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.