A method for typing Listeria monocytogenes strains by classification of listeriocins and phage receptors.

A method for typing Listeria monocytogenes (L.m.) strains was developed which is based on two different approaches. First, strains were classified according to the phage receptors specific to their mitomycin-induced prophages. The frequency of lysogenic strains ranged between 63 and 80%, depending on the serotype. For non-lysogenic strains, a reversed phage-receptor analysis was applied based on the susceptibility of the strains to be typed against bacteriophages from selected, inducible indicator strains. As a second method, typing of mitomycin-induced listeriocins was shown to be practicable as well. By this approach, it was possible to distinguish 11 different listeriocins when using a set of 7 selected indicator strains. Only two of these listeriocins were probably produced by L.m. strains of serotype 4b. A combination of the two approaches described raises the number of types. When applied to 20 human isolates of L.m. belonging to serotype 4b, 17 different types were demonstrated. 46 L.m. strains of serotype 1/2b could be differentiated into 38 types of which 18 clinical isolates represented 17 different types. 26 L.m. strains of serotype 1/2a isolated from various food products showed a large heterogeneity, with 22 different types. The power of discrimination of the method outlined recommends its application in epidemiological investigations.