Streptomycesextracellular vesicles are a broad and permissive antimicrobial packaging and delivery system

ABSTRACT Streptomyces are the primary source of bioactive specialized metabolites used in research and medicine, including many antimicrobials. These are presumed to be secreted and function as freely soluble compounds. However, increasing evidence suggests extracellular vesicles are an alternative secretion system. We assessed environmental and lab-adapted Streptomyces (sporulating filamentous actinomycetes) and found a majority of strains produce vesicles packaging antimicrobial compounds. The cargo included actinomycins, anthracyclines, candicidin, and actinorhodin, reflecting both diverse chemical properties and diverse antibacterial and antifungal activity. The production of the antimicrobials and vesicles was linked, although vesicle production continued in the absence of antimicrobials. We demonstrated that antimicrobial-containing vesicles achieve direct delivery of the cargo to other microbes. Notably, this delivery via membrane fusion occurred to a broad range of microbes including pathogenic bacteria and yeast. Vesicle encapsulation offers a broad and permissive packaging and delivery system for antimicrobial specialized metabolites, with important implications for ecology and translation. GRAPHICAL ABSTRACT IMPORTANCE Extracellular vesicle encapsulation changes our picture of how antimicrobial metabolites function in the environment, and provides an alternative translational approach for delivery of antimicrobials. We find the majority of Streptomyces strains are capable of releasing antimicrobial vesicles, and at least four distinct classes of compounds can be packaged, suggesting this is widespread in nature. This is a striking departure from the primary paradigm of secretion and action of specialized metabolites as soluble compounds. Importantly the vesicles deliver antimicrobial metabolites directly to other microbes via membrane fusion, including to pathogenic bacteria and yeast. This suggests future applications in which lipid-encapsulated natural product antibiotics and antifungals could be used to solve some of the most pressing problems in drug resistance.