Figure 3. A threshold level of bacteria is required for N. meningitidis to grow in blood.

100 µl of N. meningitidis strain FAM20 suspended in cell culture medium DMEM was mixed with 100 µl of anti-coagulated whole blood collected from CD46^+/+ mice with a final concentration of 10³ CFU/ml (circles), 10⁴ CFU/ml (squares) and 10⁵ CFU/ml (tri-angels), respectively. As a control, DMEM was mixed with bacteria in the same way. The samples were incubated at 37°C, 5% CO₂ for indicated time points and the number of surviving bacteria was determined by viable count assay. Bacterial growth index at each time point was defined as (CFU after blood incubation)/(CFU after DMEM incubation).