The emergence of antibiotic resistant microorganisms presents a worldwide problem that requires novel antibiotic and non-antibiotic strategies, and biofilm formation is a mechanism of drug resistance utilized by diverse microorganisms. The majority of microorganisms live in biofilms that help their survival against starvation, antimicrobial agents, and immunological defense systems. Therefore, it is important novel compounds be identified that inhibit biofilm formation and cell survival without drug resistance.In this study, the antimicrobial and antibiofilm activities of five prenylated flavanones (Okinawan propolins) isolated from fruits of Macaranga tanarius (L.) were investigated against 14 microorganisms including 10 pathogens.Of these five propolins, propolin D at 5-10 µg/ml significantly inhibited biofilm formation by three Staphylococcus aureus strains, a Staphylococcus epidermidis strain, and a Candida albicans with MICs from 10 to 50 µg/ml, and in C. albicans, propolin D was found to inhibit biofilm formation by reducing cell aggregation and downregulated the expressions of hypha/biofilm-related genes including ECE1 and HWP1. Interestingly, at sub-MIC concentrations (10-50 µg/ml), propolin D significantly inhibited biofilm formation by enterohemorrhagic E. coli O157:H7, uropathogenic E. coli O6:H1, and Acinetobacter baumannii without affecting planktonic cell growth, but did not inhibit biofilm formation by a commensal E. coli K-12 strain, three probiotic Lactobacillus plantarum strains, or two Pseudomonas aeruginosa strains. And, propolin D reduced fimbriae production by E. coli O157:H7 and repressed gene expression of curli fimbriae genes (csgA and csgB). Also, propolin D was minimally toxic in a Caenorhabditis elegans nematode model.These findings show that prenylated flavanones, especially propolin D from Macaranga tanarius (Okinawan propolis), should be considered potential candidates for the development of non-toxic antibacterial and antifungal agents against persistent microorganisms.
