Studies on an Indigenous Probiotic ( Shewanella algae ) Isolated from Healthy Shrimp Intestine

An indigenous probiotic ( Shewanella algae ) previously isolated from healthy shrimp ( Penaeus monodon ) intestine and found to protect Clarias gariepinus from Aeromonas hydrophila infection was screened for antibiotic susceptibility, enzymatic activities and abiotic stress tolerance. Optimization of the cultivation conditions and identification of its bioactive compounds were also carried out. The antibiotic susceptibility was performed by disk diffusion method. A total of fifteen antibiotic discs were employed. Qualitative screening for extracellular enzyme-producing ability was done using skim milk agar, starch agar, egg yolk agar, gelatin agar and cellulose agar for protease, amylase, lipase, gelatinase and cellulase activities respectively. The optimal conditions for growth of the isolate were assessed by growing on nutrient broth at various pH, temperature and salinity levels. Cell growth was estimated by standard plate count. The probiotic was screened for antimicrobial activity by agar well diffusion assay against the pathogen ( Aeromonas hydrophila ) using cell-free supernatant (crude extract) and methanolic extract. The methanolic extract was analyzed using gas chromatography- mass spectrometry (GC-MS). The isolate showed a wide range of environmental tolerance with pH (4-11), temperature (28-40℃) and salinity (0-100 ppt). The optimum conditions for cell growth were pH 6-8, temperature 37℃ and salinity 5-15 ppt. The isolate was susceptible to all tested antibiotics which supported the ideal probiotic characteristic. The isolate was capable of producing extracellular enzymes such as protease, amylase, lipase and gelatinase which could improve feed digestibility and feed utilization. The crude and methanolic extracts were active against the pathogen with inhibition zones of 13.0 ± 0.02 mm and 18.0 ± 0.03 mm respectively. Two bioactive compounds (Tromethamine and Pyrrolo [1, 2-a] pyrazine-1, 4-dione, hexahydro-) were identified. These results suggest that the Shewanella algae could probably be used as a potential probiotic in aquafeed formulation and a good source of novel drug development.