Unravelling the response of the soil microbiome to macrolactin A: A metagenomic study

The development of environmentally sustainable biopreparations for crop protection requires comprehensive assessment of their microbiome impacts. This study investigates how macrolactin A (McA)-a polyketide antibiotic produced by plant-beneficial Bacillus velezensis-shapes soil microbial communities and antibiotic resistance gene (ARG) profiles under various agricultural scenarios. Using high-throughput metagenomics and network analysis, we compared untreated soils with those exposed to two McA concentrations: a high dose (10 mg/kg soil, representing potential point-source accumulation) and a low dose (1 mg/kg, mimicking natural rhizosphere levels). While overall ARG α- and β-diversity remained stable, we observed significant taxonomic restructuring, with Pseudomonadota increasing by 1.36-2.15 % and Actinomycetota declining by 1.14-1.74 % across treatments. Resistome analysis showed dose-dependent ARG selection: control soils favored target alteration mechanism, whereas McA promoted efflux, inactivation, and protection mechanisms. Network analysis demonstrated disruption of complex ARG-host associations, as control-dominant genera belonging to Actinomycetota (Conexibacter, Baekduia, and Capillimicrobium) maintaining 16-21 ARGs per genome decreased, while genera belonging to Pseudomonadota (Bradyrhizobium, Mesorhizobium, Paraburkholderia, and Piscinibacter) with streamlined resistomes (1-2 ARGs) became prevalent. Functional gene profiling (COGs) and annotation of MAGs revealed dose-dependent restructuring: low-dose McA enriched chemotaxis systems and broad-spectrum efflux pumps, facilitating motile escape and energy-efficient resistance, whereas high-dose exposure selected for secondary metabolite synthesis, metal transporters, and cell wall remodeling genes, indicating defensive countermeasures. These findings demonstrate McA's biphasic selection: low doses favor avoidance strategies (efflux/motility), while high doses enforce biosynthetic defenses and structural resilience. The results support the hypothesis that narrow-spectrum antibiotics act as ecosystem engineers through metabolic trade-offs, highlighting the need to evaluate resistome restructuring in biocontrol agent risk assessments.