Evaluation of Gut Associated Extracellular Enzyme-producing and Pathogen Inhibitory Microbial Community as Potential Probiotics in Nile Tilapia, <i>Oreochromis niloticus</i>

Abstract

The present study aimed at evaluation of gut associated bacteria and yeasts in Nile tilapia ( Oreochromis niloticus ) as novel probiotics depending on extracellular digestive (amylase, protease and lipase) and degradation (cellulase, phytase, and xylanase) enzymes-producing ability, pathogen inhibition and bio-safety. The gastrointestinal (GI) tracts were taken out, separated into proximal and distal segments, homogenized, and enrichment culture was done on selective media plates for isolation of bacteria. Yeasts were isolated on yeast extract-peptone-dextreose media supplemented with antibiotics (150 mg L -1 ). Both, bacteria and yeasts were detected in the GI tracts of Nile tilapia. Diverse enzyme-producing microbial populations were higher in the distal segment than the proximal segment. Ten out of 97 bacteria and 5 out of 32 yeast strains were primarily selected. The bacterium ONF1P and the yeast strain ONF7.1C were noticed as the efficient exo-enzyme producing strains. Both the isolates were antagonistic against ≥2 tested fish pathogens. Both the strains were tolerant to diluted bile juice, capable to grow in fish mucus (intestinal) and compatible with previously isolated autochthonous fish gut bacteria. The isolates didn’t induce any pathological lesions or mortality in O. niloticus fingerlings. The strains ONF1P and ONF7.1C were identified as Bacillus licheniformis (KT362744) and Pichia kudriavzevii (KT582009), respectively, through 16S /18S rRNA gene fragment analyses. Extracellular enzyme-producing gut bacteria and yeast may restrain the growth of some fish pathogens and tolerate conditions within the GI tract. Further research should be directed to determine their in vivo effects on growth and disease resistance in O. niloticus .