## 3 Results and Discussion
### FNR or FMV Deposited on the MF\({}_{\text{DSM}}\) Anode as Mediator  
Due to the results obtained in the previous Section 3.3, we investigated whether FNR or FMV enhances electron collection in MF\({}_{\text{DSM}}\). Upon electropolymerization, NR forms poly(neutral red), known as FNR, which has been extensively investigated for biosensor applications [23]. As mentioned earlier, FNR can react with biological components (e.g., NADH, proteins, and enzymes) [23]. In turn, immobilized viologen-based mediators have been successfully employed to interact with biological components [25]. However, unlike FNR, FMV interacted weakly with the MF\({}_{\text{DSM}}\) anodic electrode, so the carbon chain did not increase effectively. This effect has already been described, and the fact that the reduced species \(MV^{+}\) is prone to dimerization has been reported [30]. First, we evaluated the optimal number of CV cycles for FNR and FMV (Figure S4) in the absence of _C. pasteurianum_ DSM 525 (abiotic control). In the case of FNR, the current density increased between 10 and 20 cycles and decreased thereafter. Therefore, for FNR, we adopted 20 cycles for FNR in further investigations. Concerning MV, the current density started to decrease after 10 cycles, so we limited assays with FMV to 10 cycles (Figure S5). In the latter case, decreasing current density upon increasing number of cycles was probably due to \(MV^{+}\) species forming dimers, which can further disproportionate into \(MV^{2+}\) and insoluble \(MV^{0}\)[30]. We tested MF\({}_{\text{DSM}}\) (_C. pasteurianum_ DSM 525, O.D. = 0.2) across all CV cycles for each mediator film. The combination of MF\({}_{\text{DSM}}\) with FNR or FMV increased the current density in all the CV cycles (Figure S6). Nevertheless, although FMV interacted with _C. pasteurianum_ DSM 525 on the anode, it did not interact as effectively as FNR. Figure 4A,B compare the amperometric \(i\)-\(t\) tests for the highest current density achieved with FNR (20 cycles) and FMV (10 cycles), respectively, at MF\({}_{\text{DSM}}\). After we introduced FNR into the MF\({}_{\text{DSM}}\) anodic chamber, the current density increased 12-fold compared to MF\({}_{\text{DSM}}\) (30.76 \(\pm\) 3.44 and 2.47 \(\pm\) 0.53 \(\upmu\)A\(\cdot\)cm\({}^{-2}\), respectively). As for FMV, the current density increased 3.9-fold after it was introduced into the MF\({}_{\text{DSM}}\) anodic chamber compared to MF\({}_{\text{DSM}}\) (8.91 \(\pm\) 1.92 and 2.29 \(\pm\) 0.69 \(\upmu\)A\(\cdot\)cm\({}^{-2}\), respectively). Regarding FNR introduction into the MF\({}_{\text{DSM}}\) anodic chamber, the current density doubled compared to NR introduction into the MF\({}_{\text{DSM}}\) anodic chamber (30.76 \(\pm\) 3.44 and 17.05 \(\pm\) 2.25 \(\upmu\)A\(\cdot\)cm\({}^{-2}\), respectively). This effect has not been previously reported for _C. pasteurianum_ DSM 525. This result suggested that the presence of FNR on the electrode surface enhances the electron collection efficiency and reduces electron losses [31]. Polymeric film deposition on electrodes creates effective electrode transfer mediators [31]. In addition, the presence of FNR improves the EET capacity of _C. pasteurianum_ DSM 525, indicating that the film interacts with the microorganism. On the other hand, FMV or MV introduction into the MF\({}_{\text{DSM}}\) anodic chamber elicited similar effects: 3.9- and 3.7-fold increases in the current density compared to MF\({}_{\text{DSM}}\). The MV structure does not favor the formation of polymeric films, so MV remains mostly in the dimeric form [30]. The dimer and monomer must diffuse in a similar way, which could explain the similar increase in current density. We recorded CV curves at 1 mV\(\cdot\)s\({}^{-1}\) to determine the anodic peak potentials for each mediated electron transfer system. We conducted the experiments with MF\({}_{\text{DSM}}\) and FNR or FMV (Figure 4C,D). We also recorded a control CV without the biocatalyst.