Antibody response and protection induced by immunization with smooth and rough strains in experimental salmonellosis

The antibody response of mice to a smooth strain of Salmonella typhimurium was shown previously to be extremely rapid and potent. As measured by the complement-mediated bactericidal reaction, it was also found to be highly specific as well as reproducible. Experiments which studied the effects of antigen type (live or heat-killed), antigen dose, and the route of immunization indicated that the most rapid and highest antibody response was achieved with live, smooth organisms injected by the intraperitoneal route. Living vaccines of rough strains of either S. typhimurium or S. enteritidis induced antibodies directed against the corresponding smooth organisms. The response to the rough strains was apparently due to antibody production rather than to the simple release of preformed natural antibody. The duration of protection conferred by the rough strain vaccines was closely correlated with the endotoxic content of the immunizing strain. Smooth heat-killed vaccines and a rough live vaccine protected against homologous but not heterologous challenge. In contrast, immunization with a smooth live vaccine protected mice against both homologous and heterologous challenge infections. Protection was not due to a local effect in the peritoneal cavity, since mice were also protected against subcutaneous challenge. The secondary antibody response, induced in immunized animals by the virulent challenge infection, was demonstrated to be rapid and potent, and hence a factor to be considered in protection.