Analysis of Bacterial Community Changes in Apple Trees Treated with Bacillus altitudinis KPB25, a Potential Biological Control Agent against Fire Blight

The balance of microbial communities in an ecosystem is the most important factor representing its healthy state, even when immigrant microorganisms, such as biological control agent, are introduced into agricultural fields. Thus, this study aimed to investigate the potential of the antagonistic bacterium KPB25 (Bacillus altitudinis) as a biological control agent against fire blight by analyzing the changes in the epiphytic and endophytic bacterial communities of apple tree leaves following treatment. The KPB25 treatment resulted in increased community richness and diversity in endophytic bacteria. Conversely, in epiphytic bacteria, community diversity decreased after treatment. Beta-diversity analysis revealed that the endophytic community formed distinct clusters following KPB25 treatment, indicating a shift in the community structure. Relative abundance analysis of the endophytic and epiphytic communities highlighted that some bacterial families, which increased in abundance following KPB25 treatment, oxidized sugars into organic acids or produced antibiotics, potentially creating an environment that makes it difficult for Erwinia amylovora to survive when attempting to infect its host. These findings suggest that KPB25 interacts with certain microbial taxa within apple trees, contributing to the regulation and alteration of the microbial community in a manner that promotes an environment unfavorable for E. amylovora. Overall, KPB25 may have enhanced certain microbial groups within the endophytic residual bacterial community of apple leaves that contribute to fire blight suppression, with minor structural changes but significant shifts in microbial diversity.