Low-cost system for detection of bacteria.

Researchers at Purdue University have developed a new low-cost system that analyzes scattered laser light to rapidly identify bacteria for applications in medicine, food processing, and homeland security--at one-tenth the cost of doing so with conventional technologies.

The optical-scattering that the researchers are using works by shining a laser through a petri dish containing bacterial colonies growing in a nutrient medium.

"Unlike with conventional methods, we don't have to do any biochemical staining, DNA analysis, or other types of manipulation," said Bartek Rajwa, a staff scientist at Purdue's Bindley Bioscience Center.

Particles of light, called photons, bounce off of the colony, and the pattern of scattered light is projected onto a screen behind the petri dish. This "light-scatter pattern" is recorded with a digital camera and analyzed with sophisticated software.

The work was initiated by Arun Bhunia, a professor of food microbiology in the Department of Food Science, and E. Daniel Hirleman, a professor and head of Purdue's School of Mechanical Engineering. Findings are detailed in a research paper that appeared in the May/June 2006 issue of the Journal of Biomedical Optics under the title "Feature Extraction from Light-Scatter Patterns of Listeria Colonies for Identification and Classification."

A major motivation for the research was to reduce the time it takes for industry to identify harmful organisms in food processing.

"The dairy industry, for example, grows bacteria on petri dishes to make sure products are safe, but industry is trying to develop technologies that will very quickly identify organisms," said J. Paul Robinson, a researcher at the Bindley Center and a professor at the Weldon School of Biomedical Engineering and the School of Veterinary Medicine. "The same sort of thing holds true for clinical microbiology and other laboratories. With our light-scattering method, it takes less than five minutes to identify harmful organisms after they have grown in a petri dish. The analysis is faster than any other methods in existence, and it's simple."

A mass-produced system based on the technology would consist of inexpensive, off-the-shelf hardware, such as red lasers and low-resolution digital cameras available at consumer electronics stores, and likely would cost less than $1,000, Hirleman said.

The researchers have used the new system to classify six species of Listeria, only one of which is a dangerous foodborne pathogen for humans.

"If you have a mixture of different Listeria species, you would like to know which is the one that can kill you," Rajwa said. "We took pictures of the scatter patterns from different Listeria, and we were able to classify all of them accurately." The system also was able to accurately identify other types of bacterial colonies, including Salmonella, Vibrio, and E. coli.

"We were able to classify bacterial colonies with greater than a 90 percent probability of being correct, which is as good as you could do with equipment costing more than $100,000," Rajwa said. "And, unlike conventional systems, our method is 100 percent non-invasive, which means we can carry out the procedure without staining, manipulating, or killing the biological samples."

The technique might also be used to identify staph infections that are resistant to antibiotics.

Further work will include research to develop a graphical user interface. "Now it requires a qualified, trained person to do all the recognition," Rajwa said. "We want a system in which you put a petri dish or some other container into the system, you press enter, and the computer says, 'This is Salmonella of this type and this strain,' and it does this quickly in real time. There is absolutely no fundamental reason why we wouldn't be able to do this, and we are pretty close to having an actual prototype of a product that could be commercialized."