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By application of a combined mitomycin-/heat treatment after freezing, 7 out of 29 Listeria monocytogenes strains which were found to be non-phage carriers by UV irradiation could release phages. Propagation of the obtained phages was promoted by storage at 4 degrees C. Apart from TNSA plates the application of chocolate plates appears to be necessary in order to study these phages.

The authors report on the isolation of 13 Listeria monocytogenes strains of healthy human subjects which, within serogroup 4, contain at least four variants of known Listeria antigens, but wound not fit into the grouping system that has been in use.

The aim of this multicenter study was to determine and to standardize methods in order to have common basis for comparing results of phage typing of Listeria monocytogenes. Using a common set of Listeria strains, a standardized method, including media, bacterial growth conditions, application of viruses and reading of lytic reactions, was established. 29 bacteriophages were selected according to their host range. This phage typing system allowed phagovar determination of 54% of the serogroup 1/2 strains and 77% of the serogroup 4 strains of L. monocytogenes.

A total of 5000 samples were examined for Listeria monocytogenes, and 117 strains had been isolated. Out of these, 36 strains were selected according to their low virulence. An increasing virulence could be observed, when they were cultured on optimal media at 4 degrees C, with inoculations at every third week. This increased virulence caused by temperature factors, can have an important role in the epidemiology of listeriosis.