Gromomycins: An Unprecedented Class of Triterpene Antibiotics Produced by a Novel Biosynthetic Pathway

The current situation with drug-resistant microbial pathogens is critical, dictating an acute need for novel efficient antibiotics. Herein, we report a new class of antibiotics named gromomycins with significant activity, especially against drug-resistant Gram-positive pathogens, including methicillin- and daptomycin-resistant Staphylococcus aureus. Gromomycins are pentacyclic triterpenes with a cyclic guanidino group forming the fifth six-membered ring. We have used transposon mutagenesis to identify the gromomycin biosynthetic gene cluster, since it could not be assigned by any available bioinformatics tools, highlighting its unique biosynthetic route. Using gene cluster engineering, feeding experiments, and LC-MS and NMR analyses we have proposed the biosynthetic pathway for gromomycins, which are the first bacterial triterpenes synthesized independently of the squalene pathway. They also exhibit a so far unprecedented cyclization route that utilizes a hexaprenylguanidine linear precursor. Leveraging our understanding of their biosynthesis, we have identified additional gromomycin producers, resulting in the isolation of novel bioactive derivatives.