Separation of pathogenic from apathogenic Listeria monocytogenes by three in vitro reactions.

One-hundred-twelve isolants of Listeria monocytogenes cultured from clinical and nonclinical sources were examined for hemolytic activity by means of the CAMP phenomenon and tested for acidification of xylose and rhamnose. The reactions of the isolants were noted and correlated with the pathogenicity of the organisms. All of the CAMP-negative (nonhemolytic) isolants were apathogenic, and all but one of the CAMP-positive (hemolytic) isolants were pathogenic. All xylose-acidifying isolating isolants were apathogenic, but not all apathogenic isolants were xylose acidifiers. All pathogenic isolants but one were rhamnose acidifiers, but not all rhamnose acidifiers were pathogenic. The pattern of hemolysis, rhamnose acidification, and non-acidification of xylose is associated with pathogenic L. monocytogenes; departure from this pattern is associated with nonpathogenic Listeria, which otherwise share the properties of L. monocytogenes. These in vitro tests should prove useful in screening the apathogenic isolants whose clinical and epidemiological significance should be carefully assessed.