Abstract
Streptomonospora sp. M2 has been isolated from a soil sample collected at the Wadden Sea beach in our ongoing program aimed at the isolation of rare Actinobacteria, ultimately targeting the discovery of new antibiotics. Because crude extracts derived from cultures of this strain showed inhibitory activity against the indicator organism Bacillus subtilis, it was selected for further analysis. HPLC–MS analysis of its culture broth revealed the presence of lipophilic metabolites. The two major metabolites of those were isolated by preparative reversed-phase HPLC and preparative TLC. Their planar structures were elucidated using high-resolution electrospray ionization mass spectrometry (HRESIMS), 1D and 2D NMR data as new thiopeptide antibiotics and named litoralimycin A (1) and B (2). Although rotating frame nuclear Overhauser effect spectroscopy (ROESY) data established a Z configuration of the Δ21,26 double bond, the stereochemistry of C-5 and C-15 were assigned as S by Marfey’s method after ozonolysis. The biological activity spectrum of 1 and 2 is highly uncommon for thiopeptide antibiotics, since they showed only insignificant antibacterial activity, but 1 showed strong cytotoxic effects.
Highlights
New antibiotics in general and new types of antibiotics in particular are urgently needed to counter the increasing number of pathogenic bacteria resistant against present antibiotics [1]
These rare actinobacteria are assessed as a potential storehouse for novel antibiotics due to their unique potential to produce novel metabolites [3,4]
By a fractionation of the crude extracts in 96-well plates, it was possible to link the antibacterial of the two crude extracts in 96-well it was possible link the 1antibacterial activityBy toaafractionation region containing major peaks (Figure plates, S1)
Summary
New antibiotics in general and new types of antibiotics in particular are urgently needed to counter the increasing number of pathogenic bacteria resistant against present antibiotics [1]. Actinobacteria have been the most prolific sources of novel antibiotics scaffolds, because many of the most important antimicrobials, such as β-lactames, tetracyclines, rifamycins, aminoglycosides, macrolides and glycopeptides, were discovered from them [2]. High rates of rediscovery of known compounds are observed when screening traditional producers, and the discovery of new molecules is getting more and more challenging. Current screening programs concentrate on discovering and isolating rare genera of microorganisms. Rare actinobacteria are regarded as actinomycete strains whose isolation frequency is much lower than that of Streptomyces spp. isolated by conventional methods. These rare actinobacteria are assessed as a potential storehouse for novel antibiotics due to their unique potential to produce novel metabolites [3,4]
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