Resistance to broad-spectrum cephalosporins (BSCs) in Enterobacteriaceae in companion animals has become a great concern for public health. To estimate the dissemination of BSC-resistant bacteria between dog and human, we examined the BSC-resistance determinants of and genetic similarities between 69 BSC-resistant Escherichia coli isolates derived from canine rectal swabs (n = 28) and human clinical samples (n = 41). Some E. coli isolates possessed bla(TEM-1b) (14 canine and 16 human isolates), bla(CTx-M-2) (6 human isolates), bla(CTx-M-14) (3 canine and 14 human isolates), bla(CTx-M-27) (1 canine and 15 human isolates), and bla(CMY-2) (11 canine and 3 human isolates). The possession of CTX-M-type beta-lactamases was significantly more frequent in human isolates, whereas CMY-2 was more common in canine isolates. Bacterial typing methods (phylogenetic typing, O-antigen serotyping, and pulsed-field gel electrophoresis) showed little clonal relationship between canine isolates and human isolates. Plasmid analysis and Southern blotting indicated that the plasmids encoding CMY-2 were similar among canine and human isolates. Based on the differences in the major beta-lactamase and the divergence of bacterial types between canine and human isolates, it seems that clonal dissemination of BSC-resistant E. coli between canines and humans is limited. The similarity of the CMY-2-encoding plasmid suggests that plasmid-mediated beta-lactamase gene transmission plays a role in interspecies diffusion of BSC-resistant E. coli between dog and human. (C) 2013, Japanese Society of Chemotherapy and The Japanese Association for Infectious Diseases. Published by Elsevier Ltd. All rights reserved.