## 6 Conclusions  
The extended bi-level (hybrid) HSMDM adopted in this paper was experimentally proved by [PERSON] [11] to adequately represent the dynamics of a studied FBR, for both macroscopic state variables, and for the cell key species of the CCM related to the TRP-synthesis in the FBR, which are: [a] glycolysis, [b] ATP-recovery system, [c] TRP-operon expression, and the biomass [X] growth. The hybrid structured model, linking the macro-state variables to the nano cell-scale variables, was validated by using the recorded data from a lab-scale FBR over an enough long batch time (63 h). This paper proves how such HSMDMs can be used for engineering calculations if appropriate algorithms are used. The use of a classic NLP single-objective optimization algorithm, or the use of a multi-objective Pareto-optimal front technique, can be both very effective in predicting the optimal operating strategy of a studied FBR, if the problem is correctly formulated in mathematical terms. The medium-size HSMDMs can offer a greater degree of detail and precision of the predictions compared to the un-structured (global) kinetic models. There are multiple engineering applications, detailed as follows: (a) they allow for the in silico optimization of the target metabolite synthesis (referred to as TRP herein), by optimizing the FBR operation mode; (b) able to predict the cell CCM metabolic fluxes (i.e., homeostatic stationary reaction rates), the HSMDM model can offer suggestions to genetically modify the cell strain [11; 32; 35; 41] to obtain GMOs of desired characteristics ('motifs'); (c) they can predict the reactor operating conditions when cell oscillatory processes could occur [43; 44; 45]; (d) they can perform an analysis of the substrate utilization and product synthesis closely related to the reactor operating strategy. To obtain a high productivity in TRP with an industrial FBR is a difficult problem in the literature. This engineering problem was solved here through the concomitant use of a model-based (in silico) approach, completed with a biological improvement using the modified TS _E. coli_ cell culture. Such a concomitant approach is seldom used in the literature, but it proved to be very efficient. The engineering evaluations developed in this paper can be further extended, for instance by deriving a multi-objective optimization of the FBR operating mode, by accounting for not only the product (TRP) maximization, but also minimization of the batch time, and other economic objectives. The obtained results also proved that, in addition to the cell phenotype (linked to the CCM), the FBR operation modes are the major factors determining the TRP-synthesis efficiency (that is, product maximization, with the minimization of raw material consumption). As an observation, it could be interesting to investigate, in a future work, how the heterogeneity of the cell culture is reflected in the constants of the kinetic model and in the quality of its predictions [99]. Conceptualization, G.M. ; methodology, G.M. ; investigation, G.M. and D.G. ; data curation, G.M. ; project administration, G.M. ; software, G.M. ; supervision, G.M. ; validation, G.M. and D.G. ; writing--original draft, G.M. ; writing--review and editing, G.M. and D.G. ; formal analysis, D.G. All authors have read and agreed to the published version of the manuscript. The authors did not receive support from any organization for the submitted work. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. Data Availability Statement: Experimental datasets and some of the information used in this study are imported from the literature, and from TU Hamburg, and the source was noted in the text, and in the Acknowledgements section.