Pathogenicity test for Listeria monocytogenes using immunocompromised mice

The lethality of Listeria isolates was determined with normal adult mice and mice that were immunocompromised by treatment with 20 mg of carrageenan per kg. The mean 50% lethal doses (LD50s) of the pathogenic isolates were significantly lower (alpha = 0.05) in the immunocompromised mice than in the untreated mice, with an average reduction of 5.8 log10 units. In contrast, the mean LD50s of the nonpathogenic isolates were lower in the immunocompromised mice by an average of only 0.4 log10 unit, a difference that was not significant (alpha = 0.05). When immunocompromised mice were used, the LD50s of pathogenic Listeria monocytogenes isolates were lower than those of nonpathogenic L. innocua and L. seeligeri isolates by greater than or equal to 6 log10 units and lower than those of nonpathogenic L. ivanovii isolates by greater than or equal to 4 log10 units. Pathogenic L. monocytogenes isolates could be distinguished from nonpathogenic isolates by their ability to cause deaths in immunocompromised mice in 3 days at a dose of approximately 10(4) CFU per mouse. An alternative procedure using iron-overloaded mice failed to effectively differentiate pathogenic Listeria isolates.