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Application of antibiotics to combat bacterial diseases in fish has been criticized due to likely emergence of drug resistance. Therefore, investigation of new bioactive compounds from natural sources has been taken into account. This study was designed to purify and characterize the bioactive compound in the cell free supernatant (CFSs) of autochthonous gut bacteria (Bacillus methylotrophicus KU556164, B. amyloliquefaciens KU556165, Pseudomonas fluorescens KU556166 and B. licheniformis KU556167) isolated from rohu, Labeo rohita. CFSs were antagonistic to fish pathogenic Aeromonas spp., moderately thermo-tolerant and active in wide range of pH (5-11). Antibacterial activity of the CFSs was reduced by the action of proteases (e.g., Proteinase K and Trypsin), indicating proteinaceous nature of the bioactive compound like the bacteriocins. Three-step purification procedure resulted in recovery of 16.97%, 18.04%, 33.33% and 6.38% activity of the antimicrobial protein produced by B. methylotrophicus, B. amyloliquefaciens, P. fluorescens and B. licheniformis, respectively. Purification at each step revealed decrease in protein content with gradual increase in the specific activity of the antimicrobial protein. The purified antibacterial compound ranged between 18.2 and 25.6 kDa. Identification through MALDI-TOF MS/MS and database search through Mascot search engine predicted that the bactericidal compound belonged to either alkaline proteases, or, transcriptional regulator and some hypothetical proteins. Apart from potential technological application of the antibacterial compound, the present study might show promise for application of gut-associated bacteriocinogenic bacteria to control diseases in fish caused by pathogenic bacteria., (Copyright © 2019. Published by Elsevier Ltd.)

In this study, a total of 121 bacterial strains were isolated from the gastrointestinal tract of four teleostean species, namely striped snakehead (Channa striatus), striped dwarf catfish (Mystus vittatus), orangefin labeo (Labeo calbasu) and mrigal carp (Cirrhinus mrigala), among which 8 isolates showed promising antibacterial activity against four potential fish pathogens, Aeromonas hydrophila, Aeromonas salmonicida, Aeromonas sobria and Pseudomonas fluorescens and were non-hemolytic. The isolates were further screened in response to fish bile tolerance and extracellular digestive enzyme activity. Two bacterial strains MVF1 and MVH7 showed highest tolerance and extracellular enzymes activities, and selected for further studies. Antagonistic activity of these two isolates was further confirmed by in vitro growth inhibition assay against four selected fish pathogens in liquid medium. Finally, these two bacterial strains MVF1 and MVH7 were selected as potential probiotic candidates and thus identification by partial 16S rRNA gene sequence analysis. The bacterial isolates MVF1 and MVH7 were identified as two strains of Bacillus sp.

Phytase producing autochthonous bacteria have been isolated from the proximal intestine (PI) and distal intestine (DI) of three Indian major carps, rohu (Labeo rohita), catla (Catla catla) and mrigal (Cirrhinus mrigala) and one minor carp, bata (Labeo bata). In modified phytase screening medium (MPSM), phytase-producing strains were recorded at higher densities in the PI of rohu and minimum in the PI of bata. Out of 45 isolates, 4 bacterial strains were selected as potent phytase producers according to quantitative enzyme assay. Maximum phytase activity was detected in LRF5 isolated from the PI of L. rohita followed by CCF2 isolated from the PI of C. catla. Among the selected isolates, three (LRF5, LRH2 and CMH1) were Gram positive rods, whereas CCF2 was Gram positive coccus. All the four isolates could tolerate a wide range of temperature (25-42 °C) and pH (6.0-9.0). The isolate LRH2 was most thermostable as it was able to survive up to 55 oC. On the basis of 16SrDNA partial sequence analysis, isolates LRF5 and CCF2 were identified as Bacillus cereus (GenBank Accession no. KC894957.1) and Staphylococcus caprae (Accession no. KC894956.1), respectively. Whereas, the isolates LRH2 and CMH1 were most closely related to Bacillus licheniformis (Accession no. KF011267.1) and Lysinibacillus fusiformis (Accession No. KF011266.1), respectively. [ABSTRACT FROM AUTHOR]