Differential Copy Number of Chromosomal bla

Extended-spectrum β-lactamase-producing Enterobacterales pose a significant public health threat from a One Health perspective. During an investigation of companion dogs and cats in Tokyo, we isolated seven Escherichia coli sequence type ST38 strains with characteristic genetic structures related to cephalosporin resistance, as identified by genomic analysis. All strains carried one to three copies of blaCTX−M−14 on their chromosomes, with one strain carrying one copy on a plasmid. Among the six strains with only chromosomal copies, a higher copy number correlated with increased resistance to ceftazidime and elevated blaCTX−M−14 expression, whereas the doubling times calculated from growth curves remained unchanged. Continuous passaging in the presence of ceftazidime resulted in increased copy numbers in three of the six strains. Notably, these characteristics closely resembled those reported for E. coli strains isolated from human clinical cases in South Korea. This similarity suggests that the copy number amplification mechanisms in blaCTX−M−14-carrying E. coli are more widespread than previously recognized, extend across international borders, and adapt to diverse ecological niches, including those of companion animals. Given that an increased copy number enhances resistance to antimicrobial pressure, these findings highlight the need for prudent antimicrobial use in veterinary medicine.