Flagella‐Mediated Biofilm Formation and 3D Swarming Drive Virulence in the Honey Bee Pathogen Paenibacillus larvae

ABSTRACT

American foulbrood, a devastating bacterial disease affecting honey bee larvae, represents a major threat to apiculture worldwide. Its etiological agent, the Gram‐positive, endospore‐forming bacterium Paenibacillus larvae , has therefore been extensively studied to elucidate the molecular mechanisms underlying its pathogenicity and to identify key virulence determinants. In the present study, we identified the peritrichous flagella of P. larvae as a novel and important virulence factor of the genotype ERIC II. A flagella‐deficient mutant was generated and shown to exhibit multiple defects in vitro associated with biofilm formation, including altered biofilm architecture, reduced adhesion, impaired floating capacity and loss of swarming motility. Survival assays using honey bee larvae, complemented by fluorescence in situ hybridization (FISH) analyses of infected larvae, further demonstrated the crucial contribution of flagella to virulence in vivo by mediating biofilm formation on the larval midgut epithelium as well as three‐dimensional swarming during epithelial invasion.