The cell wall of Y. pestis contains a lipopolysaccharide that func-tions as an endotoxin and is an important component of the virulence of the bacteria.
Y. pestis is antigenically homologous; hence no serotypes arepresent. The antigenic structure of the bacteria is complex and contains about 20 different antigens. Y. pestis possess following major antigens, which are of importance in the pathogenesis of disease:
V and W antigens: These two antigens are always presenttogether. V antigen is a protein of molecular weight 90 kDa, while W antigen is an acidic 145 kDa lipoprotein. These two proteins are produced by virulent strains of Y. pestis when grown at 37°C in the presence of low concentration of calcium. Production of V and W antigens is mediated by plasmid.
F1 envelope antigen: F1 envelope antigen (fraction I or F–Iantigen) is a heat-labile envelope protein generally present only in virulent strains. The antigen is formed in vivo and in vitro in culture of virulent strains at 37°C. Production of F–I antigen is mediated by plasmid. The antibody to F–I antigen is protective in mice.