Twenty years of Salmonella Infantis emergencein an export-oriented broiler food system: serovar shift and multidrug resistance in Ukraine (2006–2025)

Salmonella enterica serovar Infantis has emerged globally as a high-risk foodborne pathogen associated with broiler production and MDR, posing increasing challenges to food safety and public health. We conducted a 20-year longitudinal surveillance of Salmonella spp. in the Ukrainian broiler production system (2006–2025), analysing 1.25 million poultry meat and product samples and 1.70 million biological and pathological poultry samples. Temporal serovar dynamics were assessed, with focused phenotypic and molecular characterisation of 58 recent Salmonella Infantis isolates recovered in 2024–2025. Over two decades, a profound shift in the Salmonella serovar landscape was observed, marked by the progressive replacement of historically dominant serovars ( Salmonella Enteritidis, Salmonella Typhimurium, Salmonella Pullorum/Gallinarum) by Salmonella Infantis. In poultry meat and products, Salmonella Infantis increased from 1.9% to 66.7% of serotyped isolates, while in biological and pathological poultry samples it rose from absence or sporadic detection to 62.7%. Nearly all recent Salmonella Infantis isolates were multidrug resistant (98.3%), exhibiting resistance to multiple antimicrobial classes. Despite this MDR phenotype, all isolates were negative for the IncFIB proxy marker used for pESI-like megaplasmid screening and for the screened major ESBL genes. The resistance gene profile was dominated by tetA and sul1 , with substantial phenotypic heterogeneity. Our findings reveal the emergence and dominance of MDR Salmonella Infantis across the Ukrainian broiler production system, although the limited scope of molecular screening did not allow full characterisation of the underlying resistance mechanisms. Given Ukraine's role in international poultry trade, these MDR isolates represent a One Health–relevant risk at the interface of animal health, food safety, and public health, underscoring the need for integrated surveillance supported by whole-genome sequencing.