We report an outbreak among swine and a farmer caused by zoonotic B. bronchiseptica, and uncovered the evolutionary path during cross-species infections. Genome sequencing revealed identical chromosomal sequences in both the human and swine strains, with the swine isolate possessing an additional plasmid absent in the human variant. The entire plasmid gene was integrated into the chromosome via site-specific recombination. The human variant, devoid of the plasmid, exhibited heightened virulence, growth rate, temperature stress, and biofilm formation ability compared to its porcine counterpart.