Antibacterial activity of pimaric acid against the causative agent of American foulbrood, Paenibacillus larvae

Abstract

The number of bees has decreased drastically partly due to bacterial pathogens like Paenibacillus larvae that cause American foulbrood disease. The recent emergence of pathogenic resistant strains makes it more difficult to, successfully, treat infections. Research on alternative antibacterial agents from naturally occurring plants that also have low toxicity levels is critical to address the issue of Paenibacillus larvae in bee colonies. Five compounds were screened with a minimal inhibitory concentration (MIC) test in order to identify a compound with effective antibacterial activity against Paenibacillus larvae. Of the tested compounds, pimaric acid showed the best antibacterial activity with a MIC value of 6.25 µg/ml after 48 hours of incubation on MYPGP media. An agar diffusion test also confirmed the anti- Paenibacillus larvae activity of pimaric acid with an inhibitory zone of 10 to 14 mm. Crystal violet assay results suggested that pimaric acid disrupted the plasma membrane of Paenibacillus larvae, causing the leakage of material with an optical density of 260-nm and 280-nm, which confirmed that the compound induces the rupture of this microorganism. Pimaric acid inhibited the formation of a Paenibacillus larvae biofilm in a dose-dependent manner. An additive effect of pimaric acid with tetracycline and terramycin was shown through a checkerboard assay with a fractional inhibitory concentration (FIC) index value of 0.75. Finally, an MTT assay on insect cells of Trichoplusia ni and normal human skin cells from the HaCaT cell line, showed that pimaric acid is safe for both insect cell and human skin cells at the dosage required to treat Paenibacillus larvae.