Figure 3. Incidence of plague in immunocompetent rats (A), immunocompetent mice (B), and neutropenic mice (C) injected intradermally with ∼20 WT Y. pestis (white circles), Δypmt1.66c (black circles) or complemented mutant (grey circles).

The mutant was significantly (p<0.05) less virulent than the WT strain in immunocompetent rats (A), immunocompetent mice (B and C) and neutropenic mice (C). The survival curves of mice infected with the complemented mutant harboring the ypmt1.66c gene on a high copy number plasmid (pCR2) and the wild-type (B) were significantly different (p<0.05). The virulence of Y. pestis lacking ympt1.66c was no greater in neutropenic mice than it was in immunocompetent mice (dashed lines) (p>0.05). Data were obtained from groups of 8 or 9 animals. P values were determined using the Gehan-Breslow-Wilcoxon test.