الفهرس 
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Abstract
The Red Sea represents one of the most diverse and exclusive rich places in the world with remarkable biodiversity and geographical variability. It is well documented that all marine organisms harbour symbiotic microorganisms such as bacteria, fungi and cyanobacteria, which could be the true sources of the bioactive compounds previously isolated from their host. Nevertheless, limited research has been conducted on the isolation of bioactive compounds from symbiotic bacteria associated with the Red Sea marine organisms. In the present study eleven endosymiotic bacterial strains were isolated from the collected sponges, soft corals and seaweeds samples from the Red Sea. All the bacterial extracts were assayed for antimicrobial activities against Gram positive, Gram negative bacteria and yeast strains. The metabolites extract of the coded strains A4 and A5 & A6 were showed to be the most active ones which selected to be further studied. These bacteria identified using 16S rDNA sequencing as Enterobacter cloacae, Bacterium SRCnm and Bacillus sp. Js. The crude extract of the extracellular metabolites of the three bacterial strains exhibited an interesting effect on scavenging DPPH free radical. After the optimization of the growth conditions and selection the best fermentation medium for more bacterial productivity, the fermentation on a big scale performed to isolate the entire metabolic constituents. LC/MS of total extracts and fractions were carried out resulting in the identification of major ionized compounds as diketopiperazine derivatives. Ten compounds isolated and identified from the metabolites of Enterobacter cloacae, which selected due to its wide spectrum activity. The identified compounds were cyclo(S-Pro-S-Tyr), cyclo(S-Pro-S-Val), cyclo(S-Pro-S-Try), cyclo(S-hy-S-Pro-S-Phe), cyclo(R-hy-S-Pro-R-Phe), cyclo(S-hy-S-Pro- S-Lue), cyclo(S-hy-R-Pro- S-Lue), cyclo (S-Pro-Gly), Indole—3-aldehyde & Indole-3-acetic. By investigating their antimicrobial activity, only Indole-3-aldehyde and Indole-3-acetic acid represented a weak to moderate activity.
Keywords: Secondary metabolites, Marine bacteria, Diketopiperazines.