Y. pseudotuberculosis colonization of lungs occurs at low doses and is dependent on phoP. (A) In vivo growth of Y. pseudotuberculosis in BALB/c or Swiss Webster mice. Mice were inoculated intranasally with wild-type Y. pseudotuberculosis with 7.8 × 101 CFU (▴), 8.4 × 102 (▪), or 8.4 × 103 (•). Four days postinoculation lungs were harvested, weighed, homogenized, and plated for CFU. (B) Strains lacking PhoP are attenuated in colonization of lungs. BALB/c mice were intranasally inoculated with 8.0 × 102 CFU of IP2666pIB1, 6.0 × 102 CFU of IP2666pIB1ΔphoP, or 2.5 × 103 CFU of YPIIIpIB1. Four days postinoculation, lungs were weighed, homogenized, and plated for CFU. Each dot represents output from one mouse. Bars represent the geometric mean. P values were determined by a two-tailed Student's t test. *, P < 0.01.