Bioactive phthalate from marine Streptomyces ruber EKH2 against virulent fish pathogens

Marine Streptomyces ruber EKH2 isolated from sediments of Bardawil Lake, Egypt, was found as a promising strain for producing bioactive metabolite(s) working against some virulent fish pathogens. Some biochemical and morphological characterizations of marine S. ruber EKH2 were carried out. Cell free culture showed activities against the tested pathogens ranging from 15 to 30 mm. Optimized conditions for maximum activities were observed at neutrality and temperature 28 °C against the tested strains. Two grams of the ethyl acetate crude extract from 10 L culture supernatant was chromatographically separated into three fractions and bioassayed. One major antibacterial compound was separated exhibiting MIC average 12.5 μg/ml. Phthalic acid was structurally suggested on the basis of gas chromatography–mass spectrum (GC–MS) and infrared spectrum (IR). Phthalate activities were compared with known standard antibiotics used in fish therapy and found to be superior. A slight toxicity of phthalate against brine shrimp (LC50 = 2800 μg/ml) was observed. Dealing with pan-drug resistant bacteria in fish therapy, this study confirmed that marine S. ruber EKH2 is potentially used for extracting phthalic acid as a novel bioactive and non-toxic agent for treating bacterial fish infections.