Differential impact of Paenibacillus infection on the microbiota of Varroa destructor and Apis mellifera

The Western honey bee (<i>Apis mellifera</i>) is a vital agricultural pollinator whose populations are threatened by the parasitic mite <i>Varroa</i> destructor and associated pathogens. While the impact of <i>Paenibacillus</i> species on honey bees, particularly <i>Paenibacillus</i> larvae causing American foulbrood, is documented, their effect on the microbiota of <i>Varroa</i> mites remains unclear. This study aimed to investigate the influence of <i>Paenibacillus</i> sp. on the bacterial communities of <i>Varroa</i> mites and adult honey bees. We hypothesized that <i>Paenibacillus</i> sp. would significantly alter the microbiota of <i>Varroa</i> mites but have minimal effect on that of adult honey bees. Utilizing 16S rRNA sequencing data from a previous study, we reanalyzed samples categorized into four groups based on <i>Paenibacillus</i> sp. infection load: highly infected and lowly infected honey bees (<i>A. mellifera</i>) and mites (<i>V. destructor</i>). Infection status was determined by <i>Paenibacillus</i> sp. read counts, with more than three reads indicating high infection. Microbial diversity was assessed using alpha and beta diversity metrics. Co-occurrence networks were constructed to visualize bacterial community assemblies, and network robustness was evaluated through node addition and removal tests. Keystone taxa were identified based on eigenvector centrality and relative abundance. Highly infected <i>Varroa</i> mites exhibited a significant reduction in alpha diversity and a markedly different bacterial community composition compared to lowly infected mites (<i>p</i> < 0.05). Their bacterial co-occurrence networks showed decreased connectivity and robustness, indicating a disruptive effect of <i>Paenibacillus</i> sp. In contrast, adult honey bees displayed no significant differences in alpha diversity or network structure between highly and lowly infected groups (<i>p</i> > 0.05), suggesting a resilient microbiota. Keystone taxa analysis revealed fewer central species in highly infected <i>Varroa</i> mites, potentially impacting network stability. High <i>Paenibacillus</i> sp. infection is associated with significant alterations in the microbiota of <i>Varroa</i> mites, disrupting bacterial communities and potentially affecting mite physiology. The microbiota of adult honey bees appears more robust against <i>Paenibacillus</i> sp. influence. These findings enhance our understanding of the complex interactions within the "honey bee-mite-microorganism" system and may inform future strategies for managing <i>Varroa</i> mite infestations and associated pathogens.