



National Institute of Standards and Technology (NIST) scientists employing the use of quartz crystals to sense bacterial vibrations is a phrase one might expect to read in a new age spiritualism magazine, not in a peer-reviewed scientific journal. Yet, the NIST investigators have recently demonstrated a potential new tactic for rapidly determining whether an antibiotic combats a given infection, thus hastening effective medical treatment and limiting the development of drug-resistant bacteria. This new technique method can quickly sense mechanical fluctuations of bacterial cells and any changes induced by an antibiotic.

Findings from the new study—published recently in Scientific Reports in an article entitled “Sensing Bacterial Vibrations and Early Response to Antibiotics with Phase Noise of a Resonant Crystal”—showed that the prototype sensor can provide results in less than an hour, much faster than conventional antimicrobial tests, which typically require days to grow colonies of bacterial cells.

“The speed of conventional antimicrobial susceptibility testing (AST) is intrinsically limited by observation of cell colony growth, which can extend over days and allow bacterial infections to advance before effective antibiotics are identified,” the authors noted.

Improperly prescribed antibiotics and antibiotic-resistant bacteria pose serious threats to public health. At least 2 million illnesses and 23,000 deaths are attributed to antibiotic-resistant bacterial infections in the United States each year, according to a 2013 report from the Centers for Disease Control and Prevention.

The NIST team decided to peruse a novel microbial sensing approach, based on a quartz-crystal resonator whose vibrations vary in measurable ways when particles on the surface change. This method, which involves bacterial cells adhered to a resonator, represents a new way of using these supersensitive crystals. Moreover, the new technique senses the mechanical motion of microbes and their response to antibiotics. Other researchers previously found that some bacterial motion becomes weaker in the presence of some antibiotics, but until now such changes have been detected only with microscale sensors and generally in motile bacteria (propelled by threadlike appendages called flagella). The NIST method may be more useful in clinical settings because it collects electronic data cost-effectively and, since it senses large bacterial colonies, can be macroscopic and robust.





“Bacteria are adhered to a quartz crystal resonator in an electronic bridge that is driven by a high-stability frequency source. Mechanical fluctuations of cells introduce time-dependent perturbations to the crystal boundary conditions and associated resonant frequency, which translate into phase noise measured at the output of the bridge,” the authors wrote. “In experiments on nonmotile E. coli exposed to polymyxin B, cell-generated frequency noise dropped close to zero with the first spectra acquired 7 minutes after introduction of the antibiotic. In experiments on the same bacterial strain exposed to ampicillin, frequency noise began decreasing within 15 minutes of antibiotic introduction and proceeded to drop more rapidly with the onset of antibiotic-induced lysis.”

After the sensor measurements, the effectiveness of the antibiotics was confirmed by growth of colonies from the remaining bacteria. Both antibiotics greatly reduced the numbers of live cells.

The sensor is piezoelectric, which means its dimensions change when exposed to an electric field. A thin piezoelectric quartz disk is sandwiched between two electrodes. An alternating voltage at a stable frequency near the crystal's resonant frequency is applied to one electrode to excite crystal vibrations. From another electrode on the opposite side of the crystal, researchers record oscillating voltages of the crystal response, a signal that shows fluctuations in the resonant frequency (or frequency noise) arising from microbial mechanical activity coupled to the crystal surface.

This ultrasensitive approach enabled detection of cell-generated frequency fluctuations at a level of less than one part in 10 billion. The experiments showed that the amount of frequency noise was correlated with the density of living bacterial cells. When the bacteria were then exposed to antibiotics, frequency noise sharply decreased. Bacteria with paralyzed flagella were used in the experiments to eliminate effects of swimming motion. This enabled the researchers to conclude that the detected cell-generated frequency fluctuations arise from vibrations of cell walls.

The investigators were excited by their findings. However, they noted that to determine how broadly useful the technique might be, further studies will be needed using a number of bacterial species and antibiotics that work in different ways. “These results provide evidence that cell death can be sensed through measurements of cell-generated frequency noise, potentially providing a basis for rapid AST,” the authors concluded.























