Direct and indirect use of GFP whole cell biosensors for the assessment of bioprocess performances: design of milliliter scale-down bioreactors.

Substrate limitation responsive biosensors have been used for the development of a mini-bioreactor platform that can be used as a scale-down tool. Three green fluorescent protein (GFP) transcriptional reporters have been chosen in Escherichia coli, i.e., uspA::gfp, csiE::gfp, and yciG::gfp. Our previous studies have shown that these kinds of promoters are induced in response to substrate limitation and are significantly repressed when cultures are carried out in heterogeneous bioreactors. This sensitivity to substrate limitation has been confirmed in the case of the csiE and yciG biosensors. A mini-scale-down platform is proposed as a high throughput tool to rapidly investigate the usefulness of a given microbial biosensor. This platform is composed of shake flasks able to operate in fed-batch mode either using the slow release or the intermittent feeding principle. Local heterogeneities were reproduced at the level of these mini-bioreactors (operating under the intermittent feeding principle) and caused a decrease in GFP expression as in conventional scale-down reactors. The presence of GFP in supernatants was also noted and seems to be correlated with the substrate limitation signal for the three cultivation systems considered in this work (i.e., chemostat, conventional and mini-bioreactors) and with membrane permeability.