Atmospheric level of O
stimulates Hp growth in the presence of CO
. Hp 26695 cells collected from agar plates were inoculated into BB-NBCS at 5 × 107 CFU/ml (A and B) or 3 × 104 CFU/ml (C) and cultured under 2%, 8%, or 20% O2 tension in the absence or presence of 10% CO2. An aliquot of each culture was taken at the indicated time points to determine absorbance at 600 nm, culture media pH, and viable cell counts. For data shown in A and C, each flask was refilled with the appropriate gas mixture and incubated for measurements at later time points. For data shown in B, 15 flasks were inoculated with the preculture, filled with mixed gas, and incubated. One flask was used at each time point for measurements; flasks were used only once to prevent exposure of cultures to atmospheric oxygen. Absorbance at 600 nm and media pH data shown in A and C are expressed as mean ± SD of triplicate cultures and are representative of ten and three experiments, respectively. Data shown in B are mean ± SD of four independent experiments. Colony counting data are representative of four independent experiments with similar results.