Opportunities for visual techniques to determine characteristics and limitations of electro-active biofilms

Targeted Formation of Biofilms on the Surface of Graphite Electrodes as an Effective Approach to the Development of Biosensors for Early Warning Systems

Biofilms based on bacteria Pseudomonas veronii (Ps. veronii) and Escherichia coli (E. coli) and yeast Saccharomyces cerevisiae (S. cerevisiae) were used for novel biosensor creation for rapid biochemical oxygen demand (BOD) monitoring. Based on the electrochemical measurement results, it was shown that the endogenous mediator in the matrix of E. coli and Ps. veronii biofilms and ferrocene form a two-mediator system that improves electron transport in the system. Biofilms based on Ps. veronii and E. coli had a high biotechnological potential for BOD assessment; bioreceptors based on such biofilms had high sensitivity (the lower limits of detectable BOD5 concentrations were 0.61 (Ps. veronii) and 0.87 (E. coli) mg/dm3) and high efficiency of analysis (a measurement time 5-10 min). The maximum biosensor response based on bacterial biofilms has been observed in the pH range of 6.6-7.2. The greatest protective effect was found for biofilms based on E. coli, which has high long-term stability (151 days for Ps. veronii and 163 days for E. coli). The results of the BOD5 analysis of water samples obtained using the developed biosensors had a high correlation with the results of the standard 5-day method (R2 = 0.9820, number of tested samples is 10 for Ps. veronii, and R2 = 0.9862, number of tested samples is 10 for E. coli). Thus, biosensors based on Ps. veronii biofilms and E. coli biofilms could be a novel analytical system to give early warnings of pollution.

Examining the resistance and resilience of anode-respiring Shewanella oneidensis biohybrid using microsensors

Immobilizing electro-active microbes within polymer matrices (thereby forming biohybrids) is a promising approach to accelerate microbial attachment to electrodes and increase the biofilm robustness. However, little is known on the fine scale chemical environment that develops within the electro-active biohybrids. Herein, we develop a biohybrid by immobilizing a culture of Shewanella oneidensis MR1 in agar matrix on the surface of a graphite electrode poised at +0.25 V. The resulting bioanode (3-6 mm thick) was grown under anoxic conditions and produced a steady current of 40 μA. Oxygen and pH distribution within the biohybrid were characterized in-situ using microsensors. As Shewanella is a facultative aerobe, it will halt the current production in the presence of oxygen. Thus, in addition, we investigated the alteration of the microenvironment during and after aeration of the medium to evaluate the oxygen tolerance of the system. During aeration, oxygen was effectively consumed in the top layers of the biofilm, leaving a 400-900 μm thick anoxic zone on the anode surface, that sustained >60% of the initial current. Current production recovered to pre-oxic condition within 5 h after the aeration was stopped, showing that immobilization can promote both high resistance and resilience of the system. Despite the absence of strong buffering conditions, pH profiles indicated a maximum drop of 0.2 units across the biohybrid. Characterizing the chemical microenvironment helps to elucidate the mechanistic functioning of artificial biofilms and hold a great potential for the designing of future, more effective biohybrid electrodes.