Honeybee (Apis mellifera L.) workers act as passive vectors of Paenibacillus larvae spores, which cause the quarantine disease American foulbrood (AFB). We assessed the relative proportions of P. larvae within the honeybee microbiome using metabarcoding analysis of the 16 S rRNA gene. The microbiome was analyzed in workers outside of the AFB zone (control - AFB0), in workers from asymptomatic colonies in an AFB apiary (AFB1), and in workers from colonies exhibiting clinical AFB symptoms (AFB2). The microbiome was processed for the entire community and for a cut-off microbiome comprising pathogenic/environmental bacteria following the removal of core bacterial sequences; varroosis levels were considered in the statistical analysis. No correlation was observed between AFB status and varroosis level, but AFB influenced the worker bee bacterial community, primarily the pathogenic/environmental bacteria. There was no significant difference in the relative abundance of P. larvae between the AFB1 and AFB0 colonies, but we did observe a 9-fold increase in P. larvae abundance in AFB2 relative to the abundance in AFB1. The relative sequence numbers of Citrobacter freundii and Hafnia alvei were higher in AFB2 and AFB1 than in AFB0, whereas Enterococcus faecalis, Klebsiella oxytoca, Spiroplasma melliferum and Morganella morganii were more abundant in AFB0 and AFB1 than in AFB2.

Bee Research Institute at Dol Maslovice Dol 94 Libcice nad Vltavou CZ 252 66 Czechia

Bee Research Institute at Dol Maslovice Dol 94 Libcice nad Vltavou CZ 252 66 Czechia Department of Zoology and Fisheries Faculty of Agrobiology Food and Natural Resources Czech University of Life Sciences Prague Prague 6 Suchdol Czechia

Crop Research Institute Drnovska 507 73 Prague 6 Ruzyne CZ 161 06 Czechia

Crop Research Institute Drnovska 507 73 Prague 6 Ruzyne CZ 161 06 Czechia Czech Hydrometeorological Institute Na Sabatce 2050 17 Prague 412 CZ 143 06 Czechia

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