Analysis of Bacterial Community Changes in Apple Trees Treated with Biological Control Agent, Bacillus altitudinis KPB25, for Controlling Fire Blight

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. We used metagenomic analysis to examine the diversity and similarity of the bacterial communities before and after KPB25 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 significant shift in the community structure. However, the epiphytic community did not exhibit any clustering, and the KPB25 treatment did not result in substantial changes 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 enhanced microbial groups within the residual bacterial community of apple leaves that contributed to fire blight suppression but did not significantly disrupt or alter the pre-existing community. Therefore, KPB25 has the potential to be a biological control agent for the management of fire blight in apple orchards. Its ability to interact with and modify the microbial community without causing significant disturbance further highlights its suitability for biological control.