Abstract
Metabolic drift (antibiotics resistance) mutations were used to construct stable two (and three) marker vaccine candidate strains of the predominant Listeria monocytogenes serotypes 1/2a and 4b by stepwise selection. Derived from wild-type strains, spontaneous chromosomal streptomycin-resistant clones with their i.p LD 50 elevated from ⩽10 5.0 c.f.u. to ≈10 6.1 c.f.u. were used in the step to isolate the rifampicin-resistant mutants with i.p. LD 50 values ranging from 10 6.6 to 10 7.4. On i.p. immunization with fully tolerated doses (⩽1% LD 50), these potential vaccine strains were found to protect not less tha 95% of the mice against a lethal (⩽100 LD 50) challenge with the homologous wild-type strain. Further elevation of the i.p. LD 50 to >10 8.3 c.f.u. by means of a third attenuating fosfomycin-resistance marker resulted in over attenuation and reduced protective capacity.
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