TABLE 2.

Virulence of the mutants in BALB/c mice after intraperitoneal infection^a

Strain	Attenuation in mice
narR	0.9 ± 0.1
nnrA	2.4 ± 0.5
nnrB	0.8 ± 0.1
narG	0.5 ± 0.1
nirK	0.6 ± 0.1
norB	0.6 ± 0.1
nosZ	0.7 ± 0.1

^aVirulence is expressed as the log of attenuation of the mutant compared to the wild-type strain. Each group tested contained four mice at 4 weeks postinfection. Differences larger than 0.25 log units are statistically significant (P was <0.001 using standard ANOVA test). In each experiment, close to 4.5 log CFU was recovered from the spleen of mice infected with the wild-type strain.