High prevalence and persistence of carbapenem and colistin resistance in livestock farm environments in China.

The global dissemination of antibiotic resistance genes (ARGs) and their role as novel contaminants has garnered significant attention. However, the prevalence of ARGs in farm environments is usually underestimated and understudied. Here, colistin resistance gene mcr-1 and extended-spectrum β-lactamase (ESBL) and carbapenemase genes, all of which pose a threat to food safety and public health, were selected to investigate ARG pollution in farm environments. Target gene prevalence ranged from 22.4% to 98.8%, with broiler feedlots showing higher contamination rates than pig and layer feedlots. Moreover, the ARGs were detected in the cooperative broiler feedlot environment from 2016 to 2019, with no significant changes in mcr-1 abundance after the 2017 ban on colistin as a growth promotor. The relative abundance of target genes was also higher in solid manure than in feces and soil. Overall, our results revealed: (1) that broiler farm environments could serve as an important reservoir of mcr-1 and ESBL- and carbapenemase-encoding genes; (2) that fecal treatment processes may affect levels of ARG contamination in the environment; and (3) that residual colistin selects for the persistence of mcr-1 in farm environments. Therefore, further investigation is needed to estimate the impact of the ban on colistin as a growth promoter.