Molecular and phenotypic characterization of SG9R-like Salmonella Gallinarum field isolates from South Korea, 2019–2025

Salmonella enterica serovar Gallinarum (SG) is a well-known bacterial pathogen in poultry as the causative agent of fowl typhoid (FT) fever. SG9R live vaccines has been widely used to control outbreaks of SG in South Korea for over 20 years. Recently, molecular analysis of SG9R field strains isolated between 2019 ~ 2025 from South Korea have revealed several mutations in key genes ( aceE, rfaJ ) associated with SG-associated markers. Despite being designated as SG9R in differential multiplex PCR assays, these unique isolates harbored mutations in the molecular markers of aceE and rfaJ genes which resemble those of SG strains. We also discovered these isolates resemble the phenotypic characteristics of the SG strains. Our study emphasizes the importance of monitoring and developing a new diagnostic approach to differentiate SG, SG9R and these unique strains (SG9R-like) thereby improving disease control strategies and minimizing economic losses in the poultry industry.