Let's recycle our purification wastes !
Results in medium 284 showed that the deletion of the Cupriavidus Metallidurans pbrR gene reduces its ability to aggregate lead. We have sought through our experiments to use this loss of resistance mechanism to specialize our bacterium in order to create a heavy metal recovery system in a specific way in sludge from polluted wastewater treatment plants.
Our results in the sludge were encouraging (see Results) and more complex than initially designed.
Indeed, the mechanisms of Cupriavidus Metallidurans appear to be linked to each other by oxidation-reduction systems in response to the pH of local environments.
Associated with this specialization in heavy metal aggregation, we wanted to build a bacterium capable of producing bioethanol from cellulose digested by Trichoderma Reesei Cellulases.
For this we used the Part: BBa_K1122673, a protein for the fusion of Adh and Pdc proteins from Zymomonas Mobilis allowing the degradation of pyruvate into ethanol by alcoholic fermentation.
This modification was accompanied by the addition of a very strong promoter constitutively and inducible with heavy metals in Gram bacteria: the pan promoter of Bacillus Subtilis. This addition was intended to allow the production of bio-ethanol without any particular fermentation conditions but in the presence of heavy metals.
The modification of the bacterium in this way has been a success, our strain correctly expresses the Fusion Protein Adh-Pdc. Its expression under the control of the pan promoter appeared as strong without induction as when β-galactosidase induced the expression of the protein under the control of the LacZ promoter.
The culture of Bacteria ΔpbrR-Adh/Pdc_pan to demonstrate great difficulty in growth in the presence of high concentrations of Lead and Cadmium. In addition, the 3000 bp and 100kDa Fusion Adh-Pdc protein, being too much produced in the bacteria, further slows down the growth of the bacteria.
Under these conditions, the expected ethanol concentrations were not achieved at all and the recovery of the waste represented by sewage sludge was therefore not conclusive, either in terms of bio-ethanol production or in terms of the aggregation of heavy metals, specifically cadmium, at ΔpbrR-Adh/Pdc_pan.
On the other hand, we believe that the future use of Cellulolytic Bacteria and bio-ethanol-producing bacteria with the pan promoter could make it possible to address the problem posed by bio-ethanol because cellulose in sludge is present in large quantities and represents a major carbon source of energy. Moreover, the limited success of specialisation for metals deserves to be further developed for the various other resistance mechanisms Cupriavidus Metallidurans and could eventually allow the development of a circular economy based on the recovery of polluting metal elements from sludge, reintroduced into the polluting industry.
Finally, all these techniques could eventually solve major problems related to the pollution of our water and soil by sludge from wastewater treatment plants, agricultural spreading being the first actor to be affected by this evolution.