Fate of Pasteurella hemolytica in conventionally raised and germfree mice.

When Pasteurella hemolytica was introduced into conventionally raised ICR mice by a variety of routes (intraperitoneal, aerogenic, and oral), the inoculum was rapidly eliminated, and none of the mice died. Even when the inoculum was injected intraperitoneally into sublethally irradiated (600 rads) mice, the organisms were eliminated rapidly unless suspended in 10% hog gastric mucin. When germfree ICR mice were orally infected with P. hemolytica, the infection established itself in the intestinal tract and spread to the mesenteric lymph nodes but did not progress beyond this point. Despite the inability of P. hemolytica to establish itself systemically, the organism multiplied freely in mouse blood and a homogenate of normal mouse lung in vitro. Normal mouse peritoneal macrophages could phagocytose P. hemolytica in vitro, although not as efficiently as the control Listeria monocytogenes suspensions. The addition of hyperimmune bovine serum (opsonin) to the P. hemolytica suspension increased phagocytosis but did not greatly affect the subsequent bactericidal activity of the macrophages in vitro. The reason for the lack of pathogenicity shown by P. hemolytica in normal mice remains enigmatic.