Using Laser Fingerprinting To Identify Salmonella

(Inside Science) -- Bacteria from the genus Salmonella are a major cause of food poisoning. About 40,000 cases of salmonella food poisoning are reported in the United States every year, but according to the Centers for Disease Control and Prevention, about one million people are actually infected with the bacterium every year. Researchers have now developed a new, more rapid technology to identify food that has been contaminated with Salmonella.

Checking for Salmonella in food is now routinely done all over the world, and detection of the bacteria often results in food recalls from stores. Several methods exist to detect Salmonella, the most important of which are the polymerase chain reaction tests. They usually involve biochemical tests on bacteria obtained from food rinses -- water obtained by shaking the food in a sterile bag that contains sterilized water -- or from cultures grown on agar plates -- glass plates covered with a layer of nutrients for bacteria. The bacteria form colonies: small, round spots of multiplying bacteria. Then these colonies are subjected to biochemical tests, a process that can require 72 hours for their identification.

A team at Purdue University in West Lafayette, Ind., led by Arun Bhunia, a researcher in food science, discovered that if you shine laser light through such a colony, strange round symmetric patterns appear that are strikingly different for each type of bacterium. Bhunia began investigating how to use a laser to identify the bacteria in colonies on agar plates.

Their findings were published in the January/February issue of mBio.

They realized that they had stumbled on a new method for identifying bacteria -- when the laser struck the colonies it produced what are known as diffraction patterns, which can be read like fingerprints. And they found that it was mainly the nutrients processed by the bacteria that caused the different patterns. 

"When bacteria grow on the agar plate they use different types of nutrients, based on their genetic make-up, and they make different types of byproducts," said Bhunia. "So when the laser beam hits these different molecules, which remain trapped in the colony, you get different diffraction patterns."

However, the patterns change as the colony develops. "We wanted to find a stable time where we could consistently get the same pattern. At the end of the growth phase the colony is more stable and we see more features; after a while the cells start dying and the pattern changes again," said Bhunia.

The researchers developed an automated system, called BARDOT (bacterial rapid detection using optical scatter technology). The researchers worked with Advanced BioImaging Systems in West Lafayette to commercialize the system. BARDOT consists of an incubator and a laser scanner that can examine an agar plate in a minute. The observed patterns are then displayed on a screen. The researchers stress that this system does not supplant the current detection methods used by the U.S. Food and Drug Administration and similar organizations around the world.

Patrick Fach, a food safety researcher at the French Agency for Food, Environmental and Occupational Health (Anses) in Maisons-Alfort, France, said this probably will not happen.

"On pure [Salmonella] colonies, [polymerase chain reaction] tests can give much more information, such as virulence and antimicrobial resistance; so depending on the nature and level of information you need, you should use one system rather than the another one," said Fach.

One benefit of the new test is that it does not kill the colony, which makes further testing possible. Bhunia said that BARDOT is good for a quick scan and the polymerase test is useful for building a complete understanding of the situation.

"This is how we see the value of this technology. We are not really changing the flow of the process everybody uses in the microbiology lab; using our system would benefit them by expediting the testing," said Bhunia.

The BARDOT system matches the obtained diffraction patterns with an image library containing known diffraction patterns of microorganisms. So besides the pathogen you are looking for, you can also quickly detect other microorganisms present on the agar plates, which allows the researchers to continually improve the library by adding any organisms not yet included in the library. The system might also be suitable for other research beyond checking food, said Bhunia.

"We have tried blood samples, air samples, and water samples--anything you can grow on a plate," he said.

Alexander Hellemans is a freelance science writer who has written for Science, Nature, Scientific American, and many others.