Burkholderia pseudomallei is considered one of the most dangerous bacterial pathogens in the tropics. The disease melioidosis, caused by these bacteria, is often severe and can be fatal even with treatment. "Almost 170,000 new infections are reported worldwide every year, and about half of those affected die from it," reports Jonas Fiedler. The doctoral researcher is the first author of the publication and works in the team of Christian Hertweck, Professor of Natural Product Chemistry at Friedrich Schiller University Jena and head of the study at the Leibniz Institute for Natural Product Research and Infection Biology (Leibniz-HKI).
The pathogen is dangerous because of the toxin malleicyprol, which attacks the cells and causes the disease. "This is due to a small, highly reactive chemical structure in the molecule, the so-called cyclopropanol ring," explains Fiedler.
A previously overlooked gene codes for an enzyme that destroys the reactive part of the molecule
Although malleicyprol is an important factor in the virulence of Burkholderia species and its biosynthesis was largely understood, the function of one enzyme remained unclear: "We noticed a small gene that codes for an unknown protein. However, we were unable to assign this gene product any function in toxin formation. We wanted to close this gap and specifically switched off the gene to understand its role," recalls Fiedler.
Although the bacteria continued to produce the toxic malleicyprol, an inactive variant of the molecule was suddenly missing. "The gene must therefore encode an enzyme that converts the toxin into this harmless form," said Fiedler.
The researchers were now interested in how exactly the enzyme – called BurK – changes the molecular structure. In the process, they discovered a remarkable mechanism: BurK uses iron-containing compounds to generate highly reactive particles (radicals). These split the cyclopropanol ring, which is crucial for toxicity, and thus render malleicyprol harmless. "That was a real surprise," says Fiedler. "No enzyme in nature was previously known to specifically cleave a cyclopropanol ring." He goes on to explain: "Of course, the bacterium does not defuse the toxin to protect humans. Rather, it regulates the amount of toxin with the help of the BurK enzyme."
Protection in the model organism
To test whether BurK also works in a living system, the research team inserted the responsible gene into E. coli bacteria and then brought them together with nematodes – tiny threadworms – that were also administered the toxic malleicyprol. "The worms that ingested the toxin together with bacteria containing BurK were able to survive better," reports Fiedler. Control worms that received the toxin and bacteria without the enzyme died because the toxin remained effective. This showed that BurK can also neutralize malleicyprol in living organisms.
The researchers discovered very similar genes in other bacterial species, suggesting that the enzymes formed could play an important role in interaction with other organisms. Some microorganisms could thus potentially protect themselves against toxins from other bacteria or even protect symbiotic partners – such as nematodes – from harmful malleicyprol.
Genetically modified bacteria against pathogens?
Even though the exact function of these enzymes in nature is still unclear, practical applications for humans are conceivable: "The bacterium we have generated could be used therapeutically to neutralize malleicyprol. However, its transferability to human infections still needs to be thoroughly investigated," says Fiedler. A more realistic initial application would be in the environment, for example in regions where Burkholderia bacteria occur naturally in the soil: "Affected soils could be decontaminated to reduce toxic effects," says Fiedler. "This would also have to be thoroughly tested first."
In any case, the research team shows that nature has an amazing repertoire of tools, many of which are still hidden from humans. The BurK enzyme is a remarkable example of this. Lead researcher Christian Hertweck sums up: "Our work shows that it is possible to specifically neutralize the danger of a pathogen without having to kill it directly. This opens up new perspectives for the future treatment of antibiotic-resistant bacteria and could become part of novel therapies in the long term."
The study was conducted as part of the Cluster of Excellence "Balance of the Microverse" and the Collaborative Research Center ChemBioSys and was funded by the German Research Foundation (Deutsche Forschungsgemeinschaft, DFG).
