Effect of growth temperature on virulence of strains of Listeria monocytogenes in the mouse: evidence for a dose dependence.

Growth of Listeria monocytogenes at 4 degrees C significantly increased its virulence for mice by the intravenous route and the effect was dose-dependent. Virulence was apparent only at a dose of about or above 10(4) viable listerias. At slightly lower doses of about 10(3), no such effect was observed. Growth at 4 degrees C did not increase the virulence of the strains for mice by oral-gastric challenge when given at doses of approximately 10(10).