Advancing Antimicrobial Susceptibility Testing using Microfluidic-Based Electrical Impedance Measurements

Antimicrobial resistance (AMR) has emerged as a global health and development threat, with the World Health Organization highlighting it as a major concern. Rapid point-of-care antimicrobial susceptibility tests (ASTs) play a crucial role in combatting AMR, as they enable informed and targeted antibiotic therapy. However, the development of ASTs has faced challenges, particularly in determining which bacterial characteristics should be monitored to achieve minimal response times. Traditionally, ASTs have relied on slow methods such as the Kirby-Bauer test, which requires overnight incubation as it monitors population-averaged growth. Techniques like mass spectrometry and polymerase chain reaction (PCR) offer potential but come with limitations, including cost and complexity. Monitoring changes in cell morphology and single-cell growth are promising approaches but require high-magnification equipment. Bacterial motility is also a good indicator or antibiotic susceptivity, but its use is limited to motile strains.

In this context, bacterial metabolism emerges as a fundamental property that can serve as an indicator of antibiotic efficacy. The close relationship between bacterial metabolism and antibiotic action is well-documented, making it an ideal candidate for rapid susceptibility testing. This metabolic response can be measured using electrical impedance, a technique first employed by George Stewart in 1899. Several impedance-based methods have been developed since, with interdigitated electrode (IDE) platforms being among the most notable. However, these platforms still exhibit relatively long response times due to their reliance on diffusion processes over hundreds of microns, if not millimetres.

Now, a new study conducted by Dr. Giampaolo Pitruzzello, Professor Steven Johnson, and Professor Thomas F. Krauss at the University of York in the United Kingdom, achieves a substantial reduction in detection time, approaching the fundamental time limit for a practical AST. In the study, published in Biosensors and Bioelectronics, the authors develop an impedance-based technique that monitors bacterial metabolism at the near-single-cell level, significantly advancing the field of antimicrobial susceptibility testing.

The researchers fabricated microelectrodes on glass microscope slides and designed a microfluidic chip for bacterial trapping and electrical impedance measurements. They optimized the device to ensure rapid bacterial loading, even at low concentrations potentially compatible with clinically relevant samples. Bacterial cultures were prepared and loaded into the microfluidic channels, which allowed real-time monitoring of bacterial growth and metabolic changes.

The authors tested the microfluidic device and found it efficiently trapped bacteria, even at low concentrations, within 15 minutes, and bacterial viability was maintained inside the device for several hours. They also integrated micrometre-sized electrodes below the channels, allowing them to measure changes in electrical resistance resulting from bacterial metabolism in the immediate vicinity of single bacteria. They observed a decrease in resistance when bacteria were viable and actively growing, with the rate of change depending on the growth medium. Interestingly, when bacteria overlapped with the electrodes, the resistance increased, as the bacterial cell body acted as an insulator. This effect allowed the researchers to detect individual bacteria in the channels. Once bacteria were trapped, the researchers exposed the bacteria to various antibiotics, including kanamycin, polymyxin B, and ampicillin, and monitored the changes in electrical resistance. Their results showed distinct electrical signatures for susceptibility and resistance to these antibiotics happening as soon as 30 minutes after exposure to the drugs. Because the system probes electrical changes within a few microns around the bacteria, this detection time is limited only by the time of action of the antibiotics, therefore suggesting that 30-60 minutes represents the fundamental detection time limit for a practical AST. In addition, the electrical measurements could inform the minimum inhibitory concentration (MIC) values, demonstrating the potential clinical relevance of this technique.

In a statement to Medicine Innovates, Professor Thomas Krauss said “What excites us most about this work is that we have been able to monitor the bacterial metabolism directly, and for a single bacterium only – this provides the fundamental lower limit for antibiotic time of action, which is 30-60 mins. Understanding this lower limit is essential for the design of future antimicrobial susceptibility tests “. In conclusion, the new study introduced a new microfluidic-based method for rapid antimicrobial susceptibility testing. This technique allows for the electrical detection of individual bacteria and their metabolic responses to antibiotics, offering a near-single-cell level of sensitivity. The implications of this research are significant. It provides a universal approach to antibiotic susceptibility testing that may not be limited to specific bacterial strains or modes of action. The real-time monitoring capabilities, combined with the ability to detect changes in electrical resistance within minutes, represent an important advancement in the field by approaching the fundamental limit for an AST. Moreover, this method has the potential to eliminate the need for pre-culturing bacteria, which is a common requirement in existing ASTs. The ability to work with clinically relevant concentrations of bacteria and the capacity to inform MIC values further underscore the clinical utility of this technique. Furthermore, while the study focuses on E. coli and a select group of antibiotics, the principles established here have broader implications. This method could potentially be applied to a wide range of bacteria-antibiotic combinations, including clinical strains, paving the way for more targeted and effective antibiotic therapies. Overall, the innovation has the potential to become a state-of-the-art approach in antimicrobial susceptibility testing, offering a new level of speed and sensitivity in addressing the global threat of AMR.