Pathogen Detection at the Speed of Light

Automation of data will be a driver in the years ahead, Dr. Bailey adds. “We also need better connectivity so that for both quality indicators and pathogens we can automatically transfer laboratory results into data management systems,” he says. “We need a data management system that will allow companies to monitor the microbiology results from all their locations, even companies in different countries, in real time.”

New Technologies

Arun Bhunia, BVSc, PhD, professor of molecular food microbiology at Purdue University, focuses much of his research on the development and application of optical biosensors—including light scattering, fiber optic, surface plasmon resonance (SPR), and cell-based—all of which are used for pathogen and toxin detection.

A light scattering sensor is a laser-based entity. Shining this sensor on a bacterial colony growing on an agar plate enables researchers to record a scattered image. “That image is a fingerprint unique to a particular organism,” Dr. Bhunia says. “Without needing any DNA, reagents, or antibodies, we can identify a pathogen by comparing the fingerprint to those stored in a huge database. We already have fingerprints for many species and strains of E. coli, Listeria, Salmonella, Vibrio, and Staphylococcus.”

The time to identification depends on the amount of time needed for the colony to grow, as much as 12 to 15 hours for E. coli O157:H7, Salmonella, and Vibrio, and 24 to 30 hours for Listeria. “The benefit of light scattering is that organisms can be identified immediately, once colonies are available,” Dr. Bhunia says. This promises to be an improvement over the two to three hours now needed for PCR results.

Dr. Bhunia and his collaborators, a group that includes Purdue engineers, have built two such prototype instruments, which they call BARDOT (BActerial Rapid Detection using Optical light scattering Technology). One will soon be provided to the USDA for further testing. “We are exploring the possibility of commercializing this instrument,” Dr. Bhunia says. “It holds promise for identifying pathogens in raw and cooked product.”

Measuring the interaction between two molecules, SPR works when antibody is placed on a sensor and bacteria or toxin are added. “The pathogen will react with the antibody and change the angle of reflectance, which can then be measured,” Dr. Bhunia says.

A sensor features optical fibers with different antibodies. Binding of a target pathogen to the antibody-coated fiber can be detected using a fluorescent-labeled second antibody. The resulting fluorescent signal is proportional to the amount of target agents in a sample.

Dr. Bhunia has developed an innovative mammalian cell-based sensor in a 96-well plate format that makes it possible to test a large number of samples at once using a standard plate reader. This functional diagnostic sensor allows quick analysis—one to two hours—of the virulence potential of viable pathogens or active toxins.

“Recent developments in sensor technologies appear to be promising and sound exciting,” Dr. Bhunia says. “However, there are a few challenges we must continue to address to make this technology robust. As we push the boundaries of detection threshold, the sensor device becomes more susceptible to interferences from food matrices, background resident microflora, and inhibitors. In addition, bacterial physiology and genetic regulation of antigens that are essential for immune sensor-based detection should be well understood.”

Dr. Bhunia adds that sensor technology must be made more affordable, portable, and capable of testing multiple organisms. “Currently, light scattering instruments and fiber optic sensors cost $35,000 to $40,000. The challenge to private companies for the next level of commercial development is to make the technology as user friendly as possible.”

Leake is a food safety consultant and writer based in Wilmington, N.C. Reach her at [email protected] or (910) 799-4881.