Piezoelectric Sensor Measures Antibiotic Efficacy

National Institute of Standards and Technology (NIST; Gaithersburg, MD, USA), is extremely sensitive, and can detect the mechanical motion of microbes adhered to it, and their response to antibiotics. The sensor is composed of a thin piezoelectric quartz disk 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.

At the other electrode (on the opposite side of the crystal), oscillating voltages resulting from crystal response can be recorded; the fluctuations in the resonant frequency result from microbial mechanical activity of the several million bacterial cells coupled to the crystal surface. The ultra-sensitive approach can enable detection of cell-generated frequency fluctuations at a level of less than one part in 10 billion, with the amount of frequency noise generated correlating with the density of the living bacterial cells.

When E. coli bacteria were exposed to different antibiotics, the sensor showed that the frequency noise from the bacteria fell to zero within seven minutes of being treated with polymyxin B, and within 15 minutes of receiving ampicillin; the results mirrored the normal pharmacokinetics of the antibiotic drugs. The researchers added that since they used bacteria with paralyzed flagella, they concluded that the frequency fluctuations resulted from vibrations of cell walls. The study was published on September 22, 2017, in Nature Scientific Reports.

“Current tests require colonies of bacteria to be cultured for days, which can allow an improperly treated infection to advance and give the bacteria a chance to develop drug resistance,” concluded lead author Ward Johnson, PhD. “The NIST sensor is a quartz-crystal resonator that vibrates differently when bacterial cells on its surface change their behavior; it detects the mechanical motion of microbes to gauge a response to antibiotics…the amount of frequency noise emitted by the bacterial cells increased with the density of bacteria.”

Piezoelectricity, discovered in 1880 by French physicists Jacques and Pierre Curie, is a reversible effect in crystals that describes the internal generation of an electrical charge resulting from a mechanical force. For example, lead zirconate titanate crystals will generate measurable piezoelectricity when their static structure is deformed by about 0.1%. Conversely, the same crystals will change about 0.1% of their static dimension when an external electric field is applied to the material. The inverse piezoelectric effect is used in the production of ultrasonic sound waves.

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