Kuper is considered a natural disinfectant that reliably kills harmful germs under normal circumstances. Therefore, for example, door handles in hospitals are often coated with the precious metal. However, researchers have now shown that bacteria, including the multi-resistant hospital germ MRSA, can develop mutations under certain conditions that enable them to survive on the antimicrobial surface. For hospitals this means that regular additional disinfection measures are necessary to prevent such mutations.
Bacterial infections are usually treated with antibiotics. However, many pathogenic bacteria have developed increasingly strong resistance to common drugs over the past few decades. So-called multi-resistant bacteria, which can no longer be combated with most antibiotics, are particularly feared. This includes, for example, the germ of methicillin-resistant Staphylococcus aureus (MRSA), which is responsible for many protracted hospital infections. Copper surfaces, for example on doorknobs, are considered an effective measure against the spread of such bacteria. Thanks to their antimicrobial properties, they prevent bacteria from surviving on them and being passed on the next time they are touched.
Tolerance development in the test
A team led by Pauline Bleichert from the Bundeswehr Institute for Microbiology in Munich has now investigated to what extent bacteria can defy such actually deadly surfaces. To do this, they used two typical types of bacteria: the intestinal bacterium Escherichia coli and the hospital germ MRSA. Co-author Dietrich Nies from Martin Luther University Halle-Wittenberg explains: “Copper surfaces are real bacteria killers. Most bacteria die within a few minutes if they land on a copper surface. “
In the laboratory, the researchers specifically created conditions that are suitable for the development of resistances and tolerances: they initially only put the bacteria briefly on the surfaces, took them down again and allowed them to recover in a nutrient medium. They repeated this process several times, gradually exposing the survivors to the surface that was actually deadly for them.
And indeed: within three weeks, bacteria developed in this way that could survive on copper for more than an hour. “The conditions outside of the laboratory are of course not ideal,” says Nies. “If copper surfaces are not cleaned regularly, however, insulating layers of fat can form on them, which could enable a similar development over a longer period of time.” But how did the bacteria manage to survive on the copper surface? The mechanism was apparently not based on a reduced uptake of the deadly copper ions. These accumulated in the mutated bacteria as well as in their intolerant ancestors. But only with the non-mutated bacteria were the copper ions able to destroy the cell structures and thus kill the bacteria.
Bacteria in sleep mode
In order to clarify what is the basis of this tolerance, Bleichert and her colleagues analyzed the bacteria’s genetic makeup for indications of possible resistance. “We did not find a gene for resistance to the deadly effects of metallic copper surfaces,” reports Nies.
Instead, the team observed a phenomenon in the surviving bacteria that has been known in a similar form for some time: They reduce their metabolism to the absolute minimum and fall into a kind of hibernation.
Since antibiotics usually aim to disrupt the metabolism of growing bacteria, they are almost ineffective on these special bacteria, which are also known as persisters. “No matter how well an antibiotic works, there are always a handful of persister bacteria in every generation,” explains Nies. However, these are not antibiotic-resistant bacteria, because their offspring are very likely to be susceptible to the drugs again. Usually only a very small fraction of the bacteria will persist. In the case of the bacteria isolated in the experiment, however, it was the entire population. These could grow just as quickly as their ancestors, but under adverse circumstances save themselves into a state of persistence faster than these. “The bacteria have passed on this property for over 250 generations, even though they no longer came into contact with copper surfaces,” says Nies.
The team therefore recommends cleaning copper surfaces regularly and thoroughly with special agents so that no persister bacteria can develop. In addition, copper surfaces are only one of many means to take targeted and effective action against harmful bacteria. They can complement antibiotics and hygiene measures, but not replace them.
Source: Pauline Bleichert (Institute for Microbiology of the Bundeswehr, Munich) et al., Applied and Environmental Microbiology; doi: 10.1128 / AEM.01788-20