Screening Of Antibacterial Activity Of Marine Actinomycetes And Studies On Secondary Metabolites From Two Marine Organism-associated Atcinomycetes

The continuous emergence of drug-resistant bacteria,especially gram-negative drug-resistant bacteria,is a serious threat to human health.At present,there is an extreme lack of safe and effective drugs for the treatment of multidrug-resistant bacteria infection clinically.The abuse of antibiotics in China is serious and the situation is grim,thus it is urgent to discover new drugs with high efficiency against bacterial infection.The ocean covers a vast area with abundant biological species,which is a treasure house rich in natural products.Marine actinobacteria is an important part of marine ecosystem.Marine actinomycetes have formed unique metabolic pathways and peculiar physiological functions under long-term extreme living environment?high pressure,low temperature but relatively constant temperature,darkness,high salinity and oligonutrition,etc.?,so they can produce drug lead compounds with novel structures and diverse bioactivities.In order to find drug leads with novel scaffold and remarkable antibacterial activity,the conventional culture-dependent method was carried out to isolate culturable actinomycetes from coral and sponge samples from Nansha Islands.Their chemical and antibacterial activity were then screened.In addition,two symbiotic actinomycetes from different marine mollusks were selected as the research object and then fermented on a large scale by the method of microbial fermentation.The crude extract was subjected to silica gel,Sephadex LH-20 column chromatography?CC?and semi-preparative HPLC to afford twenty compounds including three new compounds and one new skeleton compound.Their structures were elucidated based on various spectroscopic analysis methods.The main experimental results are outlined as below:?1?23 actinomycetes were obtained from corals and sponges of Nansha Islands and they were fermented to evaluate using HPLC-DAD fingerprinting and antibacterial activity of crude extract.Seven potential isolates of marine actinomycetes with bioactivity against an array of pathogenic bacteria and drug-resistant bacteria were screened.?2?Three new compounds,along with nine known polyketides julichrome[Q₁₀?4?,Q_3·5?5?,Q_3·3?6?,Q_6·6?7?,Q₆?8?,chrysophanol?9?,4-acetylchrysophanol?10?,islandicin?11?,huanglongmycin A?12?]were isolated from Streptomyces sampsonii SCSIO 054,a putative microbial symbiont of marine gastropod mollusks Batillaria zonalis.This is the first report of julichrome family members from a marine source.Considering the relevant literature combined with the types of compounds,all secondary metabolites isolated from Streptomyces sampsonii SCSIO 054 were evaluated for the antibacterial bioactivities.In conclusion,compounds 1,2,4,7 and 8displayed different inhibitory activities against a series of methicillin-resistant Staphylococcus aureus?MRSA?,S.aureus,Micrococcus luteus and Bacillus subtilis,with MIC values ranging from 2 to 64?g/mL.The preliminary structure-activity relationship analysis revealed that julichrome compounds with unoxidized naphthalene ring fusion with cyclohexane scaffold have antibacterial activity,while anthraquinones and epoxidized julichrome compounds abolish antibacterial activity.?3?Seven compounds were isolated from sea slug associated Streptomyces sp.LCY12,comprising one novel naphthopyranofuranone endowed with fluorescent property?14?,three known anthraquinones?3,8-dihydroxy-1-methylanthraquinone,3,8-dihydroxy-1-methylanthraquinone-2-carboxylic acid,9'-hydroxyaloesaponarin II,15–17?,two known carbolines 1-acetyl-3-carbomethoxyl-?-carboline?18?,1-acetylacetyl-?-carboline?19?and one known cyclic ester peptide valinomycin?20?.The preliminary filter disk diffusion method showed that the antibacterial activity of the crude extract was mainly caused by valinomycin.The above studies enrich the structural types of secondary metabolites of marine actinomycetes and provide candidate drug molecules for the development of anti-infective drugs.