Studies on probiotics for the control of vibriosis in rainbow trout (Oncorhynchus mykiss, Walbaum)
Abstract
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.