Vaccination against Listeria infection in mice with a mutant strain of reduced virulence.

Killed Listeria monocytogenes (L.m.) or living Listeria innocua, being unable to afford noticeable immunity against L.m. challenge in mice, the efficacy of a living strain of L.m. of reduced virulence was studied. This strain was obtained from a field strain of low virulence by successive mutations for streptomycin dependence, reversion to independence and resistance to erythromycin. Its protective immunity was checked in mice against IV, SC and intra-gastric challenges. Vaccinated mice were protected against a IV challenge, lethal for controls, and they were rid of infection in a much shorter time than controls after SC or intra-gastric challenges. They were also protected against abortion when the challenge was done by SC, but not by IV route. The level of immunity was shown to be dose-dependent. Appearing very early after vaccination, immunity remained stable for at least four months, then decreased slowly afterwards, but could be restored and even enhanced by a recall. The protection was equally effective against L.m. serovars 1 or 4 challenges.