Morphology
''P. larvae'' is a rod-shaped bacterium with slightly rounded ends, approximately 2.5–5 μm long and 0.5 μm wide. The spore of ''P. larvae'' is oval, approximately 0.6 μm wide and 1.3 μm long.Classification
In 1906, G.F. White first described the bacterium that caused American foulbrood, and named it ''Bacillus larvae''. In 1950, a bacterium isolated from bee larvae and associated with the rare disease "powdery scale" was named ''Bacillus pulvifaciens'' by Katznelson. In 1993, both ''B. larvae'' and ''B. pulvifaciens'' were transferred to a new genus, ''Paenibacillus''. The two species were combined into a single species: ''Paenibacillus larvae'' in 1996, remaining differentiated as two subspecies: ''P. larvae'' ssp. ''larvae'' (formerly ''Bacillus larvae'') and ''P. larvae'' ssp. ''pulvifaciens'' (formerly ''Bacillus pulvifaciens''). In 2006, the subspecies were eliminated based on spore morphology, biochemical profile and DNA testing, and when it was also demonstrated that experimental infection of honeybee larvae with the ''pulvifaciens'' subspecies caused American foulbrood signs without causing "powdery scale". There are at least four genotypes of ''P. larvae'', named after their enterobacterial repetitive intergenic consensus (ERIC) sequences. Genotype ERIC I corresponds to the former species of ''Bacillus larvae'', and genotypes ERIC II, III and IV correspond to the former species of ''B. pulvifaciens''. A fifth genotype ERIC V has recently been discovered in honey.Epidemiology
''P. larvae'' is found worldwide. Genotypes ERIC I and II are most frequently isolated from global AFB outbreaks. In contrast, genotypes ERIC III and IV are found in bacteriology archives and are considered practically unimportant. ERIC V was found in honey and has not been isolated from infected colonies yet.American foulbrood
''P. larvae'' causes American foulbrood in honeybees.References
{{taxonbar , from= Q18088582 Paenibacillaceae Bee diseases