Technique that quickly identifies bacteria has applications in food safety, health care, and homeland security.

Researchers at Purdue University have a new technique for rapidly detecting and precisely identifying bacteria, including dangerous E. colt bacteria, without the time-consuming treatments that are usually required.

Called desorption electrospray ionization (DESI), the technique could be used to create a new class of fast, accurate detectors for applications ranging from food safety to homeland security, said R. Graham Cooks, the Henry Bohn Hass Distinguished Professor of Chemistry at the Purdue College of Science.

Use of a mass spectrometer to analyze 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 pretreatment steps, enabling researchers to use a mass spectrometer to take "fingerprints" of the bacteria in less than a minute.

Mass spectrometry works by turning molecules into ions, or electrically charged versions of themselves, inside a vacuum chamber. Once ionized, the molecules can be more easily manipulated, detected, and analyzed on the basis of their masses. The key DESI innovation is performing the ionization step in the air or directly on surfaces outside of the vacuum chamber of the mass spectrometer. DESI promises, when combined with portable mass spectrometers also under development at Purdue, to provide a new class of compact detectors. The Purdue researchers have used the method to detect living, untreated bacteria, including E. coli and Salmonella typhimurium, both of which cause potentially fatal infections in humans.

"There is always an advantage to the analysis of living systems because the bacteria retain their original properties," Cooks said.

The researchers are focusing on three potential applications: food safety, medical analysis, and homeland security. A DESI system could alert employees in the food and health care industries to the presence of pathogens and could provide security personnel with a new tool for screening suspicious suitcases or packages.

The new technique enabled researchers to detect bacteria in quantities as low as 1 nanogram. Even more important, it is possible to identify a particular bacterium down to its subspecies, a level of accuracy needed in the detection and tracking of infectious pathogens. The identifications are based on the lipids and fatty acids found in the bacteria.

"We can determine the subspecies and glean other information by looking at the pattern of chemicals making up the pathogen, a sort of fingerprint revealed by mass spectrometry," Cooks said. "Conventional wisdom says quick methods such as ours will not be highly chemically or biologically specific, but we have proven that this technique is extremely accurate."

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 ionized molecules are then vacuumed from the surface into the mass spectrometer, where the masses of the ions are measured and the material analyzed.

Much of the funding for the research was provided by the National Science Foundation, the Office of Naval Research, and Prosolia, through the Indiana 21st Century Research and Technology Fund. The findings are detailed in the paper "Rapid Ambient Mass Spectrometric Profiling of Intact, Untreated Bacteria Using Desorption Electrospray Ionization," which appeared in the journal Chemical Communications (2007, Number 1). Cooks's co-authors were Yishu Song, Nari Talaty, W. Andy Tao, and Zhengzheng Pan.