Emergent Salmonella enterica serovar Infantis forms a monophyletic lineage shaped by geographic structuring.

Multidrug-resistant Salmonella Infantis carrying pESI-like megaplasmids have disseminated worldwide representing a serious threat to public health. Previous studies have investigated its population structure and temporal dynamics above the continental level. However, their conclusions were constrained by limited datasets and sampling biases. To address these issues, we analyzed all publicly available Salmonella Infantis genomes to characterize its global population structure and phylogeographic dispersal. We selected a non-redundant dataset of 14,010 genomes representing the temporal, geographic, isolation source, and genomic diversity of Salmonella Infantis from 77 countries across five continents, collected from 1910 to 2024. Phylogenomic analyses showed that emergent megaplasmid-positive Salmonella Infantis forms a monophyletic lineage with significant geographic structuring. The megaplasmid-positive lineage was inferred to have originated in Western Asia around 1990, followed by multiple introductions into Europe and a single transmission to South America which resulted in the dissemination of this pathogen to Northern America, and from there to the rest of the continent. Multiple recent transmission events of the American lineage to all continents were observed, driving the dispersal of the blaCTX-M-65 gene encoding extended-spectrum β-lactamases. Moreover, genomic evidence also suggests that the emergence of ESBL-producing strains in parts of Asia and Africa may be associated with poultry trading from the Americas. Our findings underscore the urgent need for integrating global human, animal, and environmental surveillance data with population genomic analyses to contain the threats posed by ESBL-producing Salmonella Infantis.