Antibiotic Compounds From Four Marine-Derived Actinomycete Strains

Marine-derived actinomycetes have attracted much attention because of theirunique living environment and a variety of secondary metabolites. Many antibioticcompounds have been isolated from marine-derived actinomycetes, and they havebeen an important resource for the marine drug development. The unique environmentgave birth to the unique metabolic mechanisms, so the natural products frommarine-derived actinomycetes always possess novel structure and some antibioticactivities, such as anti-tumor and antimicrobial activities. The novel structures andactivities stimulated the enthusiasm of many marine natural products workers.In order to obtain new antibiotic compounds from marine-derived actinomycetes,many strains, isolated from marine sediments, coral and other marine environmentalsamples, were screened using the chemical and biological activity screening phase.The chemical screening focuses on the diversity, novelty and types of the secondarymetabolites. In this process, TLC and HPLC-UV-MS were used. The bioactivescreening focuses on the anti-tumor or antimicrobial activities of the secondarymetabolites. K562and MCF-7tumor cell lines, and Staphylococcus aureus, Bacillussubtilis and Candida albicans and other pathogenic bacteria model were used todetermine the cytotoxic activity and antimicrobial activity. By the comprehensiveanalysis of the chemical and biological results, four talented strains have beenselected for further study.First of all, the fermentation conditions of talented strains, including medium,fermentation time and so on, were optimized. After fermentation of these fourmarine-derived actinomycetes strains, the whole broths were extracted with ethylacetate to give ferment extracts. Then the extracts were subjected to extensive columnchromatography and HPLC to give pure compounds. Fifty-six compounds wereisolated from four strains. Twenty-seven compounds (1–27) were isolated fromActinoalloteichus cyanogriseus WH1-2216-6. Eight compounds (32–39) were isolatedfrom Nocardiopsis dassonvillei HR10-5. Twelve compounds (40–51) were isolatedfrom Nocardiopsis dassonvillei XG-8-1. Nine compounds (52–60) were isolated fromStreptomyces sp. OUCMDZ-1703.By means of modern spectral analysis (NMR, MS, CD, UV, IR), X-ray, quantumchemical calculations and chemical methods, the structures of fifty-six compounds were determined, among them thirty compounds were new. Types of new compoundsinclude bipyridine alkaloids (1–7,9–14), thiophene (19),2-pyrones (32–34,40–46),diketopiperazine (35–37), and multi-chloro substituted polyketones (52,53).The cytotoxic activity and antimicrobial activity of these compounds wereevaluated. Compound1showed moderate cytotoxicity against K562, KB and MCF-7cell lines with IC50values of1.2,4.7and9.8μM. Compound3showed moderatecytotoxicity against K562and KB cells with IC50values of0.73and4.7μM.Compound12showed significant cytotoxic activity against K562cells with IC50value of0.37μM. Compound8showed significant cytotoxic activity against HL-60and A549cell lines with IC50values of0.71and0.26μM. Compound15showedmoderate cytotoxic activity against K562and KB cell lines with IC50values of1.8and3.1μM. Compound17showed weak cytotoxic activity against A549cell linewith IC50value of15.0μM. Compounds52and53showed weak cytotoxic activityagainst MCF-7cell line with IC50values of9.9and20.2μM, respectively. Compound8showed weak inhibitory activity against Escherichia coli, Pseudomonas aeruginosaand Candida albicans with MIC values of10.9,21.8and21.8μM. Compound15showed weak inhibitory activity against Escherichia coli, Aerobacter aerogenes,Pseudomonas aeruginosa and Candida albicans with MIC values of9.7,19.3,38.6and19.3μM. Compounds32–34showed weak inhibitory activity against Bacillussubtilis with MIC values of26,14and12μM, respectively. Compounds40–46showed moderate inhibitory activity against many pathogens, such as Escherichia coli,Aerobacter aerogenes, Pseudomonas aeruginosa, Bacillus subtilis, Staphylococcusaureus, and Candida albicans. Compounds54–56showed moderate inhibitoryactivity against methicillin-resistant Staphylococcus aureus.The biosynthetic pathway of the bipyridine compounds was explored. Somedifferent aromatic formic acids were added to the fermentation medium as thebiosynthetic precursor. In this process, two new fluoric bipyridines (8F and15F) wereoatained, which confirmed the source of the first pyridine ring of bipyridines. Fournew bipyridine compounds (28–31) were isolated from three mutant strains ofWH1-2216-6(CRM001, CRM004, CRM005). The discovery of these compoundsconfirmed the biosynthetic mechanism of the second pyridine ring of bipyridines.