In our digestive tract, on our skin, in the mucous membranes and even in our eyes – bacteria are to be found everywhere and not in short supply. Measured by the number of cells in our body, we are more bacteria than humans: A study from 2016 came to the conclusion that there are around 1.3 bacterial cells for every human cell in our body. That’s a good thing, because our microbiome is essential for our health; a disturbed microbiome is often associated with conditions such as obesity, autoimmune or cardiovascular diseases.
Whether a bacterium is beneficial to our health or threatens it depends on the type of bacteria. Some strains of the notorious Escherichia coli-Bacteria are part of our natural intestinal flora. Other, pathogenic strains trigger diseases such as urinary tract infections or diarrheal diseases. Such pathogenic bacteria are often transmitted by drinking contaminated water. Especially in developing countries and remote regions it can be difficult to get access to clean drinking water. For this reason, the disinfection of drinking water is an important humanitarian task that scientists have been working on solving for decades. Although there are already established processes based on ultraviolet light, chlorine or ozone, they all have serious disadvantages: high costs, low efficiency, poor biocompatibility or even carcinogenic by-products.
Clean drinking water
A Chinese research group led by Jing Zhao from Nanjing University has now developed a new method to disinfect drinking water using quantum dots. Quantum dots are nothing more than tiny crystals with semiconducting properties that are only a few nanometers in size. They are so named because their size makes them behave somewhat like quantum objects. Zhao and his colleagues used silver sulfide quantum dots equipped with artificial proteins for their experiments. The proteins have been designed in such a way that they bind particularly strongly to silver atoms and thus cover the surfaces of the silver sulfide beads. This prevents the quantum dots from merging over time and losing their special properties.
At the Technical University of Munich, researchers led by Thomas Brück are using cyanobacteria strains to recover rare earths.
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Image: Andreas Heddergott / TUM
The synthesis is amazingly simple: The prefabricated proteins are simply mixed at room temperature with silver nitrate, caustic soda and sodium sulphide while stirring vigorously. Similar quantum dots are being investigated, among other things, with regard to their suitability as non-toxic contrast media for imaging processes in medicine.
The protein-decorated silver sulfide beads are not toxic to bacteria per se. This changes when the microbes are irradiated with a near-infrared laser. Within 25 minutes, more than 99 percent of the E. coli-Bacteria killed. The quantum dots develop an antibacterial dual effect: if the semiconducting particles absorb laser light, they produce oxygen radicals on the one hand, and on the other hand strong local heat is generated. Both effects together ensure that the cell membranes of the bacteria are destroyed and the genetic material escapes from the cell nucleus – which ends up devastating for the bacteria. The use of quantum dots could therefore be a cheap and biocompatible alternative to previous methods of disinfecting drinking water, according to the researchers led by Jing Zhao.
Mt bacteria recycle rare earths
But bacteria can also be used technically for all sorts of useful purposes, such as the biotechnological synthesis of insulin – with E. coli-Bacteria that have been given the genes to make insulin. Another useful example has now been demonstrated by a group of German scientists led by Thomas Brück from the Technical University of Munich: Cyanobacteria, also known as blue-green algae, are able to absorb and accumulate rare earth metals from their environment, as Brück and his colleagues describe in the “Frontiers in Bioengineering and Biotechnology” report. This so-called biosorption of rare earth metals is so important because these elements are difficult to break down and even more difficult to recycle. The researchers examined twelve strains of cyanobacteria from different habitats with regard to their ability to absorb cerium, lanthanum, neodymium and terbium ions from aqueous solution. After the sorption process, the bacteria are burned, resulting in a metal-enriched ash.
Once again, the bacteria have to believe in it, but not without added value: The scientists have achieved peak values of more than nine percent by weight of rare earth metals in the ash – measured in terms of the total dry matter. Brück and his colleagues have also been able to clarify how the bacteria accomplish this feat: through ion exchange. The ions of the rare earth metals bind strongly to the biomass, replacing ions that are more weakly bound, such as sodium ions. In addition, some bacterial strains produce bio-polymers outside the cell, which are also good binding partners for rare earth ions. Under optimal conditions, the process only takes a few minutes.
The researchers hope that the further development of this process, such as the targeted release of the biosorbed metals without burning the bacteria, will enable technical recovery of rare earth elements in solution in the future. Wastewater from mining or production plants in which rare earth elements are mined or used could be considered.