Electrically Connecting Bacteria to Nanoparticles for Biotechnological Applications

Combining abiotic photosensitisers such as semiconductor fluorescence emitting
nanoparticles – quantum dots (QDs), with non-photosynthetic bacteria ‘in vivo’
presents an intriguing concept into the design of artificial photosynthetic organisms
and solar-driven fuel production. Shewanella oneidensis MR-1 (MR-1) is a versatile
bacterium concerning respiration, metabolism and biocatalysis, and is a very
promising organism for artificial photosynthesis. The bacteria’s synthetic and
catalytic abilities, together with their longevity, provide a promising system for
bacterial biohydrogen production. MR-1’s hydrogenases are present in the
periplasmatic space, and it follows QDs or their electrons will need to enter the
periplasm via the Mtr pathway that is responsible for the extracellular electrontransfer
ability of MR-1. Firstly, various QDs were tested for their nanotoxicology
and further for interaction with MR-1 by fluorescence and electron microscopy.
CdTe/CdS/TGA, CdTe/CdS/Cysteamine, commercial negatively charged CdTe
and CuIn2S/ZnS/PMAL QD were examined, and it was found that the latter two
showed no toxicity for MR-1 as evaluated by a colony-forming units method and a
fluorescence viability assay. Only commercial negatively charged CdTe QDs
showed good interaction with MR-1. Detailed investigation of the above interaction
by transmission electron microscopy showed QDs were placed both inside the cell
and close to the membrane. Subsequently, the photoreduction power of QDs was
evaluated by the methyl viologen assays with different sacrificial electron donors.
It was indeed found that QDs have reduction potential sufficiently low to perform
MV photoreduction. As assessed by gas chromatography, CdTe/CdS/TGA and
negatively charged CdTe QDs supported hydrogen evolution in Shewanella
putrefaciens CN-32. The above results establish a proof of concept for
photosynthetic production of biohydrogen by CN-32. Further research should be
invested in the use of biocompatible sacrificial electron donors and the
development of appropriate bacteria mutants that would help to understand the
assisted by QDs hydrogen evolution in this bacterium.