Dietary administration of Bacillus thuringiensis on the cellular innate immune response of African catfish (Clarias gariepinus) against Aeromonas hydrophila.

AIM An experiment was conducted to evaluate the use of Bacillus thuringiensis (Bt) as a probiotic to enhance the cellular innate immune response of the African catfish (Clarias gariepinus) challenged with a bacterial fish pathogen, Aeromonas hydrophila. MATERIALS AND METHODS The bacterium (Bt) was administered orally at three different dietary doses of 0.5 x 10(7) (T2), 1.0 x 10(7) (T3) and 1.5 x 107 (T4) cfu/g feed to the fingerlings of African catfish twice daily for 15 days. The positive control group (T1) and negative control group (T5) was fed without Bt for the same period. On the 16th day, blood and serum samples were withdrawn to determine the differential leukocyte count (DLC) and serum bactericidal activity. After 15 days feeding, the treatment groups (T2, T3 and T4) and positive control (T1) were challenged intraperitoneally with the bacterial suspension of 0.2 ml (1 x 10(8) cfu/ml). The negative control group (T5) was injected intraperitoneally with phosphate buffered saline (PBS). On the 3rd day of post challenge, the DLC was assessed in all the treatment groups. RESULTS The fish fed with Bt incorporated diet showed significantly higher (P < 0.05) bactericidal activity during the pre-challenge compared with the control groups. The highest serum bactericidal activity was observed in T4, the group fed with diet containing highest concentration of Bt. Highest survival rates (90%) were recorded in fish fed with Bt supplemented group (T3 and T4). Granulocyte counts on the other hand were higher in treatment groups compared to the control in both pre and post challenge. CONCLUSIONS The result demonstrates that Bt can enhance certain innate immune responses in African catfish.