Oral Immunization in Experimental Salmonellosis III. Behavior of Virulent and Temperature-Sensitive Mutant Strains in the Intestinal Tissues of Rats.

Infection of rats via a Peyer's patch has been used as a means of studying the behavior of Salmonella enteritidis in intestinal tissues. The course of infection in the Peyer's patch and draining mesenteric lymph node is characterized by multiplication of the organism over a period of 4 days followed by a gradual decline in numbers; the organism also passes to the liver and spleen and may be isolated from these organs as well as the intestinal tissues for at least 4 weeks. Temperature-sensitive mutants derived from the virulent strain are unable to multiply and do not pass to the liver and spleen; they remain viable for periods of less than 2 weeks. A quantitative technique based on the number of viable organisms remaining in the injected Peyer's patch 48 hr after infection has been used to assess the immune state of rats. The results have clearly demonstrated that Salmonella immunity can only be induced by living vaccines and that although viable organisms remain in the reticuloendothelial tissues, organisms given in a challenge infection are immediately subject to enhanced bactericidal activity within the intestinal tissues. Under the conditions used here, humoral antibody does not seem to offer any protective effect against Salmonella infection in the intestinal lymphoid tissues.