Paenibacillus larvae, the etiological agent of the globally occurring
epizootic American Foulbrood (AFB) of honey bees, causes intestinal infections
in honey bee larvae which develop into systemic infections inevitably leading
to larval death. Massive brood mortality might eventually lead to collapse of
the entire colony. Molecular mechanisms of host-microbe interactions in this
system and of differences in virulence between P. larvae genotypes are poorly
understood. Recently, it was demonstrated that the degradation of the
peritrophic matrix lining the midgut epithelium is a key step in pathogenesis
of P. larvae infections. Here, we present the isolation and identification of
PlCBP49, a modular, chitin-degrading protein of P. larvae and demonstrate that
this enzyme is crucial for the degradation of the larval peritrophic matrix
during infection. PlCBP49 contains a module belonging to the auxiliary
activity 10 (AA10, formerly CBM33) family of lytic polysaccharide
monooxygenases (LPMOs) which are able to degrade recalcitrant polysaccharides.
Using chitin-affinity purified PlCBP49, we provide evidence that PlCBP49
degrades chitin via a metal ion-dependent, oxidative mechanism, as already
described for members of the AA10 family. Using P. larvae mutants lacking
PlCBP49 expression, we analyzed in vivo biological functions of PlCBP49. In
the absence of PlCBP49 expression, peritrophic matrix degradation was markedly
reduced and P. larvae virulence was nearly abolished. This indicated that
PlCBP49 is a key virulence factor for the species P. larvae. The
identification of the functional role of PlCBP49 in AFB pathogenesis broadens
our understanding of this important family of chitin-binding and -degrading
proteins, especially in those bacteria that can also act as entomopathogens