Molecular Characteristics of Epidemiologically Successful Salmonella Enteritidis in Poland

Since 2014, the long-term decreasing trend in human salmonellosis, caused mainly by the consumption of Salmonella-contaminated poultry products, has stagnated in the European Union (EU). As Poland has been the leading poultry meat producer in the EU since 2014, we analysed whole genome sequences of 275 Salmonella (S.) Enteritidis strains from the poultry food production chain (n = 216) and humans (n = 59) (2008-2019) to shed light on the genetic content and relatedness of the S. Enteritidis population in Poland. Most (99.3%) of the strains belonged to ST11. Overall, 5.8% of strains possessed at least one antimicrobial resistance gene (ARG), the most common being qnrB19 (n = 9). Mutations in quinolone resistance-determining regions (QRDRs) were observed in 46.9% of strains, and the most common mutation was gyrA (S83Y; n = 95). In 95.6% of strains, at least one plasmid replicon was detected, with the highest prevalence of IncFII(S)_1 (n = 263) and IncFIB(S)_1 (n = 262). The composition of Salmonella pathogenicity islands (SPIs) was uniform among 96.7% of strains carrying CS54, SPI-1-SPI-3, SPI-5, SPI-6, SPI-9, SPI-12 and SPI-14. Core genome multilocus sequence typing (cgMLST) analysis revealed no apparent clustering based on source or year of isolation. None of the genetic determinants studied here seemed to trigger changes in Salmonella epidemiology. However, other factors, such as improvements in reporting and control, could influence infection trends and are, therefore, worth further elucidation.