41st Annual Meeting of the Society for Invertebrate Pathology
and 9th International Conference on Bacillus thuringiensis

American foulbrood (AFB) is a bacterial disease affecting the brood of the European honeybee (Apis mellifera). The causative agent of AFB is the gram-positive bacterium Paenibacillus larvae that forms extremely resilient spores, serving as the transmission stage of the bacterium. We used fluorescence in situ-hybridization (FISH) performed with a P. larvae-specific, 16S rRNA-targeted oligonucleotide probe to analyze the early steps in the pathogenesis of American foulbrood. The following chain of events could be demonstrated: (i) The spores germinate in the midgut lumen, (ii) the vegetative bacteria massively proliferate within the midgut before (iii) they start to locally breach the epithelium and invade the haemocoel. Our results implicated that successful colonization of the gut may be one of the key factors in AFB pathogenesis. The paracellular route was shown to be the main mechanism for invasion contrasting earlier hypotheses of phagocytosis of Paenibacillus larvae. Invasion coincided with the death of the host implicating that the penetration of the midgut epithelium is a critical step determining the time of death.