Abstract: Listeria monocytogenes (LM) was a zoonotic foodborne pathogen that could cause severe symptoms of infection in human and animals.Measuring LD₅₀ of mice models which had immune activities was widely used for evaluation of pathogenicity of LM strains. In order to create a ICR mice model infected by LM, the N21 LM strains of the 1/2a serotype were inoculated orally into 6-week-old ICR mice groups (each group containing 10 mice) with the dose of perfusion 10⁶,10⁷,10⁸,10⁹ and 10¹⁰ CFU per mouse, respectively, the other 10 mice were inoculated by PBS as a control group to measure LD₅₀ of LM infecting ICR mice. Another 40 mice were divided into two groups by gender, and inoculated with the dose of calculated LD₅₀ of LM.Clinical symptoms, changes of histopathological, changes of body weight and tissue bacterial load were evaluated in each group, respectively.The results found that the LD₅₀ of N21 LM was 10^9.25 CFU. The cycle of infection was about 10 days, the infected mice showed the clinical symptoms of rough coat, listlessness, penis hanging out, weight loss,etc.The histopathology analysis suggested that liver had appeared bacterial clumps in parenchyma, thrombosis and necrosis in sequence; spleen mainly showed white intramedullary lymphopenia decreased;lung mainly showed fiber simply pneumonia. Colony counting and Real-time PCR found that the highest bacterial load was in the liver, spleen followed. The experimental results showed that the ICR mice could be utilized as a LM infection model. This study laid the foundation for research on LM pathogenic mechanisms, vaccine development and evaluation of antimicrobial peptides transgenic mouse's resistance to Listeria monocytogenes.

Key words: Listeria monocytogenes (LM); infection; ICR mice models