Antimicrobial diversity in plant resins used by honey bees, Apis mellifera

The use of plant resins in nest building by honey bees (Apis mellifera) is a critical yet relatively unexplored factor in bee health.  Called ‘propolis’ by beekeepers, these complex mixtures of antimicrobial compounds provide important immune benefits to the colony.  Bees utilize a variety of resinous plants that differ in antimicrobial properties, but some resins may be more beneficial than others.  The goals of this research were to 1) compare the antimicrobial activity of propolis samples collected across the U.S., 2) isolate the antimicrobial compounds, and 3) determine the plant sources of these active compounds.

           We found that propolis samples from 12 regions across the U.S. differed greatly in their ability to inhibit the bee bacterial pathogen Paenibacillus larvae and the bee fungal pathogen Ascophaera apis in culture, with anti-P. larvae activity ranging from IC₅₀ = 43 µg/mL to > 175 µg/mL.  We then used bioassay-guided fractionation to isolate a variety of flavanone-3-alkyl esters from Nevada propolis active against both P. larvae and A. apis.  Interestingly, longer alkyl esters increased anti-P. larvae activity, while shorter alkyl esters increased anti-A. apis activity.  The relative concentrations of two isolated compounds with high activity against P. larvae, pinobanksin-3-hexanoate and pinobanksin-3-octanoate, correlated very well with anti-P. larvae activity in propolis from our 12 regions.  This suggests that these two compounds are major contributors to propolis antimicrobial activity in the U.S.  These compounds were present in six different North American Populus spp. resins, supporting that poplar is a botanical source of these biologically active compounds.