Monocytosis producing activity from virulent and avirulent strains of Listeria.

Monocytosis is a hallmark of listeriosis in many species. A similar phenomenon is induced by a monocytosis-producing activity (MPA) purified from the organism. The relationship between MPA production and virulence is unclear. The purpose of this study was to measure MPA in extracts prepared from strains with known degrees of virulence and haemolysin production. Differential leukocyte counts on peripheral blood cells were performed 24, 48 and 72 h after injection of 0.5 mg MPA. In the strains we tested those known to be virulent produced monocytosis; those which were avirulent did not.