Overview of the microbiome and resistome of swine manure in commercial piglet farms and its application in grazing soils.

The environmental spread of antimicrobial resistance genes (ARGs) through the use of animal manure in agriculture has become a significant concern. This study investigated the impact of applying swine manure treated through biodigestion on the spread of ARGs in agricultural soils in the Midwest region of Brazil. Samples of untreated and treated manure, fertilized soil, and unfertilized soil were collected from three piglet production units. Bacterial communities and ARGs were characterized through metagenomic sequencing and bioinformatics. Bacterial profiles in fertilized and unfertilized soils were highly similar across all farms. In contrast, biodigestion reduced the total number of ARGs in treated manure. Of the 399 ARGs detected in fertilized soils, 67% were also found in unfertilized soils, and 12% were shared exclusively with treated manure. The presence of numerous ARGs in unfertilized soils highlights the role of environmental dissemination routes, such as runoff, dust, or wildlife, in shaping soil resistomes even in areas without manure application. These findings suggest a stable bacterial and resistome profile in soils, regardless of manure application. Although antimicrobial residues were not evaluated, the results reinforce the need for responsible antibiotic use and effective manure management to minimize environmental ARG dissemination.