Table 2.

Virulence in natural nonmutator and strong mutator isolates

No. of virulence determinants/ strainaNo. of strains in each mouse virulence groupbNo. of strains (%) with atypical virulence phenotypec
AvirulentIntermediateHighly virulent
0–138122305 (12)2 (66)d
3–522231904 (17)5 (100)d
  • a Strains with two virulence determinants were not taken into account since they are equally distributed between the avirulent and highly virulent mouse virulence groups (see Materials and Methods).

  • b The three mouse virulence groups were defined as follows: avirulent group (0 to 1 mouse killed out of 10), intermediate group (2 to 8 mice killed out of 10), and highly virulent group (9 to 10 mice killed out of 10).

  • c Atypical virulence phenotype is defined as strains with none or one virulence determinant that kill mice or strains with three to five virulence determinants that are avirulent or kill mice at an intermediate level.

  • d Significantly different (P = 0.01 and 0.0003 for strains with 0 to 1 and 3 to 5 virulence determinants, respectively).