The influence of culture medium composition on hemolytic effects produced by Listeria innocua on sheep blood
agar has been investigated. Neither alpha- nor beta-
hemolysis could be observed around L. innocua colonies grown on sheep erythrocyte
agar lacking
glucose. However, green zones of hemolytic phenomena were observed around colonies grown on such
agar which contained 0,6% (w/v)
glucose. By using L. innocua cell
suspensions and culture filtrates this
glucose-associated
hemolysis only occurred in media which were weakly buffered (1 m M Na-PBS). This hemolytic phenomenon was attributed to the resulting acidification of the culture medium caused by the metabolic breakdown of
glucose by Listeria. The results of this study demonstrate the necessity of including adequate
buffer conditions when assaying the hemolytic property of a Listeria strain. It was found that the addition of 20 mM Na-PBS to the assay medium was sufficient to eliminate artificial "acidic-
hemolysis". Moreover, the
hemolysis observed with known hemolytic Listeria strains was unaffected by this buffering, as was in fact the case even when conditions of higher buffering capacity were employed. By testing in this manner, no evidence has been found which would suggest the existence of an
hemolysin, either intra- or extracellular, produced by Listeria innocua. An accurate method for the determination of
hemolysis caused by strains of the genus Listeria is proposed. This method of assaying
hemolysis in a liquid grown medium has proven effective in determining the hemolytic properties of strains which appeared negative or questionable on blood
agar as well as strains which in virulence tests were negative. The ability to accurately assess the hemolytic properties of Listeria strains, is essential in determining the association of Listeria
hemolysin with pathogenicity of this genus. Together with current investigations on the genetics of
hemolysin production by Listeria the determination of hemolytic activities will eventually allow to understand better the pathogenic principle of hemolytic strains of Listeria monocytogenes.