3.2. Challenge tests
All unchallenged fish (in the control group) survived in the Streptococcus sp. and GIV challenge tests. For the Streptococcus sp. challenge tests of grouper fed the experimental diet after 14 or 28 days, the survival rate of fish fed the 106 and 108 cfu g−1 B. subtilis-supplemented diets was significantly higher than that of fish fed the control diet and 104 cfu g−1 B. subtilis diet after 7 days of challenge ( Table 2A, B). The survival rate of fish fed the 104 cfu g−1 B. subtilis-supplemented diet for 28 days was significantly higher than that of fish fed the control diet after 7 days of challenge. After 7 days of challenge, survival rates of fish fed the 104, 106, and 108 cfu g−1 B. subtilis-supplemented diets for 28 days were 20.9%, 37.5% and 41.7% higher, respectively, compared to the control group, and the RPS of fish were 22.8, 40.9, and 45.5, respectively ( Table 2B).