Activity of selected plant extracts against honey bee pathogen Paenibacillus larvae

The present study focuses on the antibacterial activity of selected plant extracts against Paenibacillus larvae, the causal agent of American foulbrood disease of honey bees. A gas chromatography-mass spectrometry method was used to analyze six diethyl ether extracts of two white birch species, black poplar and common aspen buds, as well as n-hexane, diethyl ether, and methanol extracts of young twigs from downy and silver birches. Among the identified extract constituents were flavonoids, phenylpropenoids, triterpenoids, and glucosides. In spite of significant differences in qualitative and quantitative composition, all tested in vitro extracts demonstrated high anti-P. larvae activity at minimal inhibitory concentration levels between < 1.0 and 125 μg/mL. To examine the cause of such disparity, the anti-P. larvae activity of some individual constituents naturally present in plant extract compounds was determined. A higher susceptibility of P. larvae (ERIC I) to relatively poorly polar triterpenoid levels compared to polar compounds, flavonoids, and glucosides was demonstrated.