Kocuria SM1 controls vibriosis in rainbow trout ( Oncorhynchus mykiss , Walbaum)

Aims:  To develop probiotics for the control of vibriosis caused by Vibrio anguillarum and Vibrio ordalii in finfish. Methods and Results:  Kocuria SM1, isolated from the digestive tract of rainbow trout, was administered orally to rainbow trout ( Oncorhynchus mykiss ) for 2 weeks at a dose equivalent to c.  10 8  cells per g of feed and then challenged intraperitoneally with V. anguillarum and V. ordalii . Use of SM1 led to a reduction in mortalities to 15–20% compared to 74–80% mortalities in the controls. SM1 stimulated both cellular and humoral immune responses in rainbow trout, by elevation of leucocytes (5·5 ± 0·8 × 10 6  ml −1 from 3·7 ± 0·8 × 10 6  ml −1 ), erythrocytes (1·2 ± 0·1 × 10 8  ml −1 from 0·8 ± 0·1 × 10 8  ml −1 ), protein (23 ± 4·4 mg ml −1 from 16 ± 1·3 mg ml −1 ), globulin (15·7 ± 0·2 mg ml −1 from 9·9 ± 0·1 mg ml −1 ) and albumin (7·3 ± 0·2 mg ml −1 from 6·1 ± 0·1 mg ml −1 ) levels, upregulation of respiratory burst (0·05 ± 0·01 from 0·02 ± 0·01), complement (56 ± 7·2 units ml −1 from 40 ± 8·0 units ml −1 ), lysozyme (920 ± 128·8 units ml −1 from 760 ± 115·3 units ml −1 ) and bacterial killing activities. Conclusions:  Kocuria SM1 successfully controlled vibriosis in rainbow trout, and the mode of action reflected stimulation of the host innate immune system. Significance and Impact of the Study:  Probiotics can contribute a significant role in fish disease control strategies, and their use may replace some of the inhibitory chemicals currently used in fish farms.