Pathogenicity test is one key criterion used in selecting probiotics for use in food producing animals. This experiment was aimed to ascertain the safety of 10 selected probiotic-potential Bacillus species (Bsp). Three hundred and sixty Clarias anguillaris juveniles were obtained from homestead fish ponds within Makurdi metropolis. The fingerlings were distributed in 10 experimental groups: Bsp1, Bsp2, Bsp3, Bsp4, Bsp5, Bsp6, Bsp7, Bsp8, Bsp9 and Bsp10 and 2 control groups viz: positive control (PC) and negative control (NC). Each group was assigned 10 fingerlings in replicate. The PC group received 0.2 x 108CFUml-1 of pathogenic bacteria Vibrio alginolyticus, the NC received 0.2mls of PBS and test groups received 0.2 x 108CFUml-1 Bacillus strains. The groups were observed for 20 days for morbidity and/or mortality from respective test groups. Survival rate of 60% (PC), 70% (Bsp8), 80% (Bsp6), 90% (Bsp2) whereas 100% were recorded for the rest of the groups. The weight gain of the PC group was significantly lower (P ? 0.05) than all groups except for Bsp6. Also, Bsp7, recorded highest weight gain (20.82±8.2g) whereas Bsp1, Bsp2, Bsp4, Bsp5, Bsp8, Bsp9 and Bsp10 were significantly higher compared to both PC and NC. All physico-chemical parameters were within the reference interval (RI) for catfish. The 100% survival from Bsp1, Bsp3, Bsp4, Bsp5, Bsp7, Bsp9, and Bsp10 compared to PC were signs that these Bacillus strains were not pathogenic to the fish used, whereas Bsp2, Bsp6 and Bsp8 were mildly pathogenic to the experimental fish, though environmental factors could be incriminated. The high weight gain by Bsp7 (20.82±8.30), Bsp1 (17.86±4.24), Bsp2 (14.48±1.65), Bsp4 and Bsp10 respectively (13.94±4.80 and 13.36±4.36) showcased the growth stimulation potentials in these isolates. The present study, showed that survival, growth performance, and regulation of physico-chemical parameters were significantly (P ? 0.05) high with Bsp7, Bsp1, and Bsp10, so can be regarded as safe and can improve growth performance in fish production. These 3 Bacillus strains were identified as B. subtilis (MN099359.1), B. subtilis MK085082.1 and B. velezensis (CP041145.1).

Keywords: Probiotics, Bacillus strains, Pathogenicity, Assay, Catfish, Juveniles