A new technique developed by scientists improves on mass spectrometry detection methods to speed up the identification of foodborne pathogens.

The technique, called desorption electrospray ionization (DESI), could help processors speed up the testing of samples, allowing managers to release products for shipping sooner than when using
current pathogen detection methods.

Using 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 take fingerprints of bacteria in less than a minute using a mass spectrometer.

DESI could be used to create a new class of fast, accurate detectors for applications ranging from food safety to homeland security, said Graham Cooks, a professor of chemistry at Purdue
University.

This is the first time we’ve been able to chemically analyze and accurately identify the type of bacteria using a mass spectrometer without any prior pretreatment within a matter of
seconds, Cooks said.

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.

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

The findings are revealed in a paper appearing on 7 January in the journal Chemical Communications. The paper was written by Cooks, along with scientists Yishu Song, Nari Talaty and Zhengzheng
Pan and Andy Tao.

Mass spectrometry works by turning molecules into ions, or electrically charged versions of themselves, inside the instrument’s vacuum chamber. Once ionized, the molecules can be more easily
manipulated, detected and analysed based on their masses. The scientists developed a method to perform the ionisation step in the air or directly on surfaces outside of the mass spectrometer’s
vacuum chamber.

Such a detector could quickly analyze foods, medical cultures and the air in hospitals, subway stations and airports, Cooks said.

When combined with portable mass spectrometers also under development at Purdue, DESI promises to provide a new class of compact detectors, they stated.

The researchers are able to detect one nanogram, or a billionth of a gram, of a particular bacterium using the system. The method also allows users 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.

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 analysed.

Song will conduct further research, including experiments to look for bacterial contaminants in foods. Ongoing work by Talaty with international E. coli expert Barry Wanner, a professor in
Purdue’s biological sciences department, will apply the method to living bacteria in biofilms.

DESI has been commercialised by Indianapolis-based Prosolia.

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

Cooks also is leading work to build miniature versions of the normally bulky mass spectrometer, creating shoebox-size instruments that weigh about 10 kilograms (22 pounds), compared to about 30
times that weight for a conventional mass spectrometer.

by Ahmed ElAmin