Identification and characterization of two novel toxins expressed by the lethal honey bee pathogen Paenibacillus larvae, the causative agent of American foulbrood.

Paenibacillus larvae is a Gram-positive bacterial pathogen causing the epizootic American foulbrood in honey bee larvae. Four so-called enterobacterial repetitive intergenic consensus (ERIC) genotypes of P. larvae exist with P. larvae genotypes ERIC I and ERIC II being responsible for disease outbreaks all over the world. Very few molecular data on the pathogen, on pathogenesis or on virulence factors exist. We now identified two genomic loci in P. larvae ERIC I coding for two binary AB toxins, Plx1 and Plx2. In silico analyses revealed that Plx1 is the third member of an enigmatic family of AB toxins so far only comprising MTX1 of Lysinibacillus sphaericus and pierisin-like toxins expressed by several butterflies. Plx2 is also remarkable because the A-domain is highly similar to C3 exoenzymes, which normally are single domain proteins, while the B-domain is homologous to B-domains of C2-toxins. We constructed P. larvae mutants lacking expression of Plx1, Plx2 or both toxins and demonstrated that these toxins are important virulence factors for P. larvae ERIC I.