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Bacteria with a chromium envelope

Heavy metals have become a huge problem for mankind. The construction of factories and the varnishing of cars leads to the contamination of soils and waters – and therefore also to the poisoning of many organisms, including people. Scientists are trying to use natural means to remove inorganic chemicals from the cycle or at least make them harmless. Dr. Johannes Gescher and his team at the Department of Microbiology at the University of Freiburg have discovered a microorganism that might be able to detoxify chromium, a toxic heavy metal.

“Chromium is in third place on the EU podium,” said Gunnar Sturm, a degree student in Dr. Johannes Gescher’s laboratory, a group of young researchers in the Department of Microbiology at the University of Freiburg. However, winning the bronze medal is no reason to celebrate. Chromium is the third worst of all heavy metals with regard to contaminating soils and waters in Europe. The hexavalent Cr(VI) ion is particularly dangerous. It dissolves easily in water and is taken up by living organisms, including humans, with the water they drink or the food they eat. “In the cells, Cr(IV) acts like a kind of adhesive,” said Gescher. “It makes DNA and proteins stick together, thereby destroying their function.” In addition, the enzyme chromium(VI) reductase reduces chromium (VI) to trivalent Cr(III) in the cell. Cr(III) is less water soluble than Cr(VI) and hence less dangerous. “However, this reaction leads to reactive oxygen species, which destroy the tissue and damage the DNA,” said Sturm adding that “cancer might be a result of chromium intoxication”.

Bacteria with a thick skin

Leucobacter spec. under the electron microscope © Work group Dr. Johannes Gescher

We humans are responsible for the growing chromium concentration in the environment. The heavy metal also occurs naturally in the soil, but the really poisonous quantities come for example from the steel-producing industry, tanneries and varnish remainders. "During a practical course we did at university, we accidentally discovered the bacterium Leucobacter spec., which tolerates high amounts of chromium," said Gescher. "This microorganism is still able to grow at concentrations that are as high as 16 gr chromium per litre of water." For comparison: Scientists normally find about 160 times less chromium in the earth's crust. The well-known bacterium Escherichia coli survives only concentrations that are at least 150 times lower than that. It appears that Leucobacter spec. somehow detoxifies Cr(VI) or at least renders it harmless. Might it be possible sometime in the future to exploit this bacterium's strategy for the bioremediation of soil using natural processes?

Gescher and his student, Sturm, have since found out exactly why their bacterium tolerates such high quantities of chromium. It is already clearly visible under the light microscope: Exposed to high chromium concentrations, the bacteria produce a slimy layer around the individual bacterial cells, they cluster together and form aggregations. The slimy layer consists of long-chain and branched sugar molecules, the so-called polysaccharides. This layer is particularly thick in Leucobacter spec. bacteria and serves as a trap for intruding Cr(VI) ions. As a result, only small amounts of chromium enter the cells, where they are reduced by the enzyme Cr(VI) reductase and rendered insoluble.

Formation of bacterial aggregations under the electron microscope: With increased Cr(VI) concentrations (yellow scale), more and more Leucobacter cells bunch together and form aggregations.
Formation of bacterial aggregations under the electron microscope: With increased Cr(VI) concentrations (yellow scale), more and more Leucobacter cells bunch together and form aggregations. © Work group Dr. Johannes Gescher

DNA as toxin trap?

Gescher and his team were very surprised to see that this adhesive layer also contained DNA molecules. This extracellular genetic material currently appears in many scientific publications. Nobody knows what kind of function it has and how it is able to access the sugar matrix. Some scientists assume that some bacterial cells die under the influence of chromium, bursting and releasing their DNA into the environment. “Under the fluorescence microscope we found that the aggregations contained only living cells,” said Gescher. Another hypothesis is that the cells are able to actively funnel extracellular DNA out of the cell. “DNA is also a kind of glue that traps Cr(VI) ions,” said Gescher. “We also assume that it helps to keep the aggregations together.” When Sturm added DNAse, an enzyme that cleaves DNA, to the aggregations the individual cells detached from each other and the slimy protective layer disintegrated.
It is still unclear which signalling mechanisms lead to the formation of the layer consisting of polysaccharides and DNA. Gescher and his team’s first goal is to find out the chemical form of chromium occurring inside and in the environment of the bacterial cells. In addition, they are also focusing on the issue as to whether their bacterium will at some stage be able to purify Cr-contaminated soils and waters. Sturm plans to carry out the first experiments as part of his degree thesis by placing the bacteria in a sand column and running Cr(VI) solutions through them. If only small heavy metal concentrations come out of the column, this would indicate that the microorganisms retained a major proportion of ions in their slime layer. “In a second step we will then need to find out how this contaminated layer can be removed so that the bacteria can form a new layer and be used again as decontaminators,” said Gescher.

Further information:

Dr. Johannes Gescher
Institute of Biology II
Schänzlestr. 1
79104 Freiburg
Tel.: +49 (0)761/203-2685
Fax: +49 (0)761/203-2626
Email: johannes.gescher(at)biologie.uni-freiburg.de

Website address: https://www.biooekonomie-bw.de/en/articles/news/bacteria-with-a-chromium-envelope