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Improved Monitoring in Microdroplet Reactors

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Microfluidic systems can provide a novel platform for an array of biological and chemical research, including the cultivation of cells. The successful long term use of these microdroplet reactors relies on the ability to accurately and unobtrusively monitor conditions necessary for the optimum growth of the cells.

Recent research published in Analytical Chemistry1 highlights a new method to rapidly and accurately monitor the rate of oxygen consumption of bacteria growing in individual microdroplets. To learn more about the new method and the implications it could have for the future of drug discovery and resistance, we spoke to Professor Piotr Garstecki, Dr Ott Scheler and Dr Tomasz Kaminski of the Research Group of Microfluidics at the Institute of Physical Chemistry, Polish Academy of Sciences.

AM: What are some of the advantages of cultivating bacteria in microfluidic systems, rather than bulk cultures?

PG: Microfluidic systems are like small compact versions of test-tubes and other laboratory devices we often see in laboratories. There are several advantages of using microdroplets – the most important are following: small volume ranging from pico- to single microliters, large number of compartments (hundreds to millions) and prevention of cross contamination and biofilm formation thanks to the presence of water-oil interface stabilized by surfactants.

AM: How is bacterial growth in microdroplets currently measured, and what challenges are there with these methods?

OS: The growth of bacteria in such microfluidic systems is usually measured optically. For example we can measure changes of fluorescence or optical density in the microfluidic systems that are caused by growth of bacteria. One of the challenges regarding fluorescent detection is the leakage of bacteria viability indicators - fluorescent dyes to the organic continuous phase. In the case of using absorbance or scattering we have to deal with much lower sensitivity and specificity and usually lower throughput.

AM:  Can you tell us more about the oxygen monitoring method you have recently developed and how it overcomes some of these challenges?

PG:  We have developed a system with our friends in Austria that helps us to measure the level of oxygen inside microfluidic droplets. This method helps us to see if bacteria have optimum conditions to grow inside microdroplet reactors.

The technology itself is based on special oxygen-sensitive nanoparticle dyes that have several advantages over previously used techniques for measuring the oxygen concentration in microfluidic systems. Nanoparticles do not leak from the droplets and they are excited by red light and near-infrared (NIR) wavelengths – in this way the background autofluorescence from biological matter is minimized. We do not measure the intensity of phosphorescence but its lifetime which is an intrinsic property of oxygen sensor nanoparticles and is not affected by instrument variations, sample turbidity, or ambient environment.

AM:  What implications does this have for drug discovery?

TK: In general microfluidic systems help us to conduct more experiments at the same time and they are also faster. From the perspective of drug discovery, it helps us to make systems that are useful in the development of new antibiotics by testing new drug candidates and combinations of compounds. Also the technology of nanoparticle oxygen sensors might be of great interest in studies of bacteria adaptation and evolution of drug resistance and sensitivity. We hope that in the future we will be able to execute these long term experiments in microdroplet chemostats circulating in the automated microfluidic chips.

Further information about Prof. Garstecki and his research can be found here.

1Horka, M., Sun, S., Ruszczak, A., Garstecki, P., & Mayr, T. (2016). Lifetime of Phosphorescence from Nanoparticles Yields Accurate Measurement of Concentration of Oxygen in Microdroplets, Allowing One To Monitor the Metabolism of Bacteria. Analytical Chemistry, 88(24), 12006-12012. doi:10.1021/acs.analchem.6b03758

Professor Garstecki, Dr. Scheler, and Dr. Kaminski were speaking to Anna MacDonald, Editor for Technology Networks