This exquisitely sensitive machine can hear a single bacterium die

Extremely sensitive listening devices could be a critical tool for doctors facing the growing crisis of antibiotic-resistant bacteria. 

Imagine looking at a cross-section of a single strand of hair under a microscope. At its widest point, you could place roughly 100 E. coli bacteria end-to-end on the cut surface. These microbes are so small. They're also incredibly dangerous. Each year, E. coli kills hundreds of thousands of people and sickens hundreds of millions more. And like plenty of other pathogens, E. coli is growing immune to our stock of antibiotics far faster than we're developing new ones.

That’s why it could be big news that a team of researchers has developed a new way to determine if an antibiotic has succeeded in killing a sample of E. coli. The super-sensitive listening device relies on a material called graphene to capture the unbelievably faint sound of E. coli bacteria trying to move around — or of dead E. coli bacteria sitting in silent stillness.

The team describe the technique in a paper published Monday in the peer-reviewed journal Nature Nanotechnology.

A thin sheet of atoms can hear the impossibly faint sound

Graphene has been hailed as a revolutionary material for everything from solar cells to smartphone screens. It’s “a form of carbon consisting of a single layer of atoms,” says researcher and engineer Farbod Alijani, who is one of the inventors of the listening device. Graphene is a big deal in the material sciences world. The researchers who made the first potentially useful version of the stuff in 2004 won the Nobel Prize for their discovery just six years later.

“It’s very strong with nice electrical and mechanical properties,” Alijani says. Some have called graphene this century’s “wonder material” because of those qualities, but Alijani and his colleagues were interested in something else. “It’s also extremely sensitive to external forces,” he says. That makes it perfect for detecting movement from the tiny, hair-like appendage — called a flagellum — that an E. coli bacterium uses to move through the world.

“To understand how tiny these flagellar beats on graphene are, it’s worth saying that they are at least 10 billion times smaller than a boxer’s punch when reaching a punch bag,” Alijani explains. “Yet, these nanoscale beats can be converted to sound tracks and listened to.”

The invention could one day help doctors navigate antibiotic resistance

Alijani collaborated with nanobiologist Cees Dekker to design the system. Even their early experiments showed the researchers they were onto something.

“What we saw was striking! When a single bacterium adheres to the surface of a graphene drum, it generates random oscillations with amplitudes as low as a few nanometers that we could detect," Dekker says. "We could hear the sound of a single bacterium!” 

As the researchers continued working, it became clear that the graphene-enabled listening device would be useful in figuring out if an antibiotic had what it took to clear a bacterial infection. If the strain of E. coli is resistant to the drug in question, then graphene drum registers no change in the sound of the flagella beating against the thin layer of carbon atoms. If the antibiotic does its job, the vibrations get slower and fainter until the noise has completely stopped.

Alijani says the team plans to optimize the platform and “validate it against a variety of pathogenic samples. Their ultimate goal is to create “an effective diagnostic toolkit for fast detection of antibiotic resistance in clinical practice.”