Stress-Driven Discovery of Novel Cryptic Antibiotics from a Marine Fungus sp. BB1122.

Standard laboratory cultures have long been known to hinder activation of specific gene clusters which in turn hamper production of secondary metabolites with unique properties due to lack of innovation or the inability to trigger cryptic gene clusters' expression. Due to challenges related to the avoidance of the isolation of replicated metabolites, resistance-developing pathogens are to be addressed by the scientific community worldwide in order to progress with novel and potent compounds which could further be developed in the future for pharmaceutical usage. This study reports the isolation of novel cryptic antibiotics from a marine fungus sp. BB1122 collected from Zhoushan coast by applying the "metal-stress" strategy, here referring to the heavy metal cobalt (6 mM). High-performance liquid chromatography-guided isolation of four novel and four known compounds belonging to the polyketide class has been carried out where their relative as well as absolute configurations have been determined using spectroscopic analysis techniques as well as by the comparison of theoretically calculated ECD spectrum and the experimental ECD spectrum, respectively. The structures of novel compounds and represent the first example of 2,5-dioxabicyclo[2.2.1]heptane pyrone backbone bearing a migrated polyene chain. The novel compounds , and exhibited impressive antibiotic properties against methicillin resistant (MRSA) with MIC value of around 0.5 and 1 μg/mL, respectively. Moreover, the new compounds , and displayed potent antibiotic activities with MIC values of around 4 μg/mL against the pathogenic . Moreover, the MBC of the different potent compounds ranged from 1 to 128 μg/mL against MRSA, , and . In addition, the cytotoxic activities were also evaluated where new antibiotics and were not obviously harmful toward normal liver cell lines LO2, showing IC values above 100 μg/mL. As a consequence, the results from this study unveiled that cobalt stress is an effective strategy to discover novel antibiotics from microorganisms.