Rapid detection of bacterial proliferation in food samples using microchannel impedance measurements at multiple frequencies

Abstract

We present a novel method for detecting viable bacteria in suspensions such as milk and apple juice. Underlying the technique is the fact that bacteria in aqueous suspensions can store a large amount of charge, and thus act like (non-ideal) capacitors. Thus increased numbers of bacteria due to proliferation increases the capacitance of the bulk of the suspension. However, this increase cannot be directly measured since the capacitance of the solid-liquid interface (''double layer'') in effect ''screens'' the latter. We present a method (derived from an earlier one) that is able to discern such changes with high sensitivity and robustness. We also demonstrate its ability to monitor food quality/safety by detecting bacterial proliferation in ''real world'' liquid food samples like milk and apple juice. We are able to detect * 1, 10, 100, and 1000 CFU/mL of E. coli in milk in about 4.5, 3, 2, and 0.5 h, respectively. For the same initial loads, the corresponding times to detection (TTDs) for Lactobacillus in apple juice are approximately 8, 6, 4, and 1 h. These represent a greater than 4-fold reduction in TTD when compared to automated systems on the market such as RABIT, Bactometer etc. We can achieve such low TTDs for low initial loads since, due to the much greater effective charge holding capacity of bacterial cells (compared to surrounding media), we are able to detect a change in the overall bulk capacitance of the suspension as the bacterial numbers cross a threshold of around 500 CFU/mL.

Electrical impedance spectroscopy (EIS) for biological analysis and food characterization: a review

“…The obtained results showed that, compared with those based on liquid media, the proposed biosensor had comparable performance in spite of its simpler fabrication and portability. Puttaswamy and Sengupta produced a capillary microchannel with gold-coated electrodes (Puttaswamy and Sengupta, 2010) that, thanks to the increased electrolyte resistance, was able to monitor the medium capacitance C m with a multi-frequency approach (1 kHz to 1 MHz) allowing a drastic reduction in response time (4 times faster than traditional IM systems on the market). A microfluidic biochip was presented by GomezSjoberg et al (2005) that uses dielectrophoresis (DEP) to separate bacterial cells from the supporting electrolyte and trap them inside a small chamber (400 pL) by DEP forces.…”

Novel Electrical Method for Early Detection of Viable Bacteria in Blood Cultures

“…The technique needs to be refined and made more user-friendly (by automating the aliquot collection, impedance measurement, and data analysis) and tested extensively against a panel of organisms encountered in bloodstream infections before it can be actually tested in a clinical setting. Our earlier work with multiple bacteria in food substrates (8) suggests that the threshold concentration is likely to be similar for other bacteria, but because their doubling times are different, their TTPs (for the same initial loads) will differ, with slower-growing bacteria having correspondingly longer TTPs (as is also the case for the Bactec system). However, since bacteria with lower metabolic rates also have correspondingly longer doubling times (2,14), we expect to see a similar 4-to 10-fold decrease in TTP for other bacteria as well.…”

“…Conversely, an increase in the bulk capacitance over time can serve as a signature for the presence of proliferating (viable) bacteria. We have previously demonstrated our ability to detect multiple species of bacteria in food substrates (milk and apple juice) by using this approach (8). In the present work, we aim to show that it can be applied to blood culture broth as well.…”

“…The latter was loaded into a Bactec 9240 continuous monitoring blood culture system, and the TTP was recorded. The former was assayed electrically by using our method (8). Appropriate controls were also included with each set of experiments conducted.…”

“…The other was loaded into a specially designed microfluidic cassette and assayed electrically. Details of the cassette design and electrical assay method are provided in our earlier work (8). Briefly, the key aspect of the cassette design is the confinement of the sample in a long, narrow channel with electrodes at the ends that enables the charge storage in the bacteria to contribute significantly to the measured impedance.…”