Insertion mutagenesis with the help of the plasmid pFS23 was used to generate mutants. The results of pFra- strain production under non-selective conditions suggested that such variants may be generated in natural plague foci at high frequency and may participate in supporting the epizootic process. Present data suggest that the reduction of virulence in Fra- strains reported by the majority of investigators was connected with the use of variants carrying additional unidentified mutations. It was shown that the loss of the ability to produce capsular antigen (FI) alone or in combination with absence of murine toxin production did not lead to an increase in LD50 absolute values. Simultaneous loss of these two virulence determinants did not influence the duration of survival of the infected animals. However, absence of only FI antigen production in the infecting strain resulted in prolonged survival of the infected animals. Conversion of plague infection from acute to chronic form is probably dependent on the animal host species and the parent strain subjected to mutagenesis.


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