Erysipelothrix rhusiopathiae is an intracellular bacterium that represents the class Erysipelotrichia, a new member of the phylum Firmicutes. E. rhusiopathiae is phylogenetically close to Mollicutes and shows genome reduction (JB, 2011). E. rhusiopathiae expresses an unusual type of peptidoglycan, suggesting that this is an evolutionary characteristic of the organism (JB, 2011; IAI, 2012).
Among the four species of the genus Erysipelothrix, E. rhusiopathiae is the only species that causes disease in swine and poultry and has also been isolated from human patients. E. rhusiopathiae has been differentiated from other Erysipelothrix species by their serovars, determined based on peptidoglycan antigens. Specific E. rhusiopathiae serovars (1a, 1b, and 2) are associated with disease in pigs, poultry, and humans; however, the molecular basis for the association between these serovars and virulence remains unknown.
We examined the 15-kb chromosomal region encompassing a putative pathway for polysaccharide biosynthesis previously identified in E. rhusiopathiae Fujisawa strain (serovar 1a). Transposon mutants possessing a mutation in this region lost serovar 1a-specific antigenicity. Sequence analysis of this region in wild-type strains of serovars 1a, 1b, and 2 and serovar N, which lacks serovar-specific antigens, revealed that gene organization was relatively similar among the strains and serovar N strains possessed certain mutations in this region. In two of the analyzed serovar N strains, restoration of the mutations via complementation with sequences derived from serovar 1a and 2 strains recovered serovar 1a and 2-specific antigenicity, respectively. Induced mutations in this region resulted in altered capsule expression and attenuation of virulence in mice. These results indicate a functional connection between the biosynthetic pathways for the capsular polysaccharide and peptidoglycan antigens used for serotyping (IAI, 2018). A PCR assay targeting the serovar-specific sequences in this region enabled simultaneous detection and differentiation of the serovar 1a, 1b, and 2 strains of E. rhusiopathiae.