The next step towards usable microbial bioelectrochemical sensors?

Microbial bioelectrochemical systems have been in the full spotlight for over a decade due to the promise of being a platform for sustainable electrical power generation from wastewaters. This promise has not yet been fulfilled, but the research on bioelectrochemical systems (BES) has received a tremendous boost (Schr€ oder, 2011; Arends and Verstraete, 2012). The field has diversified into exploring various configurations of BESs for energy production/storage, bioremediation/waste cleanup and bioproduction/electrofermentation, as a tool for fundamental research and for biosensor development (Schr€ oder et al., 2015). The fact that many bacteria, with Geobacter spp. as one of the model organisms, are (putatively) able to directly deposit electrons on an electrode (Koch and Harnisch, 2016) opens the possibility to translate biologically relevant environmental signals directly into an electrical current. This allows the creation of a low current, amperometric microbial bioelectrochemical sensor system, applicable in many environments. However, to come to practical applications of BES-based sensors, several issues need to be overcome. Amongst others, one can think of signal/response ratio, signal specificity i.e. minimizing interferences, measurement range, need for calibration, signal stability over time, use of pure cultures or mixed communities, mechanical stability, storage and shelf life of the sensor probe. The work by Estevez-Canales et al. (2017) as described in their recent manuscript makes a step forward in prolonging the shelf life and optimizing storage conditions of pre-colonized bioanodes. They have developed a method to colonize an anode with Geobacter sulfurreducens in a silica gel matrix embedded in a carbon felt electrode. The microorganisms remained alive within the silica matrix in which they were encapsulated, and their concentration did not change for a minimum of 4 days as assayed with fluorescence microscopy. Activity could be maintained over 18 days in a lactate-fed system as determined by the current response. Acetate pulse experiments indicated a fast response time, in the order of minutes, of the bacteria encapsulated in the silica matrix. Using pre-colonized electrodes which take ~1 h to produce (Estevez-Canales et al., 2017) and can be stored offers a great advantage over producing (in the order of days) and possibly storing electroactive biofilms. As the authors indicate, several issues remain to be addressed to develop storable microbial electrodes for fast, selective and reproducible recordings. For example, optimal storage conditions for these electrodes need to be determined. From the manuscript, it becomes clear that the electrodes were stored under favourable conditions for Geobacter sulfurreducens. However, the most extreme conditions for storing the encapsulated microorganisms, in terms of temperature, humidity, time without electron donor/acceptor and exposure to atmospheric oxygen (Lin et al., 2004) still need to be understood. It would be interesting to compare the durability of a precolonized electrode with an electroactive biofilm of the same dimensions with regard to their storage conditions but also regarding the mechanical strength of biocatalyst attachment. An intriguing follow-up question is the impact of invasion, or colonization, of the silica surface by other microorganisms on the performance of the electrodes with immobilized electroactive microorganisms. Generally, microbial bioelectrochemical sensors are envisioned to be used in ‘dirty’ environments such as (waste)waters, fermenters and anaerobic digesters (Yang et al., 2015). These environments are characterized by a very diverse microbial community of which several members might possibly be capable of colonizing the silica matrix and thus hinder analyte mass transfer towards the embedded sensing microorganisms. On a more positive note, interaction of environmental microorganisms with encapsulated microorganisms (not Received 28 November, 2016; accepted 8 December, 2016. *For correspondence. E-mail [email protected]; Tel. +32 (0)9 264 59 76; Fax +32 (0)9 264 62 48. Microbial Biotechnology (2018) 11(1), 20–21 doi:10.1111/1751-7915.12590 Funding information European Research Council.