Isolation of Vi antigen and a simple method for its measurement

Vi antigen was purified from saline extract of Citrobacter strain 5396/38 by enzymatic digestion, concentration with ethanol, and precipitation with hexadecyltrimethylammonium bromide. The Vi antigen prepared by this method was 18 and 230 times more protective in mice than two other preparations isolated by an earlier method utilizing acid hydrolysis. A sensitive and specific method of electroimmunodiffusion was described for measurement of Vi antigen.