Vi antigen is present in all except-

Vi antigen is present in all except S.paratyphi A. Vi Antigen: It is the surface polysaccharide envelope or capsular antigen covering the O antigen. Expressed in :- S.typhi, S. paratyphi C, S. dublin and some strains of Citrobacter freundii. When Vi antigen is present, it renders the bacilli inagglutinable with the O antiserum. Poorly immunogenic and antibody titres are low, not helpful in diagnosis , hence not employed in the Widal test. Complete absence of the Vi antibody in a proven case of typhoid fever- Poor prognosis. Note: Vi Antibodies are not seen in normal population.