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Batch cultures of E. coli TB1/pUC13 were carried out at different oxygen transfer rates (OTR) enhanced by the increase of stirring rate and by the increase of air total pressure of the bioreactor. These two variables showed to have little effect on cell growth but a negative effect on cytochrome b5 (recombinant protein) production. However, this effect was more significant of high stirring rates than for values of pressure up to 0.4 MPa. The effects of stirring and pressure were also investigated for fed-batch mode operation. In this type of cell cultivation high cell densities are reached, thus a high capacity of oxygen supply of the system is required. To compare the two ways of improving OTR, cell behaviour was followed in two bioreactors at different operational conditions giving the same maximum OTR value. The first one operated at a high stirring rate (500 rpm) and at atmospheric pressure (0.1 MPa) and the other one at high air pressure (0.48 MPa) and low stirring rate. The increased pressure seemed to be a better way of ensuring an adequate oxygen supply to a culture of E. coli TB1 cells than an increased stirring rate. For the high pressure experiment a higher cellular density was reached, as well as a higher cyt.b5 expression which led to a 4-fold increase in final productivity. These experiments showed that bioreactor pressurization can be successfully used as a means of enhancing oxygen mass transfer to shear sensitive cell cultures.

Batch and fed-batch cultures of E. coli TB1 at different oxygen transfer rates