**5.3 The virulence antigen (Vi Ag)**

It is an antigen of the envelope; it was identified in three types of serovar: Typhi, Paratyphi C and Dublin, but all strains of these serotypes do not necessarily have this antigen (Humbert et al., 1998). This antigen is considered a surface antigen (Dumas, 1958), it is distinct from the somatic antigen and the flagellar antigen. The Vi antigen makes germs inagglutinable by antibodies O when it is abundant. It does not develop if the cultures are carried out below 25 ° C and above 40 ° C. Heating at 100 ° C destroys the germs and become agglutinating antibodies by O. It is likely glucidolipidopolypeptidique. In addition to these antigens exists in the genus Salmonella, the protein structures from surface pilis pilis which differentiate into common (occurring in mannose-dependent haemagglutination) and sexual pilis (involved in bacterial conjugation) and whose presence is encoded by plasmids (and Gledel Corbion, 1991).
