Seasonal dynamics of the gut microbiota in Apis mellifera ligustica: a two-year longitudinal study

The honey bee gut microbiota plays a crucial role in host nutrition, immunity, and colony health, yet the relative influence of seasonal and colony-specific factors on its long-term dynamics remains incompletely understood. This study investigated temporal variation in the gut bacterial community of three Apis mellifera ligustica colonies maintained in the same apiary and monitored over two consecutive years (2022-2023). Worker bees were sampled during eight seasonal periods, and gut microbiota composition was characterized using 16S rRNA gene amplicon sequencing. Across all sampling periods, the microbiome was consistently dominated by the characteristic honey bee-associated genera Gilliamella, Snodgrassella, Bartonella, Frischella, Commensalibacter, and Lactobacillus, indicating the persistence of a conserved core bacterial community. Seasonal variation was primarily associated with changes in the relative abundance of dominant taxa rather than with major changes in community composition. In particular, Gilliamella apicola and Snodgrassella alvi exhibited complementary seasonal patterns, with Gilliamella reaching its highest abundance during autumn, particularly in autumn 2023, whereas Snodgrassella predominated during spring and winter. Alpha-diversity metrics (Observed OTUs, Chao1, Shannon, and Simpson indices) showed limited seasonal variation, whereas beta-diversity analyses detected significant differences in community composition among seasons. Principal Coordinates Analysis and PERMANOVA identified season as the factor most strongly associated with microbiome variation, while colony identity did not significantly influence bacterial community composition under the standardized experimental conditions adopted in this study. Overall, these findings show that the gut microbiome of A. mellifera ligustica maintains a conserved core bacterial community while exhibiting reproducible seasonal variation in the relative abundance of its dominant members. This study provides a longitudinal baseline for future investigations aimed at understanding the ecological mechanisms underlying seasonal microbiome dynamics and their relationship with honey bee biology and environmental change.