A patient admitted to a hospital with a serious bacterial infection may have only a few hours to live. Figuring out which antibiotic to administer, however, can take days. Doctors must grow the microbes in the presence of the drugs and see whether they reproduce. Rush the process, and they risk prescribing ineffective antibiotics, exposing the patient to unnecessary side effects, and spreading antibiotic resistance. Now, researchers have developed a microscopic "tuning fork" that detects tiny vibrations in bacteria. The device might one day allow physicians to tell the difference between live and dead microbes—and enable them to recognize effective and ineffective antibiotics within minutes.
"It's a brilliant method," provided subsequent investigations confirm the researchers' interpretation of their data, says Martin Hegner, a biophysicist at Trinity College Dublin who was not involved in the work.
The research involves tiny, flexible bars called cantilevers that vibrate up and down like the prongs of a tuning fork when they receive an input of energy. Cantilevers are an important part of atomic force microscopy, which is useful for making atomic scale resolutions of surfaces for use in nanotechnology or atomic physics research. In this technique, a minute needle attached to a cantilever moves across a surface, and the deflection of the cantilever gives information about how atoms are arranged on the surface. It can even be used to shunt atoms around. More recently, however, they have been used without the needle as tiny oscillators, allowing scientists to investigate matter directly attached to the cantilever.
Biophysicist Giovanni Longo and colleagues at the Swiss Federal Institute of Technology in Lausanne and the University of Lausanne in Switzerland immersed these cantilevers in a liquid bacterial growth medium and monitored their movement using a laser. They found that the bare cantilever moved very slightly as a result of the thermal movement of the liquid molecules in the medium. They then covered both sides of the cantilever with Escherichia coli bacteria, which can cause food poisoning, and immediately found that the oscillations became much more pronounced. The researchers believe that chemical processes that occur inside the bacteria as they metabolize energy are driving the oscillation. "What we see is that if you have some sort of a moving system on the cantilever, you are going to induce a movement on the cantilever itself," Longo explains. "Exactly what kind of metabolic movement we see is something that we are still studying."
To determine if the cantilevers could detect the impact of drugs, the team added ampicillin, an antibiotic that the cultured bacteria were sensitive to. The size of the cantilever's oscillations decreased almost 20-fold within 5 minutes, the researchers report online today in Nature Nanotechnology. Fifteen minutes later, the scientists flushed the antibiotic out with fresh growth medium, but the movement of the cantilever did not increase again. This, the researchers say, suggests that the antibiotic had killed the bacteria. When they used an ampicillin-resistant strain of E. coli, however, they found that the oscillations initially decreased but returned to normal within about 15 minutes, indicating that the microbes had recovered.
Hegner cautions that the research is still "basic science. … It's not yet an applied tool which is robust enough to be used in an ER or something." That, he says, might take another 5 or 10 years.
Before that happens, Hegner says, researchers need to determine what the sensors are picking up and whether that signal can be conclusively linked to the bacteria and their antibiotic resistance. They also need to find out if properties of the medium affect the results, he says. "If you inject a bacterium into a medium with different viscosity and different density, this also might affect the vibration of the sensor."
The Swiss researchers are continuing to investigate clinical applications of their system. They have recently obtained access to a more secure lab licensed to handle highly pathogenic bacteria and are working on confirming their results in these microbes. They are also thinking beyond the clinic. "Our dream is to send something like this to Mars to see if there is life," Longo says. "It's much faster than any other technique one can imagine—you just put some of the martian dirt inside the liquid and whatever attaches to the cantilever, if it moves it's alive."