Capsular polysaccharide determines the serotyping of Riemerella anatipestifer.

Abstract

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. IMPORTANCE Riemerella anatipestifer (R. anatipestifer) is one of the most important veterinary pathogens with at least 21 serotypes. However, the exact polysaccharide(s) that determine R. anatipestifer serotype is still unknown. This study has provided a preliminary exploration of the relationship between capsular polysaccharides and serotyping in R. anatipestifer and suggests possible directions for further investigation of the genetic basis of serotypes in this bacterium.