Characterization of toxin systems of Paenibacillus strains isolated from honeybees

Paenibacillus is a large genus of Gram-positive bacteria isolated from a wide range of sources. Members of this genus have attracted significant research as plant growth promoters as well as pathogens of both plants and animals. Extensive studies have been conducted on Paenibacillus larvae, the causative agent of American Foul Brood in honeybees, but little is known about other closely related species associated with honeybees, such as Paenibacillus apiarius or Paenibacillus thiaminolyticus. These bacterial species have been isolated from dead honeybee larvae and are believed to be secondary invaders as opposed to primary pathogens. We utilized genome mining efforts to explore the number and diversity of toxins present in multiple species of Paenibacillus, with major emphasis on a potentially entomopathogenic clade that includes Paenibacillus alvei, P. apiarius, and P. thiaminolyticus. Our results show that strains of these species contain a variety of potentially insecticidal toxins, including highly conserved Spp-like and Mpp-like toxins. These toxins are absent in P. larvae. We also found several ADP-ribosylases which were shared between P. larvae and several strains from this entomopathogenic clade. Tc toxins, a class of insecticidal toxin not often found in Gram-positive bacteria were found in in P. apiarius and P. alvei. These findings suggest the potential for Paenibacillus species in this clade to be harnessed as sources of host specific and pollinator-friendly biopesticides.