Studies on probiotics for the control of vibriosis in rainbow trout (Oncorhynchus mykiss, Walbaum)
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Kocuria SM1 and Rhodococcus SM2 were administered to juvenile rainbow trout as dietary supplements dosed at ~107–108 cells g−1 of feed for two weeks, and conferred protection against Vibrio infections. Thus, use of SM1 and SM2 led to a significant (P < 0.05) reduction in mortalities, i.e. 12–15% (relative percent survival, RPS = 81– 85%) and 15–20% (RPS = 73–80%) after challenge with V. anguillarum and V. ordalii, respectively, compared to the 80% and 74% mortalities among the respective controls. A two-week feeding regime, compared with 1–4 weeks, for SM1 led to the maximum reduction in mortalities after challenge with V. anguillarum. The use of an equi-mixture of SM1 and SM2 led to significantly enhanced survival against vibriosis, but the result was not better than the use of single cultures. Moreover, use of SM1 for two weeks led to protection for up to 4 weeks. Fish inoculated with cell wall proteins (CWPs) and whole cell proteins (WCPs) of SM1 and SM2 demonstrated better protection against challenge with V. anguillarum, although extracellular proteins (ECPs) fared less well. The mode of action reflected competitive exclusion (= antibiosis), nutrition (= positive effects on growth), and stimulation of cellular and humoral innate immunity, notably greater head kidney macrophage phagocytic, respiratory burst, peroxidase and bacterial killing activities, and elevation of leucocytes, globulin, protein, complement and lysozyme levels. These results demonstrate the efficacy of dietary bacteria as probiotics for the control of vibriosis in rainbow trout.