Endosymbiotic pathogen-inhibitory gut bacteria in three Indian Major Carps under polyculture system: A step toward making a probiotics consortium

The gastrointestinal (GI) microbiome in fish plays significant roles in health and disease resistance. This investigation was accomplished to enumerate, characterize and identify the potential probiotic bacteria from three Indian Major Carps (IMCs), viz., rohu (Labeo rohita), catla (Catla catla) and mrigal (Cirrhinus mrigala) using culture dependent methods. Altogether, 105 pathogen-inhibitory bacteria (out of 1216 isolates) were detected from three IMCs by double layer assay. 16S rRNA partial gene sequence analyses and BLAST search in the NCBI GenBank unveiled that 94.29% of the pathogen inhibitory bacteria were bacilli (99 strains) and Bacillus licheniformis by far the most common (28%). The primarily selected 27 pathogen-inhibitory strains (cumulative inhibition score ≥13) produced extracellular enzymes, while 15 of them produced all the six exo-enzymes studied (amylase, protease, lipase, cellulase, phytase and xylanase). Gut stability of the strains became apparent by their ability to grow in fish mucus and tolerance to diluted bile-juice. Finally, 14 strains were noticed as γ-hemolytic and susceptible to the commonly used antibiotics. Further, intra-peritoneal injection with γ-hemolytic strains did not induce any pathological signs or mortalities in fish, and thus were considered as safe. These 14 γ-hemolytic isolates were represented by the genus Bacillus (13) and Stenotrophomonas (1), which might form probiotic consortia for prospective use in carp culture.