Food safety labs rely on a popular testing method to distinguish between living and dead bacteria, but new research shows the technique is unreliable in real-world conditions, potentially triggering false recalls and massive food waste. The findings raise urgent questions about how the U.S. Food and Drug Administration (FDA) and food companies decide when to pull products from shelves.

How Does Food Safety Testing Actually Work?

When a potential pathogen is detected in food, labs use DNA-based tests called nucleic acid amplification tests (NAATs), such as polymerase chain reaction (PCR), to identify dangerous bacteria like E. coli, Salmonella, and Listeria. These tests are powerful and fast, but they have a critical flaw: DNA can survive long after the bacteria that produced it dies. A facility might have eliminated a contamination problem completely, but traces of dead bacterial DNA could still trigger a positive test result weeks later.

To solve this problem, scientists developed a workaround about 15 to 20 years ago using a dye called propidium monoazide (PMA). The idea was elegant: PMA would bind to DNA from dead bacteria and lock it away so it could not be detected by PCR, while leaving living bacteria unaffected. In theory, this would eliminate false positives and prevent unnecessary recalls.

What Did the New Research Discover About PMA Testing?

Recent research has exposed serious problems with PMA-based viability assays. Scientists found that PMA does not work reliably in real food samples where the number of living and dead bacteria is unknown. The dye only functions properly under very specific, controlled conditions that rarely exist in actual food safety testing.

The research identified several critical failures in how PMA performs:

• Insufficient Dye Coverage: When there are too many dead bacterial cells in a sample, there is not enough PMA to block all the leftover DNA, so dead bacteria still show up as a positive result on the test.
• Interference With Live Cells: If too much PMA is used to compensate for high numbers of dead cells, the dye can interfere with detecting actual living bacteria, potentially missing real contamination threats.
• Pathogen-Specific Variability: PMA does not affect all foodborne pathogens equally; a treatment that works well for Salmonella might undercount live E. coli or Listeria, making the test unreliable across different types of bacteria.
• Test Sensitivity Matters: The dye performed differently depending on whether labs used highly sensitive tests like quantitative PCR (qPCR) or less sensitive methods like loop-mediated isothermal amplification (LAMP).

The bottom line is stark: PMA-based viability assays are unreliable when the concentration and composition of bacterial mixtures are unknown, which is the case in most real-world food safety investigations.

Why Does This Matter for Food Safety and Your Wallet?

The implications are significant for both food safety and the economy. A single food recall can cost a company over $10 million in direct costs alone. Beyond the financial burden, the waste is staggering. In just the first half of 2024, the FDA ordered the destruction of nearly 85 million units of food over safety concerns, much of which may have been safe to eat.

When a lab test suggests a pathogen might be present, food companies must err on the side of caution and issue a recall. But if some of those positive test results are false alarms driven by unreliable PMA testing, companies and regulators are destroying food unnecessarily. This creates a dilemma: how can labs distinguish between a genuine contamination threat and a false positive caused by dead bacterial DNA?

What Are Experts Saying About Better Solutions?

The research report plainly states that PMA is unreliable for viability assays when the concentration and composition of the bacterial mixtures are unknown. It is suitable only in carefully controlled cases, essentially if a rough estimate is known of how many dead cells are present, which defeats the purpose of using it in unknown samples.

Scientists are exploring alternative approaches to better distinguish living from dead pathogens. Some researchers are investigating RNA-based detection methods, since RNA degrades much faster after a bacterial cell dies compared to DNA. Others are developing entirely new sensor technologies that detect metabolic activity as proof of life, or using advanced techniques like microfluidics to identify viable pathogens more accurately.

Some testing workflows have added extra steps to mitigate false positives, such as physically removing or "cleaning" residual DNA from dead cells before running PCR tests. However, none of these alternative approaches is a perfect solution, and the challenge remains significant for food safety labs nationwide.

What Should Food Companies and Regulators Do Now?

For food producers and testing labs, the findings suggest that relying on PMA-PCR alone to confirm a pathogen's viability could be problematic. An assay designed to prevent false positives might instead be introducing new uncertainties into the food safety system. The research underscores an urgent need for better methods to differentiate live and dead pathogens in rapid testing, particularly as food safety regulations evolve and companies face pressure to act quickly when contamination is suspected.

The challenge is significant because food safety testing must be both sensitive enough to catch real threats and specific enough to avoid destroying safe products. Until better methods are widely adopted, labs and regulators may continue to struggle with the question of whether a positive test result represents genuine danger or a false alarm triggered by the limitations of current technology.