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Researchers at Purdue University have used a new technique to rapidly detect and precisely identify bacteria without time-consuming treatments usually required.

The technique, called desorption electrospray ionisation, or DESI, could be used to create a new class of fast, accurate detectors for applications ranging from food safety to homeland security, according to R Graham Cooks, the Henry Bohn Hass Distinguished Professor of Chemistry in Purdue''s College of Science.

Using a mass spectrometer to analyse bacteria and other microorganisms ordinarily takes several hours and requires that samples be specially treated and prepared in a lengthy series of steps. DESI eliminates the pre-treatment steps, enabling researchers to take "˜fingerprints' of bacteria in less than a minute using a mass spectrometer.

"This is the first time we''ve been able to chemically analyse and accurately identify the type of bacteria using a mass spectrometer without any prior pre-treatment within a matter of seconds," Cooks said.

New findings show how the Purdue researchers used the method to detect living, untreated bacteria, including E. coli and Salmonella typhimurium, both of which cause potentially fatal infections in humans.

Mass spectrometry works by turning molecules into ions, or electrically charged versions of themselves, inside the instrument''s vacuum chamber. Once ionised, the molecules can be more easily manipulated, detected and analysed based on their masses.

The key DESI innovation is performing the ionisation step in the air or directly on surfaces outside of the mass spectrometer''s vacuum chamber. When combined with portable mass spectrometers also under development at Purdue, DESI promises to provide a new class of compact detectors.

The researchers are able to detect one nanogram of a particular bacterium. More importantly, the method enables researchers to identify a particular bacterium down to its subspecies, a level of accuracy needed to detect and track infectious pathogens. The identifications are based on specific chemical compounds, called lipids and fatty acids, in the bacteria.

The procedure involves spraying water in the presence of an electric field, causing water molecules to become positively charged "˜hydronium ions', which contain an extra proton.

When the positively charged droplets come into contact with the sample being tested, the hydronium ions transfer their extra proton to molecules in the sample, turning them into ions. The ionised molecules are then vacuumed from the surface into the mass spectrometer, where the masses of the ions are measured and the material analysed.

Such a system could alert employees in the food and healthcare industries to the presence of pathogens and could provide security personnel with a new tool for screening suspicious suitcases or packages.

"This method could be applied very soon because the hardware is already available," Cooks said.