Screening, partial purification of antivibriosis metabolite sterol-glycosides from Rhodococcus sp. against aquaculture associated pathogens.

The present study probed the antimicrobial potential of a rare mangrove associated actinomycetes against an array of aquatic bacterial pathogens causing disease outbreak in fin and shellfish. Antibacterial activity results implied that the mangrove associated actinomycetes RAS7 exhibited striking inhibitory activity against the tested aquatic bacterial pathogens. Identification of strain RAS7 through polyphasic and 16S rRNA sequencing affirmed that the strain belongs to Rhodococcus sp. Optimization of culture conditions for antibacterial activity by Rhodococcus sp. inferred that it grew well and exerted notable antagonistic activity in medium supplied with 1% galactose and peptone as carbon and nitrogen sources. Similarly, the strain grown in 0.1% tyrosine, 1% NaCl, pH 7.5 and temperature 35 °C recorded maximum bioactivity against the test pathogens. The crude ethyl acetate extract of Rhodococcus sp. at 200  µg/ml recorded markedly pronounced growth inhibitory activity ranged between 14 and 29 mm. The cytotoxic effect of crude extract against brine shrimp Artemia salina nauplii registered LC50 value of 134.294 µg/ml after 24 h of exposure. The secondary metabolite was separated using Ethyl acetate: Methanol (7:3) as solvent system through TLC. The TLC autobiogram mapped the active spot in TLC with Rf value of 0.84. Analysis of chemical constituents and FT-IR spectral analysis substantiated that the active principle in bioassay guided fraction was sterol-glycosides.