SERS Susses Out Salmonella

In the future, even the smallest traces of salmonella and other foodborne pathogens may be easily detected using a technology known as surface-enhanced Raman scattering, or SERS.

Agricultural scientist Bosoon Park of the US Department of Agriculture’s (USDA) Agricultural Research Service’s Quality and Safety Assessment Research Unit is leading exploratory studies of this analytical technique’s potential for quick, easy and reliable detection of harmful bacteria in food.

Salmonella causes more than 1 million cases of illness in the US each year, according to the US Centers for Disease Control and Prevention.

Transmission electron microscope image of biopolymer spheres coated with silver nanoparticles, which might make an ideal surface for SERS detection of foodborne pathogens in food and beverage samples. (Image: Jaya Sundaram)

If SERS can successfully detect salmonella, the technique may soon be used at public health laboratories around the nation to rapidly identify this or other pathogens responsible for outbreaks of foodborne illness, Park said. To ensure that their products are free from unsafe levels of these harmful bacterias, the foodmakers of tomorrow might also opt to use SERS at their in-house quality control labs.

To perform the simple SERS analysis, a specimen is placed on a surface — such as a stainless steel plate — that has been “enhanced” or changed from smooth to rough. For some of their experiments, Park’s team enhanced the surface of stainless steel plates by coating them with a tiny sphere of biopolymer encapsulated with silver nanoparticles.

Rough surfaces and colloidal metals such as silver enhance the light scattering that occurs when a specimen, placed on this nanosubstrate, is scanned with a Raman spectrometer’s laser beam. The scattered light that returns to the spectroscope forms a distinct spectral pattern known as a Raman spectral signature, or a Raman scattered signal.

Postdoctoral research associate Jaya Sundaram (left) and agricultural engineer Bosoon Park are determining whether SERS signatures can be used to reliably identify pathogenic foodborne bacteria. (Image: Jerry Heitschmidt)

“The idea of using a substrate of silver nanoparticles for Raman spectroscopy is not new,” Park said. “But in SERS studies to detect foodborne pathogens, our use of a substrate of silver nanoparticles that encapsulate a biopolymer is novel, to the best of our knowledge.”

The researchers expect to prove the concept that all molecules, such as those that make up salmonella, have their own unique Raman spectral signature. In the future, Raman signatures of unidentified biological specimens could be compared with known Raman signatures to identify unknown specimens.

In experiments with comparatively large concentrations of two pathogenic kinds, or serotypes, of Salmonella enterica — enteritidis and typhimurium — Park’s team showed, for the first time, that SERS can differentiate these two serotypes collected from raw chicken.

In follow-up studies, Park intends to use less-concentrated serotype samples to challenge the technology’s ability to detect and quantify the pathogen. These tests could reveal SERS sensitivity to determine the smallest number of salmonella cells that can be detected in a given concentration of solution.

Bosoon Park (right) and Jaya Sundaram examine a hyperspectral microscope image of an experimental substrate developed for SERS food-safety analyses. (Image: Jerry Heitschmidt)

In earlier experiments, Park’s team differentiated live salmonella cells from dead ones using SERS. This discovery could help quality-control labs evaluate food sterilization methods, such as irradiation, thermal processing and high-pressure processing. Reliable differentiation also could speed up and simplify research and testing of new techniques to keep food safe to eat.

“SERS is one of several candidate technologies we are currently testing,” Park said. “If we find that it continues to offer major advantages over other options, our goal would be to develop a SERS-based system that wouldn’t require extensive training and would use affordable SERS instruments.”

The work appeared in the April 2012 issue of Agricultural Research magazine.

For more information, visit: www.ars.usda.gov