Selection and characterization of a Listeria monocytogenes vaccinal strain of reduced virulence for mice.

In an attempt to reproduce resistance to Listeria monocytogenes induced by primo-infection, a vaccinal strain of low virulence capable of affording a good immunity was looked for. After a double mutation in regard to streptomycin, i.e. dependence then reversion to independence, nine strains were obtained from a Listeria monocytogenes field strain of low virulence. In addition, five of these reverse strains were made resistant to erythromycin. Measure of virulence of the resulting 14 strains was performed on mice by spleen counts three days after subcutaneous inoculation of 10(5) bacteria. A general decrease of virulence after the mutations was observed. Immunogenicity was estimated by spleen counts, three days after a challenge with parent strain on mice vaccinated one month previously with 10(6) organisms of each strain. A strain, resistant to erythromycin, was selected as potential vaccine, as having good immunogenicity, sufficiently reduced virulence, good growth ability and evident marker. This strain was shown resistant to 16 micrograms/ml of streptomycin and to 1000 micrograms/ml of erythromycin when respective values for parent strain were 4 micrograms/ml and 0.25 microgram/ml. Its metabolic activity in regard to glucids did not differ from that of parent strain but requirements in aminoacids increased at each step of the mutations leading from parent to selected strain. These characters were recognized as stable after two independent series of 10 passages on mice.