The antibiotic resistance is natural in bacteria and predates the human use of antibiotics.Numerous antibiotic resistance genes (ARGs) have been discovered to confer resistance to a wide range of antibiotics.The ARGs in natural environments are highly integrated and tightly regulated in specific bacterial metabolic networks.However,the antibiotic selection pressure conferred by the use of antibiotics in both human medicine and agriculture practice leads to a significant increase of antibiotic resistance and a steady accumulation of ARGs in bacteria.In this review,we summarized,with an emphasis on an ecological point of view,the important research progress regarding the collective ARGs (antibiotic resistome) in bacterial communities of natural environments,human and animals,i.e.,in the one health settings.We propose that the resistance gene flow in nature is "from the natural environments" and "to the natural environments";human and animals,as intermediate recipients and disseminators,contribute greatly to such a resistance gene "circulation.".

Emergence of the colistin resistance gene,mcr-1,has attracted worldwide attention.Despite the prevalence of mcr-1-positive Escherichia coli(MCRPEC)strains in human carriage showing a significant decrease between 2016 and 2019,genetic differences in MCRPEC strains remain largely unknown.We therefore conducted a comparative genomic study on MCRPEC strains from fecal samples of healthy human subjects in 2016 and 2019.We identified three major differences in MCRPEC strains between these two time points.First,the insertion sequenceISApll1 was often deleted and the percentage of mcr-1-carrying IncI2 plasmids was increased in MCRPEC strains in 2019.Second,the antibiotic resistance genes(ARGs),aac(3)-Ⅳa and blaCTX-M-1,emerged and coexisted with mcr-1 in 2019.Third,MCRPEC strains in 2019 contained more viru-lence genes,resulting in an increased proportion of extraintestinal pathogenic E.coli(ExPEC)strains(36.1％)in MCRPEC strains in 2019 compared to that in 2016(10.5％),implying that these strains could occupy intestinal ecological niches by competing with other commensal bacteria.Our results suggest that despite the significant reduction in the prevalence of MCRPEC strains in humans from 2016 to 2019,MCRPEC exhibits increased resistance to other clinically important ARGs and contains more virulence genes,which may pose a potential public health threat.