Bacillus subtilis, a Gram-positive, aerobic, endosporeforming bacterium, was evaluated for its probiotic potential in Indian major carp, Labeo rohita. Labeo rohita (15 AE 2 g) were fed a feed containing B. subtilis in three concentrations for 2 weeks, e.g., 0.5 (T 2 ), 1.0 (T 3 ) and 1.5 (T 4 ) Â 10 7 CFU g À1 feed. The control group (T 1 ) was fed feed without B. subtilis for the same period. Haematological and serum parameters were monitored at weekly intervals. The response variables were total erythrocyte count, total leucocyte count (TLC), haemoglobin, total protein, albumin, globulin, albumin^globulin ratio, alkaline phosphatase activity, alanine aminotransferase activity and aspartate aminotransferase activity. Fish were challenged intraperitoneally with a virulent strain of Aeromonas hydrophila after 2 weeks of feeding to the treatment groups and positive control group, while the negative control group was challenged with phosphate-bu¡ered saline only. Clinical signs and symptoms, and mortality/survival percentage were noted in each group. The haematological and serum parameters were monitored each week and during post challenge on the third and tenth day. The B. subtilis-treated ¢sh (T 4 ,1.5 Â 10 7 CFU g À1 feed) showed maximum per cent survival (87.50%), weight gain (35.5%), TLCs (3.23 Â 10 4 cells mm À 3 ), haemoglobin content (7.4 g%), total protein (2.37 g dL À1 ) and globulin content (1.28 g dL À1 ) during the pre-challenge. Enzymes showed higher activities during post challenge (Po0.05). The result suggests that B. subtilis can be used e¡ectively as a commercial product for use in aquaculture.

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