Age‐Associated Antimicrobial Resistance in Escherichia coli Isolates From Companion Dogs and Cats Linked to CTX‐M Gene Carriage

ABSTRACT

As companion animals live in close contact with humans, the presence of antimicrobial‐resistant organisms in these animals is an important issue in veterinary medicine from a One Health perspective. In this study, we isolated a total of 385 Escherichia coli strains from fecal samples collected from 218 companion dogs and 167 companion cats that visited veterinary clinics in Tokyo, Japan, from 2021 to 2024. Antimicrobial resistance to third‐generation cephalosporins and fluoroquinolones increased significantly with host age, even though fecal abnormalities were low relevance. A total of 60 isolates were resistant to at least one third‐generation cephalosporin, and whole‐genome analysis was performed to further characterize these isolates. The isolates were classified as CTX‐M gene‐carrying or AmpC‐associated isolates, including those carrying acquired AmpC genes and those harboring chromosomal ampC promoter mutations. The age‐associated increase was more pronounced among CTX‐M gene‐carrying isolates than among AmpC‐associated isolates. CTX‐M gene‐carrying isolates frequently exhibited concurrent levofloxacin resistance and were predominantly represented by ST131. Core‐genome multilocus sequence typing revealed that these CTX‐M gene‐carrying isolates were phylogenetically similar to previously reported isolates of both human and animal origin. These findings suggest that the age‐associated increase in resistance is related to the expansion of specific CTX‐M gene‐carrying lineages rather than to a uniform increase across all cephalosporin‐resistance mechanisms. Our results support the need for continuous investigation within a One Health framework and further epidemiological and genomic surveillance.