TABLE 2.

Virulence in natural nonmutator and strong mutator isolates

No. of virulence determinants/ straina	No. of strains in each mouse virulence groupb						No. of strains (%) with atypical virulence phenotypec
	Avirulent		Intermediate		Highly virulent
	Nonmutator	Mutator	Nonmutator	Mutator	Nonmutator	Mutator	Nonmutator	Mutator
0–1	38	1	2	2	3	0	5 (12)	2 (66)d
3–5	2	2	2	3	19	0	4 (17)	5 (100)d

^aStrains 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). 

^bThe 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). 

^cAtypical 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. 

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