Preparation of antigens a Salmonella typhi is used to prepare S. typhi O and S. typhi H antigens aO antigens for S. paratyphi a and S paratyphi b are not taken as they cross-react with Styphi O antigen a H antigen suspension is prepared by treating overnight broth culture or saline suspension of Salmonella with 0. 1% formalin a For preparing antigen suspension, Salmonella are grown on phenol agar (1: 800)to inhibit flagella. The growth is then emulsified in small volume of saline. mixed with 20 times its volume of alcohol heated at 40C 50C for 30 minutes and centrifuged a The antigens are treated with chloroform (preservative) and appropriate dyes are added for easy identification of antigens Widal test7  Salmonella typhi is used to prepare S. typhi O and S. typhi H antigens.  O antigens for S. paratyphi A and S.paratyphi B are not taken as they cross-react with S.typhi O antigen.  H antigen suspension is prepared by treating overnight broth culture or saline suspension of Salmonella with 0.1% formalin.  For preparing O antigen suspension, Salmonella are grown on phenol agar (1:800) to inhibit flagella. The growth is then emulsified in small volume of saline, mixed with 20 times its volume of alcohol, heated at 40 ℃ 50 ℃ for 30 minutes and centrifuged  The antigens are treated with chloroform (preservative) and appropriate dyes are added for easy identification of antigens. Preparation of antigens Widal test