Rapid Microbial Detection Can Pay Big Dividends

As new regulations adopted under the Food Safety Modernization Act (FSMA) begin taking effect, more scrutiny is being placed on food growers, processors, distributors, and service providers to assure regulators and the public that their food products are free from pathogens that could cause foodborne illness.

On a daily basis, product contaminations are discovered by food processors. Companies in food service are also subject to problems involving foodborne pathogens.

Because FSMA requires food producers, processors, and service providers to certify that their products are free from pathogens such as Listeria, Salmonella, coliforms, and E. coli, obtaining results using valid pathogen testing protocols is now—and will continue to be—a necessary step for companies in all parts of the food industry.

Culturing and Traditional Methods

Culturing has been the traditional method of testing for the presence of pathogens in food for more than a century. Requiring sophisticated medical-grade laboratories and highly-trained technicians, these tests are considered tedious and expensive. While culturing is moderately accurate, it’s frustratingly time-consuming and, since only about 10 percent of all bacteria are able to be cultured, its applicability is limited.

“The majority of bacteria will not grow on nutrient medium in the lab,” says Kim Lewis, professor of biology and director of the Antimicrobial Discovery Center at Northeastern University. He finds it fascinating that, while the microbes continue to grow in nature, 100 years of effort to grow most bacteria in controlled settings has yet to prove successful.

On average, culturing takes between three and nine days to yield results, creating significant dilemmas for food producers. As a result, QA managers must determine whether the risks of keeping “good” product out of distribution for several days—pending results of testing—outweighs the risks associated with recalling product that might test positive for contamination later.

The Biology Encyclopedia states, “The limitations of cell culture include the finite doubling potential of most normal cells, the possibilities for unexpected infections with viruses or microorganisms, or even cross-contamination with other cell types.”

Alternative Testing and Enhancements to Culturing

Several enhancements to culturing have been developed. A Fourier transform using infrared spectrometry speeds up the time-to-results to approximately 20-24 hours, however, this new and improved method can only test for a limited number of microbes.

Growth-based carbohydrate utilization tests provide results based on changes to a culture caused by a microbe digesting sugars contained in the culture media. These tests can provide results within 2 to 72 hours. A wide range of bacteria as well as some yeasts and molds can be tested with these methods.

Polymerase chain reaction (PCR) tests are the most common alternative to traditional culturing, however, they also rely on culturing as a step in the process. PCR tests are available for the major foodborne bacteria, but generally require between one and five days to provide results. Advanced PCR tests include quantitative tests and real-time tests.

Raman Spectroscopy Imaging relies on the molecular vibration of a sample to scatter laser light into patterns that are unique for each microbial species. It requires the previous mapping of scattered light patterns, a concentrated sample, and specialized equipment operated by a well-trained technician. Raman spectroscopy may be combined with viable staining techniques and can provide results in a matter of minutes. A limited number of prominent bacteria have been categorized.

Ribotyping uses restriction enzymes to digest the DNA in bacteria, creating fragments that can then be hybridized, digitized, and analyzed by comparison with reference organisms in a database to determine the species present. These tests take approximately eight hours to yield results and can be applied to a wide group of bacteria.

Low-cost test strips are available for most major foodborne microbes and provide results within a few minutes when the test strip is dipped into a sample solution. Quick solution test kits are available for identification of certain pathogens. Drops of a sample are placed into a certain test solution, which will change colors in the presence of the pathogen. Unfortunately, test strips and quick solution kits are susceptible to contamination during testing and are not highly accurate. Due to accuracy issues, providers of such kits recommend a secondary culturing or PCR test to confirm results, thus incurring major delays in the time required for a definitive result.

The Rapid Testing Methods

Rapid microbial testing platforms that use fluorescent DNA markers to identify species power some of the most advanced technologies currently in operation. Using combined scientific methods and well-established microbiological techniques, these systems can be automated as well as mobile—in some circumstances. Some of these culture-independent platforms use fluorescent in situ hybridization, fluorescent microagglutination, and filter cytometry to identify the pathogens rapidly. Some systems can do so in as little as two hours. Using such methods, in combination with others, can yield very accurate results and effectively eliminate the need for expensive labs and highly trained technicians. Quantitative data and detection of dead microbes are also possible using such rapid testing methodology.