Description:
<jats:title>Summary</jats:title><jats:p><jats:italic>Paenibacillus larvae</jats:italic> is the causative agent of the notifiable epizootic American foulbrood, a fatal bacterial disease of honey bee larvae. The species <jats:italic>P. larvae</jats:italic> has been classified into four differentially virulent and prevalent genotypes (ERIC I‐IV), which also differ in their virulence factor equipment. Recently, a novel <jats:italic>P. larvae</jats:italic> toxin, the C3‐like C3larvin, has been described. Genome analysis now revealed that the C3larvin gene is actually a part of a toxin locus encompassing two genes encoding a binary AB toxin with the A subunit being C3larvin (C3larvinA) and a putative B subunit (C3larvinB) encoded by the second gene. Sequence and structural analyses demonstrated that C3larvinB is a homologue of the <jats:italic>Bacillus anthracis</jats:italic> protective antigen (PA), the B subunit of anthrax toxin. The C3larvinAB toxin locus was interrupted by point mutations in all analysed <jats:italic>P. larvae</jats:italic> ERIC I and ERIC II strains. Only one <jats:italic>P. larvae</jats:italic> ERIC III/IV strain harboured an uninterrupted toxin locus comprising full‐length genes for C3larvinA and B. Exposure bioassays did not substantiate a role as virulence factor for C3larvinAB in <jats:italic>P. larvae</jats:italic> ERIC I/II. However, the PA homologue C3larvinB had an influence on the virulence of the unique <jats:italic>P. larvae</jats:italic> strain expressing the functional C3larvinAB locus.</jats:p>