Using (microchannel) electrochemical impedance spectroscopy to detect bacterial activities
Even though rapid detection of molecular methods has been used in bacterial detection, (automated) liquid culture methods are still the "Gold Standard" for the detection of living bacteria, especially in cases where there may be similar dead bacteria also present in the sample. However, the current liquid culture methods rely on the effects of bacterial metabolism (changes in O2/CO2 levels, pH, etc.) to bring about measurable changes to the suspension. Hence, for slow-growing microorganisms like M. tuberculosis (Mtb), they take a long time (up to 6 weeks) to yield results. To cut the time to positivity, we applied the detection concept of "detection by death" and using our high sensitivity microchannel Electrical Impedance Spectroscopy (m-EIS) method to detect bacteria in various samples rapidly. Viable bacterial cells can be killed in a few hours with very high doses of cidal drugs. m-EIS relies on the fact that high-frequency AC fields cause transient charge-accumulation at live cells' intact membranes. Cell-death in the microchannel causes charges to no-longer be accumulated, and the bulk capacitance of the microchannel will decrease. By picking up this change, we can rapidly detect viable bacterial cells in a testing sample. For viable TB bacteria detection, we use synthetic sputum with gram-positive bacteria, gram-negative bacteria, and M. tuberculosis H37ra. After decontamination (kill all non-TB bacteria), we are able to discern decreases in bulk-capacitance for suspensions containing ~500 CFU/ml of Mtb cells (but not for ~50 CFU/ml). Thus, our turnaround-time (~4 hours) and limit-of-detection ([less than]500 CFU/ml) are comparable to those of GeneXpertTM. Using our m-EIS method, we can also detect Lactic Acid Bacterial (LAB) contamination for ethanol fermentation with a low concentration of 5000 CFU/ml in less than 3 hours. Our detection of limit for LAB has a three-order reduction than the HPLC method, but with a similar detection time. Aside from investigating the bacterial activities in the solution, we also observed the biofilm formation on the electrodes. We have observed that the interfacial capacitance also increases in proportion to the number of cells in the biofilm matrixes. In the future, we believe that we could also record cell death within biofilm using our method, and so researchers will be able to use it to screen new anti-biofilm material and drug rapidly.