti-E. tarda antibody revealed a larger number of
positive signals in pyogranuloma-participating cells, indicating an enhanced phagocytic ability. Alternative complement activity
was significantly higher in the probiotic groups than in the control. These results suggest that L. rhamnosus enhanced the alternative
complement system of the fish, enabling phagocytic cell aggregation, increasing phagocytic activity and subsequently protecting
the fish from acute septicemic death by E. tarda infection. Prevention of thymic necrosis by the probiotic supplement seems to
minimize immunosuppression and to initiate an immune response against edwardsiellosis.
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