Comparative studies on Salmonella typhi grown in vivo and in vitro III. The immunizing potencies of acetone-killed vaccines prepared from in vivo and in vitro grown bacteria and the immunizing potency of substances isolated from infected organs

1. Salmonella typhi, strain Ty2, grown in vivo and employed as acetone-dried vaccine possessed a higher immunizing potency than the descendants of the same parent strain grown in vitro and employed as vaccine. 2. When 2 × 108in vitro-grown bacteria were employed as challenge, the immunizing effects of both types of vaccine were more marked than after administration of 2 × 108in vivo-grown bacteria as challenge. 3. The higher potency of the in vivo-grown vaccine was apparent in all experiments, whether the challenge strain was grown in vivo or in vitro. 4. Immunogenic substances were isolated from infected organs of mice and guinea-pigs, and an immunogenic substance from the peritoneal fluid of the infected guinea-pigs was concentrated by precipitation with ethanol.