Spleens were aseptically isolated from mice previously infected with the different Listeria strains as mentioned above and examined for morphological alterations. Tissues were fixed in 10% neutral buffered formalin, embedded in paraffin, sectioned, and 5 mm sections were stained with hematoxylin and eosin (HE). Pathoplogical foci in spleen sections were then microscopically examined (Keyence).