Prevalence of antibiotic resistance in nitrogen-fixing Bradyrhizobium communities from South Korea

Abstract

Antimicrobial resistance is a critical global threat that interconnects human, animal, and agricultural health under the One Health paradigm. While agricultural soils are recognized as major reservoirs of antibiotic resistance genes, the resistance profiles of key soil microorganisms, such as Bradyrhizobium, remain insufficiently characterized. This study examined the antibiotic resistance patterns of 152 indigenous Bradyrhizobium strains isolated from soybean nodules across South Korea. Phenotypic antimicrobial susceptibility testing, combined with PCR-based screening for selected antibiotic resistance genes, provided a representative overview of the resistance landscape in soybean fields. The findings revealed an alarmingly high nationwide prevalence of resistance to chloramphenicol, gentamicin, erythromycin, and vancomycin, across both species and regions. Notably, B. elkanii displayed a distinct resistance profile, characterized by exceptionally high resistance to cephalexin and rifampicin, suggesting the presence of unique intrinsic mechanisms or novel resistance gene variants in this species. B. diazoefficiens, the most predominant species, exhibited apparent geographical heterogeneity of phenotypic resistance patterns and arr gene prevalence, which may reflect localized selection pressures and limited gene flow between regional populations. These findings highlight the urgent need for systematic resistome mapping to monitor the emergence and dissemination of antibiotic resistance and to establish more stringent regulatory frameworks for antibiotic use across the agricultural and public health sectors.