Figure 1. The output of our fuel cell when powered with S. cerevisiae. We observed a consistent output of ~500 mV over the course of 60 mins
These results show a steady voltage output of around 500mV over the hour. After carrying out this experiment, we started to move towards using E. coli instead. We modified the Reading Univeristy protocol in order to this, see below.
Microbial Fuel Cell with E. coli
After discussing the ethical issues of using yeast with PEALS, we decided to build novel genetic constructs for E. coli. However, we used the same protocol as above and made only slight changes, such as dissolving 1 g of arabinose as well as the 9 g of glucose into 50 ml of potassium phosphate buffer solution. 4 ml of the BBa_K1895004 and the BBa_K1895005 cell cultures were added to individual fuel cells during the experiments.
The fuel cells were left to run for an hour and the voltage taken every 3 minutes.
Figure 2. Reading microbial fuel cell output (mean±SE, mV). BBa_K1895005 caused overexpression of large porins, BBa_K1895004 caused overexpression of small porins. Separately, BBa_K1895004 with glucose only (no porin expression induction by arabinose) and LB broth only were used as negative controls. For both negative controls error bars are negligible and therefore difficult to distinguish on the graph. Voltages were measured every 3 minutes via digital voltmeter and the experiment stopped after 60 minutes.
Figure 2. Growth curves of the different battery constructs at 37°C for 20 hours. These were grown in a 96 well-plate and measured using a BIOTEK Powerwave HT. OD600 readings were taken every five minutes and the cultures were shaken inbetween measurements. The cells grown in the prescence of arabinose were found to have a lower growth rate due to the overexpression of the porins. However, the cells grown in LB broth without arabinose have a much higher growth rate.
Figure 3. Output of our microfluidic microbial fuel cell (mean±SE, mV) using the BBa_K1895004 construct undergoing porin expression. Solutions were made up as per the larger fuel cell, thoroughly mixed and injected by syringe to fill each chamber following insertion of the cation exchange membrane. Voltages were measured every 3 minutes via digital voltmeter and the experiment stopped after 60 minutes. For more information on how we designed the miniature fuel cell, please see our hardware design page
Conclusion
As evidenced by the given growth curves above, we can confirm that by placing the constructs under the pBAD arabinose-induced promoter, we can increase the overall growth of the battery constructs. Therefore we believe we have improved upon the part developed by Bielefeld 2013 who had reduced growth of E.coli containing their construct which had a constitutive T7 promoter. This may have been as a result of increased translation pressure.
