Contribution to the study of live streptomycin–dependentSalmonellavaccines: the problem of reversion to a virulent form

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Abstract
The recovery of virulence by means of reversion of a live streptomycin-dependent (Sm D)Salmonella typhimuriumvaccine was studied in CD–I Swiss mice. Initially, a one–step Sm D mutant was obtained from a virulent streptomycin-sensitive (Sm S)S. typhimuriumstrain. Afterwards, two pools of streptomycin-independent (Sm I) revertants were prepared from the Sm D strain. The virulence of the Sm D strain and of the Sm I revertants was tested intraperitoneally. In the virulence testing the original suspension of the Sm I revertants, as well as their 1st and 10th passages on plain medium, medium + 50 μg. streptomycin/ml, and medium + 1000 μg. streptomycin/ml, were used. The results show that the Sm D mutant was avirulent, its avirulence being due to an intrinsic, genetic quality. The Sm I revertants, compared to the original Sm S strain, also displayed a lack of virulence. However, afterwards, the Sm I revertants behaved quite differently, according to their subsequent passages. Indeed, there was an increase in virulence after passages on plain medium, whereas after similar passages on medium containing the drug, the virulence not only failed to increase, but disappeared almost completely. Moreover, the passages on medium containing 1000 μg. streptomycin/ml, induced a return to the status of drug–dependence. The danger of recovery of virulence by means of revertants is evaluated.