LAUSR

Abstract Salmonella enterica serovar Typhimurium (S. Typhimurium) has two serological variants: one that expresses the O:5 antigen (1,4,5,12:i:1,2) and one that lacks O:5 antigen (1,4,12:i:1,2). For serotyping, S. Typhimurium is agglutinated by diagnostic O:4 antigen serum. This study was carried out to compare the antigen‐antibody affinity of O:4 antigen in S. Typhimurium χ3306 O:5‐positive and S. Typhimurium χ3306 O:5‐negative strains. The affinity of O:4 antigen with O:4 antigen serum was found to be stronger in the O:5‐negative strains compared to O:5‐positive strains. Next, we investigated the antigen‐antibody affinity of O:4 antigen with O:4 antigen serum in field strains of S. Typhimurium, which showed the same tendency in affinity as seen with S. Typhimurium χ3306 O:5‐positive and negative strains. This study suggests that the presence or absence of O:5 antigen causes differences in O:4 agglutination reactions with different field strains of S. Typhimurium.