In Figure 2, the profiles of total cell density and viability, glucose and lactate concentrations, and SEAP activity during a cultivation time of 13 days are given. Starting from initial cell density of 0.6 x 106 cells/mL-1, the cells grew with a mean growth rate of 0.893 d-1 corresponding to a doubling time of 18.6 h.
About 36 h after starting the cultivation, 1 L fresh growth medium was added. The growth rate after the media addition was slightly lower with 0.871 d-1 so that the total cell density prior to the medium exchange was 4.64 x 106 cells/mL-1. The maximum cell density of 7.44 x 106 cells/mL-1 was achieved after 161 h of cultivation.
The cell viability remained high (over 96 %) until the end of the stationary phase, where all substrates were depleted. Afterward, it dropped rapidly to zero within one day, when the cultivation was stopped.
The substrate consumption and metabolite production were comparable to our experiences with similar stirred benchtop-scale bioreactors. About 0.9 g glucose was consumed for the production of 106 cells/mL-1 and glucose was depleted after 192 h of cultivation. At the end of the exponential growth phase, the cells started to consume the lactate, whereas the maximum lactate concentration after the media exchange was 2.15 g·L-1 (Figure 2B).