Crowd control

A device that can concentrate, manipulate and separate a wide class of swimming bacteria has been developed by a team of scientists at the US Department of Energy's Argonne National Laboratory, Illinois Institute of technology, University of Arizona at Tucson and Cambridge University.

It could have applications in biotechnology and biomedical engineering, including use in miniaturised medical diagnostic kits and bioanalysis.

The device uses a small electric current in a very thin film sample cell containing a colony of bacteria. The current produces electrolysis that changes the local pH level in the vicinity of the electrodes. The bacteria, uncomfortable with the changes in pH, swim away from the electrodes and ultimately congregate in the middle of the experimental cell. Concentrated bacteria form self-organized swirls and jets resembling vortices in vigorously stirred fluid.

The method, which is suitable for flagellated bacteria such as E. coli, Bacillus subtilis, among many others, relies on the ability of bacteria to swim toward areas of optimal pH level.  The bacteria live in an environment of a specific pH level, so that an increase or decrease of pH stimulates the bacteria to avoid areas of non-comfortable pH and swim in the direction of pH gradient.

Since only living bacteria respond to the pH stimulation, the device can separate living and dead cells or bacteria with different motility. It can be used for the purposes of express bioanalysis, diagnostic and identification of small bacterial samples, and separation sicken/live cells.  A patent for the device is currently pending.

'Using this method, our research succeeded in dramatically increasing the concentration of microorganisms in tiny fluid drops and films. Unlike traditional centrifuging techniques, the new approach allows selective concentration of healthy cells,' said Andrey Sokolov, PhD student from Illinois Institute of Technology and contributor to the research.

A thin film containing bacteria spans two adjustable platinum wires and two dielectric fibres and is stretched by a control screw. Electric current is transmitted between the platinum wires and a small platinum ring lowered onto the film.