Study On Secondary Metabolites Produced By Two Strains Of Nematode-symbiotic Bacteria

Microbial secondary metabolism produces natural products with structuraland bioactive diversities, which are important sources of pharmaceuticals andpesticides. Among them, about80%of antibiotics are derived from the soilmicroorganism-actinomycetes. However, the rate of the discovery of newcompounds was significantly decreased in recent years, which will encouragepeople to turn their attention to microbial resources from marine and specialecological environment for new compound discovery. In this thesis, we selectedgram-negative bacteria of the genus Xenorhabdus associated with nematodes asthe targets, isolated and identified the secondary metabolites of two strains ofXenorhabdus spcies.Firstly, we optimized the fermentation condition based on the UV-visspectra of the secondary metabolites of the strains PacYellow and GcGill5.Subsequently,50L-scale fermentation of each strain was performed. After thecrude products were obtained by extraction with acetic ether, we used thecombination of thin-layer chromatography, silica gel column chromatography,gel chromatography, semi-preparative high-performance liquid chromatographyto finally purified8compounds from each strain. Their chemical structures wereelucidated by using spectroscopic technologies including nuclear magnetic resonance (NMR), infrared spectroscopy (IR), and mass spectroscopy (MS). The8compounds from the strain PacYellow were identified as the indole derivativesand names P-1to P-8that might be derived from tryptophan and valine orisoleucine,4of which are new compounds. Among8compounds from the strainof GcGill5,6compounds were identified as pyridone derivatives that are newcompounds, and two compounds has not been identified.The antimicrobial and antitumor activity assays were carried out for thenew compounds (P5-P8) and showed that these four compounds exhibited veryweak activity against the tested strains including Staphylococcus aureus,Bacillus anthracis, Mycobacterium smegmatis2155and the compound5didn’tshow obvious inhibitory activity against tumor Hela cells (IC50=183.6μmol/L).However, the selected three compounds (G-1, G-3and G-5) exhibited strongactivity against the all tested strains and significant inhibitory activity againstcervical cancer (Hela) and lung cancer (H460) cells with IC50value from0.5Mto1.3M, especially the IC50value of G-1is lower than0.5M.In summary, we isolated and purified16compounds from the two strains ofXenorhabdus, and eventually identified14compounds,10of which are newcompounds counting for higher than70%rate of new compound discovery.Although the compounds from the strain of PacYellow didn’t show goodbiological activity in our tested conditions, the compounds from the strain ofGcGill5have strong biological activity against the tested microbial strains andtumor cells. Our findings suggest that microbes from special habitats sources may indeed have the potential to generate new structure of the compounds,providing opportunities for the new drugs discovery.