Routine test for in vitro differentiation of pathogenic and apathogenic Listeria monocytogenes strains.

The exosubstance of Rhodococcus equi in a prepurified form strongly enhanced the hemolytic effect of certain strains of Listeria monocytogenes. The strains which produced positive synergic hemolysis with this exosubstance were also pathogenic for guinea pigs and white mice. The other strains, which remained nonhemolytic in the presence of the R. equi. exosubstance, were apathogenic for those animals. A routine test was devised for the in vitro determination of the pathogenicity of L. monocytogenes strains.