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Use of a probiotic to control lactococcosis and streptococcosis in rainbow trout, Oncorhynchus mykiss (Walbaum)


Prof B Austin, School of Life Sciences, Heriot-Watt University, Riccarton, Edinburgh EH14 4AS, UK


From a comparison of 125 bacterial isolates recovered from the digestive tract of rainbow trout, Oncorhynchus mykiss, and carp, Cyprinus sp., a culture was obtained which was effective at preventing clinical disease caused by Lactococcus garvieae and Streptococcus iniae when used as a feed additive. The culture, Aeromonas sobria GC2, was incorporated into the feed and fed to rainbow trout (average weight = 20 g) for 14 days at a dose equivalent to 5 × 107 cells g−1 of feed. Whereas the untreated controls experienced losses of 75–100% when challenged intraperitoneally with L. garvieae and S. iniae, the probiotic-treated groups remained healthy with total mortalities of only 0–6%. Formalized and sonicated preparations of GC2 and cell-free supernatant fared less well. The mode of action reflected stimulation of innate immunity, namely an increased number of leucocytes and enhanced phagocytic and respiratory burst activity.