Capsular polysaccharide determines the serotyping of Riemerella anatipestifer

Riemerella anatipestifer (R. anatipestifer) is a Gram-negative pathogen that causes significant economic losses to the farming industry. The polysaccharides that determine serotype are well understood in other bacteria; they have not yet been fully characterized in R. anatipestifer. In this study, we unexpectedly discovered a strain, RCAD0392, that is capable of cross-agglutination. Whole genome sequencing revealed base mutations disrupting a gene on its capsular polysaccharide synthesis gene cluster. By constructing homologous gene deletion mutants in CH-2, we demonstrated that deletion of this gene leads to altered serological phenotypes. India ink and transmission electron microscopy experiments revealed the disappearance of capsule on the surface of the bacteria, indicating the association of the gene with capsule synthesis. In addition, agar-gel precipitin tests showed that lipopolysaccharide binds to antisera of multiple serotypes, while the capsular polysaccharide only binds to the corresponding antisera. This suggests that capsular polysaccharide is the specific antigen that determines the serotype of R. anatipestifer.