Immunity to enteric infection in mice.

Specific pathogen-free CD-1 mice infected orally with sublethal doses (10(4) to 10(6) viable organisms) of Salmonella enteritidis rapidly developed extensive bacterial populations in the liver, spleen, and mesenteric lymph nodes. Although the pathogen did not multiply extensively in the gut, the infection persisted in the intestine at between 10(4) and 10(5) viable organisms throughout the experiment. S. gallinarum was less invasive than S. enteritidis when given by mouth; S. pullorum failed to survive in the intestine or to invade the tissues of orally infected mice. Vaccination with a sublethal dose of living S. enteritidis, either orally or intravenously, completely prevented the establishment of liver and spleen populations of a drug-resistant, virulent strain of S. enteritidis. Vaccination with an ethyl alcohol-killed vaccine given by various routes delayed the spread of the orally introduced challenge population to the liver and spleen by 1 to 2 days but was unable to prevent the subsequent growth of the pathogen in vivo, although the vaccinated mice survived the infection. The importance of these findings in relation to vaccination against typhoid fever in man is discussed.