Infection with a sublethal dose of L. monocytogenes induces high-level, long-lasting specific immunity in immunologically normal mice. Thus, the LD50 of immune mice is increased by a factor of 103 to 104 over naive mice. In order to determine the degree of protection afforded by immunization, it is faster and more practical to determine the bacterial number in the spleen and liver of reinfected mice than to determine the LD50. This section describes a protocol to determine the number of bacteria in the spleens and livers of L. monocytogenes-infected mice.
The severity of L. monocytogenes infection can also be monitored in unimmunized mice. In normal mice, L. monocytogenes infection induces a nonspecific inflammatory response involving neutrophils, natural killer cells, and macrophages during the first 24 to 72 hr of infection. This response substantially decreases the infectious burden and is essential for the survival of infected mice. The specific immune response to L. monocytogenes occurs 5 to 7 days after infection and ultimately clears the last bacteria from the spleen and liver of infected mice. Therefore, following infection of mice with sublethal doses of L. monocytogenes, it is possible to assess the competence of innate and specific immunity by quantifying bacteria 48 to 72 hr and 7 to 10 days after infection, respectively.
Additional Materials (also see Basic Protocol) Naive (nonimmunized) mice (optional) 0.05% (v/v) Triton X-100
Brain heart infusion (BHI) agar (Difco) plates, sterile
Stainless steel wire mesh screens 50-mm sterile Petri dishes Dissecting forceps and scissors, sterile 5-ml plastic syringes, sterile 12 x 75-mm polystyrene tubes, sterile Glass spreader, sterile
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