Purdue News

January 17, 2007

New technology accurately identifies E. coli for food safety

WEST LAFAYETTE, Ind. — Researchers have shown that a new low-cost system to quickly identify bacteria by analyzing scattered laser light can accurately distinguish between different strains of E. coli, a potentially valuable way to screen the food supply.

Euiwon Bae
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The technique, which works by passing a laser beam through bacterial colonies growing on a nutrient medium, also promises to have future applications in medicine and homeland security, identifying dangerous organisms far more quickly and at much lower cost than conventional technologies, said E. Daniel Hirleman, a professor and William E. and Florence E. Perry Head of Purdue's School of Mechanical Engineering.

Laser light passing through and around the colony is redirected by the bacteria and produces a scattering pattern. This light-scatter pattern is recorded and analyzed to identify the types of bacteria growing in colonies.

"We have learned that slight genetic differences between strains of E. coli create subtle differences in the micro- and macrostructure of the respective colonies," Hirleman said. "Our scattering instrument, in effect, amplifies these slight differences to produce remarkably different scattering phenomena."

The light-scattering project was initiated by Hirleman, working with Arun K. Bhunia, a professor of food microbiology in the Department of Food Science, and other researchers, including J. Paul Robinson, a professor in the Weldon School of Biomedical Engineering.

Hirleman has specialized in research to develop new types of sensors that work by analyzing light scattering off objects for applications such as detecting impurities on silicon wafers in computer chip manufacturing and measuring the size and speed of burning fuel droplets in jet engines.

A major motivation for the bacteria-detection research is to reduce the time it takes to identify harmful organisms in food processing. The industry generally collects food samples or swabs, places them first in a nutrient broth and then on a plate coated with solid nutrient to allow the bacterial growth to reach detectable levels. E. coli bacterial cultures take about 18-24 hours to grow. Then, subsequent biochemical analyses and other time-consuming and expensive techniques, such as polymerase chain reaction, take four to seven days to complete the bacteria identification.

The light-scattering method works immediately after the colony is grown.

"Within a second after the colony is full grown we can identify by its scattering fingerprint a certain strain of E. coli, for example," Hirleman said. "Or we might see a new scattering fingerprint and only be able to say that something is growing on the same growth medium as E. coli. We've never seen it before, but there is something here. That means we are warned within seconds instead of days."

A mass-produced system based on the technology would consist of inexpensive, off-the-shelf hardware, such as computers, red lasers and low-resolution digital cameras available at consumer electronics stores, and likely would cost less than $10,000. Instruments used for conventional methods can exceed 10 times that cost, Hirleman said.

Another advantage is that the light-scattering fingerprints of bacteria can be added to a library for quick reference in future outbreaks of food-borne pathogens.

One conventional approach requires sophisticated methods to "label" the bacterium with antibodies that attach a fluorescent dye to the target.

"But that means you have to use a designer antibody specifically suited for the strain of E. coli in question," Hirleman said. "What if food is contaminated with, say, a new strain of E. coli? You won't see it because the label will not attach to it."

The light-scattering technique, however, would enable researchers to detect bacterial contaminants they were not specifically looking for, making it less likely to miss an unsuspected culprit.

"Our team has done experiments where we've spiked ground beef and spinach with known pathogens, found those but also found another bacterium that was not in our library," Hirleman said. "So then we went back to the colony and did further analyses to find out what it was."

The researchers have studied growth characteristics of bacterial colonies and the corresponding evolution of the scattering patterns for a wide variety of pathogens relevant to food safety, developing a mathematical model that predicts the light-scattering signatures for given pathogens.

"We have found that different strains generally have unique forward scattering fingerprints, and those fingerprints can be used for rapid detection and classification of the bacteria," Hirleman said. "The light-scattering method has high sensitivity and speed and shows great promise for identifying bacterial colonies of a wide range of organisms relevant to infectious disease, homeland security and food safety."

This research was supported through a cooperative agreement with the Agricultural Research Service of the U.S. Department of Agriculture and Purdue's Center for Food Safety Engineering.

A provisional patent has been filed for the data-processing technique, and a full patent is pending on the underlying light-scattering technology.

Writer: Emil Venere, (765) 494-4709, venere@purdue.edu

Sources: E. Daniel Hirleman, (765) 494-5688, hirleman@ecn.purdue.edu

Arun Bhunia, (765) 494-5443, bhunia@purdue.edu

J. Paul Robinson, (765) 494-0757, jpr@flowcyt.cyto.purdue.edu

Purdue News Service: (765) 494-2096; purduenews@purdue.edu

Related Web sites:
E. Daniel Hirleman: http://tools.ecn.purdue.edu/ME/Fac_Staff/hirleman.whtml

Arun Bhunia: http://www.foodsci.purdue.edu/research/labs/bhunia/

J. Paul Robinson: http://www.cyto.purdue.edu/flowcyt/staffpgs/robinson.htm

Center for Food Safety Engineering: http://www.cfse.purdue.edu/

Note to Journalists: Video B-roll is available by contacting Emil Venere, Purdue News Service, at (765) 494-4709.

PHOTO CAPTION:
Euiwon Bae, a mechanical engineering postdoctoral research associate, positions a bacterial culture being viewed using a new low-cost system that quickly identifies bacteria by analyzing scattered laser light. Laser light passing through and around the bacterial colony is redirected by the bacteria and produces a scattering pattern. The light-scatter pattern appears on a computer monitor in the background. The system can accurately distinguish between different strains of E. coli, a potentially valuable way to screen the food supply. The technique also promises to have future applications in medicine and homeland security. (Purdue News Service photo/David Umberger)

A publication-quality photo is available at http://news.uns.purdue.edu/images/+2007/bae-bacteria.jpg

 

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