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Evolution in a test tube: These bacteria live on deadly copper surfaces

Descendants of normal, wild-type bacteria can develop to survive for a long time on metallic copper surfaces that usually kill them within a few minutes. An international research team led by Martin Luther University Halle-Wittenberg (MLU) and the Bundeswehr Institute for Microbiology were able to produce these young survivors in the laboratory and study them more closely. The team reports on its findings in Applied and Environmental Microbiology.

Bacterial infections are usually treated with antibiotics. However, in recent decades, many disease-causing bacteria have developed increased tolerance to common drugs. The so-called MDR bacteria are of particular concern as they can no longer be controlled with most antibiotics. Copper surfaces – for example on door handles – are a good weapon to fight these germs. Copper surfaces are a surefire way to kill bacteria. Professor Dietrich H. Ness, a microbiologist at MLU, explains that most bacteria die within minutes after landing on a copper surface. Copper is a vital trace element for bacteria – but only in very small amounts. On copper surfaces, bacteria are literally drenched to death with copper ions because they can no longer fend off them using normal defense strategies.

Nies’ research team wanted to see if two typical bacteria, Escherichia coli and Staphylococcus aureus, were theoretically able to survive on copper surfaces, and how quickly. So the team placed the bacteria on the surfaces for only a few minutes before returning them to a normal culture medium where they were allowed to recover. This process was repeated many times, with survivors gradually exposed to the dead surface for longer and longer periods of time. Within three weeks, the researchers produced bacteria that could live for more than an hour on the surface of the copper. Outside the lab, conditions are clearly not ideal. But if copper surfaces are not cleaned regularly, insulating layers of grease can start to form on them, which could result in a similar development over time, ”says Ness.

Using comprehensive genetic analyzes, the team sought to understand why bacteria on surfaces did not die. “We couldn’t find the gene that makes it resistant to the lethal effect of metallic copper surfaces,” says Ness. Instead, the team noticed a phenomenon among the surviving bacteria that had already been known for some time, albeit in a slightly different way: the bacteria’s metabolism slowed to a minimum, and they fell into a kind of hibernation. Since most antibiotics aim to disrupt the metabolism of the developing bacteria, they are almost completely ineffective against these special bacteria, which are also known as “inhibitors”. “No matter how effective an antibiotic is, there are always a few inhibitors in every generation,” explains Ness. However, these bacteria are not considered resistant to antibiotics, because their offspring are again vulnerable to the drugs.

Usually only a small percentage of bacteria become episodes. However, in the case of isolated bacteria, the population was all. Although they were able to grow at the same speed as their ancestors, they also managed to save themselves by rapidly turning into an early state of perseverance under adverse conditions. Scientists were concerned about one additional thing they noticed: “The bacteria also inherited this ability over 250 generations, even though the offspring did not come into contact with the surface of the copper,” says Ness. So the team recommends that you clean copper surfaces regularly and thoroughly using special agents so that no buffer bacteria will develop in the first place. At the same time, Ness notes that using copper surfaces is just one of many ways – including antibiotics – to effectively combat harmful bacteria.

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Bleshert et al. Generation and analysis of mutant strains of Escherichia coli and methicillin-resistant Staphylococcus aureus obtained by laboratory selection for survival on metallic copper surfaces. Applied and Environmental Microbiology (2021). Doi: 10.1128 / AEM.01788-20

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