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Both traditional methods of microbiology and PCR provide high sensitiv- ity, says Jean-François Mouscadet, R&D manager at Bio-Rad’s Food Science Division outside Paris, but “microbiological methods take usually 24 hours more than PCR.”
PCR-based tests also provide high specificity. “These assays can be de- signed to detect a genus, a species, a serovar or a strain,” says Mouscadet. “The specificity of PCR is unmatched by other methods.” Scientists can also design PCR assays that detect multiple types of bacteria in one test.
The PCR approach comes with some challenges. “The DNA extraction has to be robust to show good efficacy with all kinds of food matrices,” says Mouscadet. “We developed a universal lysis buffer for DNA extraction, but suppliers like us need to remain very reactive to constantly adapt and validate their solutions for unique matrices.”
PCR can be used to develop very specific tests for microbes in foods. (Image courtesy of Bio-Rad.)
microbes, but various additives and other possible contaminants. “This adds complexity to the testing parameters and methods,” Spitznagel says. “The workflow can be very long, but fast answers are required.” She adds, “Testing should be very fast to be able to recall products which have already gone to the market.”
For fast testing of complicated samples, experts need a variety of tools. Merck Life Science makes a wide range of culture media, as well as what Spitznagel calls “cheap and simple immunoassays, which require no au- tomation for pathogen detection.” The production of the culture media is ISO (International Organization for Standardization) controlled, she says, which “reduces the customer’s incoming quality control tests.” For the im- munoassays, she adds, “No special training is needed, no instrumentation is necessary, and they shorten traditional workflow by one day.”
To keep track of contamination in work areas, Merck Life Science provides a range of air samplers, including a handheld device that counts microbes.
The power of PCR No matter what experts have tried, food testing remains imperfect.
According to Donaghy, “The production of safe food is not guaranteed by testing alone—absence of evidence is not evidence of absence.” Microbial testing has statistical limitations, and when companies test they must use the best tests available. The need for more testing also fuels the need for faster methods. “Many traditional culture methods are slow and labor intense, taking up to five days to confirm a positive finding,” says Donaghy.
Rather than using culture methods, foods can be tested for microbes with the polymerase chain reaction (PCR). “There have been many advances in the area of molecular diagnostics for foodborne pathogens,” Donaghy says. “Reverse transcriptase—RT-PCR—and quantitative—qPCR—are now widely applied for detection of the many bacterial pathogens and are also used to circumvent the issues with nonculturable foodborne viruses.” Scientists develop PCR tests to target specific genes.
AMERICAN LABORATORY 47 MARCH 2016
To expand the use of PCR-based tests in food analysis, the technology needs even easier workflows. Bio-Rad’s real-time PCR-based iQ-Check assays can be automated with the iQ-Check Prep automation platform. “Using this instrument, it is possible to run all food testing assays from Bio-Rad with a true walk-away solution and unmatched throughput,” Mouscadet says.
Stretching the science Dangerous microbes must be identified and characterized, and instru-
ments must be developed to analyze them more quickly. The analysis process must also adapt to the changing microbes of concern.
Beyond safety, the study of how microbes end up in and impact foods is an important area of science in itself. Gutierrez says, “My favorite part of microbiology is how microorganisms can change the environment they grow in and create something completely different.”
Mike May is a freelance writer and editor living in Ohio. He can be reached at
mikemay1959@gmail.com.
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