SUMMARY: Cured derivatives of and showed reduced virulence following oral infection of mice (10-10-fold for , 10-fold for ). Large plasmids from and independently restored virulence to the cured but truncated plasmids with deletions in a previously identified virulence region did not. This common virulence region identified in plasmids from and was shown to be carried on plasmids from 11 other serotypes of but was absent from 10 plasmid-containing serotypes. Tn and Tn were transduced from the virulence region of two Tn-insertion mutants of and one Tn-insertion mutant of that showed diminished virulence to recipient wild-type strains of and . Each transductant showed a decrease in mouse virulence within the range 10-10. It is therefore proposed that similar virulence determinants are expressed in different serotypes. It was also shown that integration that occurred during curing was Tn dependent.


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