Culturable autochthonous gut bacteria in rohu, Labeo rohita. In vitro growth inhibition against pathogenic Aeromonas spp., stability in gut, bio-safety and identification by 16S rRNA gene sequencing

Abstract

Autochthonous endosymbiotic gut bacteria antagonistic against pathogenic Aeromonas spp. have been evaluated in rohu, Labeo rohita for characterization of putative probiotics. Four promising pathogen inhibitory bacteria (23 strains out of 225 isolates showed antagonism) were selected by double layer assay, following which inhibition pattern was examined through in vitro growth curve and statistical analyses. Cell free supernatant (CFS) of the selected gut bacteria significantly inhibited the growth of pathogenic aeromonads. While, CFS of strain LR3FG26 was the most efficient among them. Selected strains were γ haemolytic and susceptible to most of the common antibiotics that demonstrated their likely non-pathogenic and eco-friendly nature. Additionally, selected bacteria produced different exo-enzymes (digestive and antinutritional factors degrading), could grow better in skin mucus than intestinal mucus (exceptionally, LR3FG26 grew better in skin mucus) and tolerated diluted bile juice (2–20%). 16S rRNA partial gene sequence analyses and Blast search in NCBI GenBank unveiled that the strains LR1FG1, LR2HG13, LR3FG26 and LR3HG4 were similar with the type strains of Bacillus methylotrophicus (NR116240), Bacillus amyloliquefaciens (NR117946), Pseudomonas fluorescens (NR113647) and Bacillus licheniformis (NR118996), respectively. Application of these symbiotic pathogen-inhibitory bacteria in pathogen challenge studies is required to appraise their probiotic effects in vivo.