By listening to bacteria, you can find out whether they are alive or not. This way you can quickly determine whether antibiotics are working. Delft researchers have developed a technique to make this possible by absorbing the minimal noise of bacteria with graphene.
‘We know that sheets of graphene are very sensitive to external influences,’ says the Delft PhD student Irek Roslon† ‘We wondered what would happen if you put something live on it, like a bacteria. Would we then be able to pick up the movement of such a bacterium?’ Started with that question the Delft research to eavesdropping on bacteria.
Graphene drumhead
Biologists usually study bacteria under a microscope, in a vacuum. Because bacteria cannot survive those conditions, researchers look at dead cells. Graphene makes it possible to study living bacteria. Graphene is an exceptionally thin and strong material consisting of a single layer of carbon atoms, arranged in a chicken wire structure. This makes it possible to place ‘drum skins’ of graphene in a liquid containing living bacteria. ‘The graphene is thin enough to pick up the ‘sound’ of the bacteria’, Roslon explains. “And at the same time it’s strong enough not to tear under the pressure of the liquid.”
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The researchers tested their technique with a liquid containing the common bacteria E. coli† ‘We put a sticky substance on the graphene sheets that traps the bacteria if they get too close to the graphene,’ says Roslon. They poured the liquid containing the bacteria onto the greased drum heads. ‘The bacteria then descended on their own and stuck to the graphene.’
Drumming bacteria
Once stuck, the movement of the bacteria causes the drumhead to vibrate. Those vibrations are tiny: the sheet moves only a few millionths of a millimeter. By shining a laser on it and collecting the reflected light, the researchers can detect these mini-vibrations. They then convert that signal into sound. “It was really cool to hear the sound of a single bacteria for the first time,” Roslon says.
The vibration of the drumhead is mainly caused by the movement of the flagellum, the whiptail of the bacteria. It’s as if the bacteria is hitting the drum with its tail. But bacteria without tails can also make themselves audible, albeit slightly weaker, the researchers discovered. Roslon: ‘We are still investigating how this can be done. All kinds of processes take place in a bacterium; it grows and divides. They may be responsible for the weak sound.’
The researchers are sure that the sound is caused by living bacteria. When they add an antibiotic, the sound signal disappears within one to two hours – the time it takes for the antibiotic to kill the bacteria.
Antibiotic resistance
This means that with the graphene technique you can also test whether bacteria are resistant to a certain antibiotic. If they are resistant, the drug no longer works and they remain alive. This is relevant for the treatment of patients with a bacterial infection.
With current technology, it takes two to three days to find out which antibiotics work for a patient. With the graphene technique this could be done within a few hours. The researchers therefore want to further develop this technology in a start-up company into a commercial machine for hospitals, for example.