Acquired resistance of BCG-vaccinated red mice to infection with Listeria monocytogenes.

Infection experiments have shown that red mice, belonging to the vole family, could be infected with Listeria monocytogenes and that they were almost as susceptible to intravenous infection as Cf1 mice. Vaccination of red mice with BCG induced a resistance which could be demonstrated by prologation of the survival time of the animals after challenge with Listeria. The resistance was greatest in the second and third week after vaccination and was considerably higher after intravenous injection of BCG vaccine than after intraperitoneal or subcutaneous injection. On the basis of studies concerning the dose of vaccine, the route of vaccination, and the interval between vaccination and challenge, a method by which to evaluate the potency of a strain of BCG has been elaborated. By this method it was found that there was a significant difference between the resistance produced by a strain of BCG which is weakly virulent and one which is strongly virulent for hamsters.