# Bielectroanalytical Detection of Lactic Acid Bacteria  
[PERSON]  
1 Singapore Centre for Environmental Life Sciences Engineering (SCELSE), Nanyang Technological University, 60 Nanyang Drive, Singapore 637551, Singapore; [EMAIL_ADDRESS] (E.J.Y.H. ); [EMAIL_ADDRESS] (S.W)2 Singapore Eye Research Institute (SERI), 20 College Rd., Level 6 Discovery Tower, Singapore 169856, Singapore  
organism by exploiting a phenomenon known as extracellular electron transfer (EET)--the metabolic process that transports electrons from the cytosol to the exterior of a cell [8,9]. When the electrochemical reporter, or redox mediator, is released into the medium, it is indicative of the presence of the target organism; the redox mediator is reduced metabolically and subsequently reduces the electrode. EET has been extensively studied, particularly in the model Gram-negative electrogens _Geobacter_ spp. and _Shewanella_ spp., and is usually described as a type of anaerobic respiration achieved by heme-containing electron transfer protein complexes [9,10]. The environmental significance of EET is in biogeochemical cycling of metal oxides in the subsurface and is achieved biologically, as microbes transport electrons across their membranes and reduce solid terminal electron acceptors, such as Fe (III) or Mn (V), in a process that yields metabolic energy in the form of ATP [11,12]. The practical role of respiration in achieving a good detection signal in bioelectroanalytical systems was recently deduced from the protracted detection times for _Escherichia coli_ (_E. coli_) strains with deficient respiratory chains that are only able to grow by fermentation [13]. Therefore, the utility of bioelectroanalytical systems to _rapidly_ detect lactic acid bacteria is unknown, as LAB are unable to respire and instead gain their metabolic energy exclusively by fermentation. EET is well described in Gram-negative organisms but has typically remained more obscure in Gram-positive organisms, except for a few notable exceptions, and despite Firmicutes regularly turning up in the phylogeny of bioelectrochemical systems [14,15,16]. Recent research suggests that EET in Gram-positive bacteria is evolutionarily more ancient than in Gram-negative organisms [17]. Lately, it has become commonplace to describe EET mechanisms in Gram-positive organisms that include LAB. [PERSON] et al. (2019) recently described a flavin-based EET mechanism in _Listeria monocytogenes_ and electrode-dependent growth that is distinct from more well-described mechanisms of EET [9]. Similarly, EET has been described in another clinically important LAB, _Enterococcus faecalis_ (_E. faecalis_), and although the exact mechanism is yet to be elucidated, its ability to reduce an electrode and Fe (III) has been demonstrated [8,18,19]. In addition, for both _Listeria monocytogenes_ and _E. faecalis_, EET has been implicated in virulence either through increased competitive capabilities, enhanced biofilm potential, or through EET mediated synergistic interactions with commensals. In these systems, however, basic quantitative assessments of the EET process is not always reported. In light of the recent insights into the EET in LAB, we decided to revisit the idea of their compatibility in bioelectroanalytical detection. The aim of this contribution is to determine whether a clinically relevant member of the LAB, _E. faecalis_, is amenable to rapid detection and enumeration in a bioelectroanalytical system (Figure 1). A secondary aim of this contribution is to comment on the quantitative importance of EET mechanisms in _E. faecalis_ by looking at EET efficiency. To our knowledge, this is the first report describing the potential for specific and swift bioelectroanalytical LAB detection and to quantitatively assess the efficiency of the EET process and how this relates to EET mechanisms of _E. faecalis_ in the context of biosensing.