A new method of testing for Salmonella could shorten the time it takes to detect the bacteria in food samples.


Researchers at the Agricultural Research Service’s Quality and Safety Assessment Unit in Athens, GA are using a technique called surface-enhanced Raman scattering (SERS), in which light from a laser is directed at a sample specimen, whose interaction with the light produces a unique spectral pattern called a “Raman spectral signature.”

Scientists postulate that each strain of bacteria has its own unique signature that acts as a badge of identity for the bug.

Currently, bacteria are most often identified by their DNA fingerprint using a technique called PFGE, or pulsed-field gel electrophoresis. These PFGE patterns are then uploaded onto PulseNet, a national database that can be used to see if the strain matches any others in the system.

But PFGE analysis usually takes at least 24 hours to complete in a lab, whereas a test using SERS takes less than 30 minutes from start to finish, according to lead researcher Bosoon Park.

Park says that if SERS proves effective, a worldwide Internet database could be created using Raman spectral signatures to find a match for bacteria more quickly, which could help investigators pinpoint the source of contamination earlier in an outbreak.

One reason SERS detection is faster than PFGE and many other leading methods is that bacteria do not need to be isolated from a food or drink sample before they’re analyzed. Most techniques require that the pathogen be separated out first using magnetic particles or other extraction agents.

While this project is not the first one to use SERS to identify foodborne pathogens, nor the first to test samples on a metallic surface, which enhances light refraction and leads to a more defined pattern, it is the first to use a silver substrate that encapsulates a biopolymer.

Park breaks down this concept: A biopolymer is a biodegradable polymer. “The biopolymer helps eliminate the aggregation of silver nanoparticles. Aggregation of particles gives poor SERS signal enhancement and non-uniformity of signals from spot to spot,” he explained in an e-mailed statement.

Another problem often encountered with the use of silver nanoparticles is oxidation of the metal during storage. “In our research,” Park explains, “silver particles bind with biopolymers that help avoid the silver oxidation.”

While the team has so far only tested SERS on higher concentrations of Salmonella enterica, the next step is to analyze samples with lower levels of bacteria to see how sensitive the test is. Researchers predict that the technique will be able to detect even small amounts of Salmonella, which is important because as few as 10 cells can cause illness in humans.

And while the team’s testing has focused on Salmonella, “[SERS] is easy to expand to other types of foodborne pathogens,” says Park. “We have already been testing with Listeria innocua and Staphylococcus haemolyticus. They showed promising results to proceed further.”

Park expects the technology will likely to take 1-2 years before it is validated for use in testing labs.

More information on this research can be found in Agricultural Research Magazine.