Isolation of Vi Antigen from Escherichia coli by a New Method.

Vi antigen was isolated from a trichloroacetic acid-deproteinized saline extract of acetone-killed and -dried cells of Escherichia coli (5396/38) by ethyl alcohol precipitation and absorption on diethylaminoethyl-Sephadex. Stepwise elution with distilled water, 0.15 m NaCl, and 0.5 m NaCl resulted in recovery of Vi antigen and two additional antigenic fractions (designated antigens 1 and 2). Vi antigen was an acidic polymer which flocculated readily with albumin, contained 6.0% nitrogen, and had a sedimentation coefficient of 14.7S. Vi was shown to be antigenic in the rabbit by the indirect hemagglutination test and in mice by tests for protection against challenge with Salmonella typhosa, wherein it was more protective by the intraperitoneal (ip) than by the subcutaneous (sc) route. Vi antigen prepared by this method was much more protective for mice than was the Vi antigen used in previous studies. The other antigens, not fully characterized, were also capable of protecting mice. Antigen 1 was more effective by the sc than the ip route, whereas antigen 2 was more effective by the ip route. The relationship between Vi antigen and antigens 1 and 2, as demonstrated by hemagglutination tests with rabbit antisera, remains unclear.