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NIST Scientists Measure Cell Mechanical Activity to Determine Antibiotic Efficacy, Resistance

NEW YORK (360Dx) – Researchers from the National Institute of Standards and Technology have devised a method to determine the efficacy of an antibiotic by measuring its mechanical activity, offering a potentially new strategy in the fight against antibiotic resistance.

Citing statistics from the Centers for Disease Control and Prevention, the NIST scientists noted that in the US, at least 2 million illnesses and 23,000 deaths are attributed to antibiotic-resistant bacterial infections each year,

Their approach, described in a study published Friday in Scientific Reports, is based on measuring the mechanical fluctuations of bacteria, and the effect of antibiotics on those fluctuations. A decrease in fluctuation frequency, after the bacteria is exposed to an antibiotic, correlates to an increase in drug efficacy.

The strategy uses quartz crystal resonators that measure surface vibrations caused by microbes and changes to the frequency of the vibrations when the microbes are exposed to an antibiotic. In the study, the scientists said that previous research has suggested that exposure to antibiotics reduced the motion of bacteria, and a few studies demonstrated that the presence of adhered motile E. coli on a microcantilever led to "fluctuating forces and associated displacements of the cantilever."

That led them to hypothesize that "measurements sensitive to mechanical fluctuations of cells" could elucidate the response of bacteria to antibiotics more rapidly than traditional microbial tests, which depend on growing bacterial cell colonies and typically take days.

The NIST approach involved adhering bacterial cells to a resonator, which the researchers said in a statement represented a new way of using "these supersensitive crystals." It works by using a sensor that is piezoelectric, meaning its changes its dimensions when it is exposed to an electric field.

They placed a thin piezoelectric quartz disk between two electrodes, and applied an alternating voltage at a stable frequency near the crystal's resonant frequency to one electrode to "excite the crystal vibrations." At the electrode on the opposite side of the crystal, the NIST team recorded oscillating voltages of the crystal response, which indicated frequency noise resulting from microbial mechanical activity on the crystal surface.

They then conducted proof-of-concept work for their approach by using two quartz crystals coated with millions of bacterial cells. One resonator was used to test the effect of an antibiotic on the cells, while the other was used as a control without an antibiotic.

The team said that they were able to detect frequency fluctuations at a level of less than 1 part in 10 billion. The amount of frequency noise they detected correlated with the density of living bacterial cells, and after the cells were exposed to antibiotics, the frequency noise decreased, the researchers added.

They tested their method on Escherichia coli and two antibiotics with different modes of actions, polymyxin B and ampicillin, and reported that cell-generated frequency noise dropped to almost zero within 7 minutes after the E. coli was exposed to polymyxin B. Meanwhile, frequency noise began to drop 15 minutes after ampicillin was introduced and dropped more rapidly as cells broke apart and died.

"These time scales reflect the normal speeds at which these antibiotics work," the researchers said in a statement.

They determined that their strategy "offers the potential of broad applicability to sensing efficacy of antibiotics through their effects on mechanical cellular activity," they wrote in the study, adding that though their work was limited to one nonmotile gram-negative bacterial species, "the perceived potential of the measurement approach is enhanced when considered in conjunction with results from other labs that demonstrated antibiotic-induced reductions in cellular fluctuations in motile gram-negative (E. coli) and nonmotile gram-positive (S. aureus) species."

However, more work is needed to determine the broad usefulness of their approach, they acknowledged, including testing it on additional bacterial species and antibiotics that work in different ways.

The NIST team has been awarded a US patent for the approach.