Image credit: Burrus/NIST

Waiting for conventional antimicrobial test results can take days, giving bacterial infections time to progress and develop resistance to drug therapies. A prototype sensor developed by researchers at the National Institute of Standards and Technology (NIST) delivers such results in under an hour, paving the way toward speedier, more appropriate treatments for patients. The sensor uses a quartz-crystal resonator whose vibrations pick up the mechanical fluctuations of bacteria and their response to antibiotics. Researchers in the Scientific Reports journal detailed the technology’s ability to sense fluctuations of nonmotile Escherichia coli (E. coli) after exposure to two types of antibiotics: polymyxin B (PMB) and ampicillin.

E. coli was chosen as the model species partly to compare results from other studies that demonstrated the effects of antibiotics on mechanical fluctuations of motile E. coli, explained the study’s lead author Ward L. Johnson, PhD, a physicist with the Applied Chemicals and Materials Division of NIST’s Material Measurement Laboratory, in Boulder, Colorado. “The use of ampicillin and polymyxin B (PMB) also facilitated comparisons with previous studies.  However, the principal reason for using PMB was that this antibiotic leaves cellular structure mostly intact for an extended period of time after cell death, and this situation enables stronger conclusions to be drawn about the nature of the observed cell-related noise,” Johnson told CLN Stat.

To test the theory that bacterial motion declines with antibiotic exposure, researchers employed the use of two quartz-crystal resonators coated with bacterial cells. One served as a control, Johnson and his colleagues used the other to test the antibiotic’s effects on the cells. They found that frequency noise did correlate with cell death. “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,” according to a statement from NIST.

Ampicillin and PMB were both highly efficient in reducing live E. coli cells. Within 7 minutes of exposure to PMB, cell-generated frequency noise dropped close to zero. With ampicillin, it started to decline within 15 minutes of exposure—dropping even more rapidly once antibiotic-induced lysis took place. Overall, researchers were able to detect cell-generated frequency changes at a level less than 1 part in 10 billion.

Phase noise arising from cells on crystal resonators is an interesting phenomenon that could prove useful for rapid antimicrobial susceptibility testing (AST), Johnson said. “Much remains unknown, with respect to the basic biophysical and mechanical processes involved in the generation of the observed cell-related noise. The applicability of the approach for AST needs to be assessed through extensive studies on a variety of cell types exposed to antibiotics with various modes of action,” he said.

Next steps are to explore differences in noise spectra from motile and nonmotile E. coli before and after antibiotic exposure, to identify relative magnitude of contributions from cell-wall motion, motility, and thermally generated movement, Johnson said.