Real-Time PCR in Food Microbiology: Challenges and Applications

　Real-Time PCR is widely used in various fields, but its quantitative capabilities remain underutilized in food microbiology. This article explains why this is the case and highlights the unique challenges in applying Real-Time PCR for food testing.

Real-Time PCR, in principle, is capable of detecting even a single DNA copy in a reaction tube if primers, reaction systems, and inhibitory substance removal are optimized. However, the challenge lies in transferring that single DNA copy from the food sample to the PCR tube due to unavoidable dilutions during preparation.

The minimum detection threshold for Real-Time PCR is 100 colony-forming units (cfu) per gram or milliliter of sample. Let’s calculate why this is the case:

１．Sample Dilution: Typically, 25 grams of food is mixed with 225 mL of diluent and homogenized, resulting in a 10-fold dilution.

２．DNA Extraction: Further steps during DNA extraction introduce another 10-fold dilution.

Thus, to ensure at least one DNA copy is present in the PCR tube, the original sample must contain 100 cfu per gram.

In contrast, food microbiology often requires detecting 1 cfu per 25 grams (equivalent to 0.04 cfu per gram) to meet international microbiological safety standards. Direct Real-Time PCR lacks the sensitivity needed to achieve this level of detection.

To overcome sensitivity limitations, food microbiology relies on enrichment culturing. This step amplifies target microorganisms but sacrifices the ability to quantify the original bacterial concentration, which is essential for Real-Time PCR’s quantitative application.

The difference between food and medical microbiology further clarifies the challenge:

• Medical Testing: Focuses on measuring higher concentrations of pathogens (≥100 cfu/mL or gram), aligning well with Real-Time PCR’s capabilities.

• Food Testing: Aims to detect the mere presence of pathogens (1 cfu/25 grams), making enrichment culturing indispensable.

Despite its quantitative potential, Real-Time PCR is primarily used in food microbiology for qualitative purposes:

• It determines the presence or absence of pathogens by detecting fluorescence exceeding a set threshold.

• This simplifies the testing process, making it a practical tool for routine pathogen screening.

Real-Time PCR’s role in food microbiology is fundamentally different from its use in medical fields. While its quantitative abilities shine in clinical diagnostics, food microbiology relies more on its qualitative detection capabilities due to stricter sensitivity requirements and the necessity of enrichment culturing.