# Bielectroanalytical Detection of Lactic Acid Bacteria
## 3 Results and Discussion
faecalis_ EET in our microscale detectors using BHI as a medium and the redox active aglycone used in the proposed detection approach, resorufin, to effect electron transport. The charge generated by _E. faecalis_ was substantially greater (20 mC) when mediated with resorufin than the control current which barely increased above a baseline current of around 2 mC (Figure 4A). Over a 20 h period, around 600 mg L\({}^{-1}\) COD more was consumed in the mediated system (\(\Delta_{\text{COD}}\) = 2020 mg L\({}^{-1}\)) compared to the unmediated  
Figure 3: (**A**) Growth of _E. faecalis_ as endpoint measurements of turbidity (OD\({}_{600}\)) and (**B**) endpoint pH measurements after growth with different supplements to stimulate respiratory behavior. system (\(\Delta_{\text{COD}}\) = 1420 mg L\({}^{-1}\)), which is suggestive of an increase in EET by _E. faecalis_ upon the addition of the mediator (Figure 3B). However, the response of _E. faecalis_ to mediator addition in terms of COD consumption was similar in both benchtop controls and in bioelectrochemical systems. The \(\Delta_{\text{COD}}\) in equivalent reactors but without the electrode and incubated under identical conditions was 1160 and 1920 mg L\({}^{-1}\) for the unmediated and mediated systems, respectively, equating to 760 mg L\({}^{-1}\) more COD consumed in the benchtop system that was supplemented with resorufin compared with the unsupplemented control. Thus, while the electrochemical behavior of _E. faecalis_ is stimulated with the addition of redox mediators, so too is a planktonic culture, and this is reflected in the observation in Figure 4A as well as with the growth studies in Figure 3A, where the addition of menaquinone only resulted in some growth stimulation under anaerobic conditions. This is also in keeping with observations by other researchers [25]. Coulombic efficiency (CE) of the electrochemical response of _E. faecalis_ is low: 1.6% in the mediated system and 0.1% in the unmediated system (Figure 4B). Figure 4: **(A)** Mean charge and (**B**) mean coulombic efficiency of _E. faecalis_ in systems grown in electrochemical conditions. **(C)** Comparison of COD consumption by _E. faecalis_ in benchtop incubations and electrochemical conditions. The shaded area in **(A)** and the error bars in **(B,C)** represent the absolute error of duplicate measurements. EET is increasingly being described in Firmicutes isolated from the gut [8; 9; 27; 28] and, more recently, in situ directly by inserting electrodes into a mouse gut and comparing the output with germ-free organisms [29]. These studies, in the main, have lacked a quantitative assessment of the electron flux as a function of the carbon utilization in the system. Where columbic efficiency (CE) is reported, it is low (typically \(<\)1%) and of a comparative magnitude to that reported here. For example, similarly diminutive CE observations were made by [PERSON] et al. (2019), where a CE of only 0.02% was reported for a gut-isolated _Entrococaceae_ sp. with a 99% 16 S RNA sequence similarity to _E. avium_[27]. The question therefore remains whether EET in many LAB-based systems, either to an electrode or in the reduction in metal oxides (e.g., the commonly reported Fe (III) reduction), is a respiratory phenomenon or if it is a strategy for Firmicutes to exert redox control over their environment, e.g., to enable nutrient uptake or even mitigate metal toxicity, or even if it is just redox leakage. Since the first option is involved in energy conservation, and the other comes at a metabolic cost, the distinction is an important one. However, the low CE of EET in LAB systems reported to date suggest that such a strategy, if it used to conserve energy, is a minor one. Mechanisms for EET in Firmicutes have been reported to be flavin dependent [9; 28], although this does not necessarily mean that the flavins are mobile redox mediators as proposed in _Shewanella_ spp. [PERSON] et al. (2018) and [PERSON] (2020) identified key genes for EET in Firmicutes. In _L.