Old Compound, New Context: Naphthocyclinones Mediate Microbial Competition in Wild Bee Nests
Although thousands of microbial specialized metabolites have been identified, the ecological functions of most remain unknown. Naphthocyclinones are aromatic polyketide antibiotics discovered more than five decades ago, yet their biological role has remained elusive. Investigating the microbial community of the oligolectic ivy bee (Colletes hederae), a bacterial community that included several Streptomyces species and members of the class Bacilli was recovered from fermented nest provisions. One Streptomyces isolate, B-004T (DSM 121525T = LMG 34598T), was identified as representing a novel species based on polyphasic taxonomic studies, for which the name Streptomyces hederae sp. nov. is proposed. This strain showed potent antibacterial activity against co-isolated Gram-positive bacteria as well as Paenibacillus larvae, the causative agent of American foulbrood. Bioassay-guided fractionation coupled with liquid chromatography-mass spectrometry analysis led to the identification of naphthocyclinone congeners as the active metabolites. Whole-genome sequencing and genome mining revealed a type II polyketide synthase biosynthetic gene cluster closely related to that of the characterized naphthocyclinone pathway. Targeted deletion of the cluster-encoded ketosynthase gene (B-004_07360) abolished naphthocyclinone production and resulted in the complete loss of antibacterial activity, whereas heterologous overexpression of the Streptomyces antibiotic regulatory protein gene papR2 significantly increased antibiotic production, confirming the function of the naphthocyclinone biosynthetic gene cluster. Furthermore, naphthocyclinone biosynthesis was found to be induced by sympatric bacterial isolates from bee nests. Together, our findings provide the first evidence for an ecological function of naphthocyclinones as mediators of bacterial competition in wild bee nests and highlight their potential as antimicrobial substances against P. larvae.