Characterization of Bacillus spp. from the gastrointestinal tract of Labeo rohita--towards to identify novel probiotics against fish pathogens.

The aim of the present study is to screen and characterize endogenous microbiota Bacillus spp. from the gastrointestinal (GI) tract of Labeo rohita in order to evaluate their probiotic attributes. A total of 74 isolates from the GI of L. rohita were evaluated for their antimicrobial properties by agar well-diffusion method against fish pathogens. Based on the better antibacterial features, three isolates (KADR1, KADR3, and KADR4) were selected for further delineation. The three selected isolates exhibited higher tolerance to bile salt, moderate tolerance to low pH, high surface hydrophobicity to solvents, and capable to autoaggregate. All three isolates demonstrated notable proteolytic, catalase activity and susceptibility to various antibiotics. Partial 16S rRNA sequencing revealed that the isolates exhibited 99 % sequence homology with Bacillus subtilis, Bacillus aerophilus, and Bacillus firmus of the database substantiating morphological and physiological characterization. Survivability in low pH and bile salt ensures their adaptability in the fish intestinal microenvironment. The ability to autoaggregate reveals colonization potential in the GI of the fish. Absence of hemolytic activity, antibiotic susceptibility to certain antibiotics, presence of protease and catalase activity, and non-pathogenic caliber of the above-mentioned isolates could be feasible characteristics when considering them as probiotics in the aquaculture industry.