Passive Immunity Experiments in Mouse Typhoid *

Experiments involving active immunization have shown' that infection with a non-virulent B. aertrycke produces in mice a high degree of immunity to infection with virulent B. enteritidis. It also appeared that a non-virulent enteritidis organism conferred a cross protection to infection with B. aertrycke. Furthermore, in the immunities so disclosed the protection failed to bear any relationship to the somatic agglutinin present. Thus, if a mild infection were a necessary factor in the establishment of the cross protection, and if the usual antibody directed against the bacterial protoplasm were of no consequence, the question might well be raised as to whether or not the two enteric organisms may not possess some substancepossibly of the nature of a toxin-in common, and thus the immunities exhibited, if humoral at' all, might be merely the expressions of a single antitoxic immunity. Tests of passive protection offered an approach to this problem of the nature of crossed immunity to these two infections, although such earlier work as had been reported offered but scant encouragement. Raistrick and Topley2 state that an immune serum developed to a fraction of B. aertrycke merely served to prolong the life of their animals infected with the homologous organism. For the preparation of the immune sera 200 mice, 100 in each of two groups, were used, the procedure with each group being as follows: Group A: The mice were given three feedings of living nonvirulent B. enteritidis 904 in 0.05 cc. amounts of an 18-hour broth culture. The feedings were given 52, 45, and 15 days before the animals were bled to death from the heart. Serum was collected, 29 cc. being obtained. This immunizing procedure leads to a high degree of active immunity, protecting at least 96 per cent of the mice so treated against a surely fatal dose of virulent B. enteritidis culture orally administered. Group B: The mice were given three feedings of non-virulent