04 March 2026
Salmonella Infantis has emerged as a significant public health threat, ranking among the top serovars linked to foodborne outbreaks in the United States. Recently designated as a "recurring, emerging, and persistent" (REP) strain by the CDC, this pathogen is notorious for its rapid global spread and increasing resistance to critical antibiotics.
A new study published in BMC Genomics analyzed nine S. Infantis strains isolated from the U.S. poultry production line in 2022, including chicken carcass and comminuted meat samples. Using whole-genome sequencing (WGS) and phenotypic testing, they uncovered several alarming trends: Most isolates were phenotypically resistant to five or more antibiotics, with some strains showing resistance to as many as 16 different antimicrobials. Every strain carried a massive IncFIB mega-plasmid (ranging from 289 to 327 kb). These plasmids, identified as pESI-like, act as the primary hubs for antibiotic resistance genes (ARGs). All strains belonged to sequence type 32 (ST32), a major global lineage of S. Infantis.
The researchers concluded that the evolution of S. Infantis along the poultry production chain is not a random process but is systematically driven by mobile genetic elements (MGEs). These elements facilitate the horizontal transfer and reorganization of resistance genes, allowing the bacteria to adapt rapidly to the selective pressures of the agricultural environment. The pESI-like mega-plasmids are the "primary factors" contributing to the unique genomic features and fitness of these emergent clones.
