Salmonella enterica subsp. enterica serovar Infantis is the most prevalent serovar found in broilers and broiler meat and is among the top five serovars responsible for human infections in Europe. In 2008, a multidrug-resistant S. Infantis isolate emerged in Israel with a mosaic megaplasmid named pESI, associated with increased virulence, biofilm formation, and multidrug resistance. Since then, S. Infantis clones with pESI-like plasmids have been reported worldwide, replacing pESI-free clones. Here, we typed 161 S. Infantis isolates of poultry (n = 133) and human clinical (n = 28) origin using whole-genome sequencing. The isolates were collected between 2007 and 2021. In addition, we performed PacBio/Illumina sequencing for two representative pESI-like plasmids and compared them with publicly available sequences. All isolates belonged to sequence type 32 (ST32), except for one isolate that represented a novel single-locus variant of ST32. Core genome MLST (cgMLST) analysis revealed 14 clusters of genetically closely related isolates, of which four suggested broiler-to-human transmission of S. Infantis. pESI-like plasmids were present in 148/161 (91.9%) isolates; all were highly similar to the publicly available pESI-like sequences but lacked extended-spectrum beta-lactamase (ESBL) genes. PacBio/Illumina hybrid assembly allowed the reconstruction of two novel complete pESI variants. The present study revealed that the multidrug-resistant, pESI-positive S. Infantis clone became the predominant S. Infantis clone in Slovenian broilers and humans during the last decade. Continued surveillance of resistant S. Infantis clones along the food chain is needed to guide public health efforts. IMPORTANCE Salmonella Infantis clones with pESI-like plasmids harboring several virulence and resistance genes have been reported worldwide. In the present study, we compared the population structure of 161 Salmonella Infantis isolates obtained from humans and broilers in Slovenia from 2007 to 2021. Whole-genome sequencing showed that most human isolates clustered apart from broiler isolates, suggesting an alternative source of infection. Most isolates were multidrug resistant due to the presence of pESI-like plasmids, of which two variants (pS89 and pS19) were fully reconstructed using long-read sequencing. Both exhibited high similarity with the original Israeli pESI plasmid and German p2747 plasmid. The prototype plasmid pS89 harbored the typical pESI-associated resistance genes aadA1, qacEΔ1, sul1, and tet(A), which were absent in the truncated plasmid pS19.
Keywords: ST32; Salmonella Infantis; broilers; humans; multidrug resistance; pESI.