Colonization of the respiratory tract of C57BL/6 mice by RB50 and RB50Δ sigE. Groups of three 4–6 week-old C57BL/6 mice were inoculated with 5 × 105 CFU of RB50 (filled squares) and RB50ΔsigE (open triangles). Groups of three mice were sacrificed at each time point. The bacterial load in the indicated organ is expressed as log10 CFU ± SE. The dashed line indicates the limit of detection. The experiment was performed twice with similar results and a representative dataset is shown.