Microbial Testing of Cannabis: Why Culture and qPCR Can Produce Different Results

Reliable microbial testing is essential for cannabis products, particularly those used by medical patients who may be more vulnerable to infection. Cannabis flowers can carry bacteria, yeasts, molds, and fungi capable of causing disease or producing toxins, but the method used to detect them strongly influences the results.

A 2016 study, “Metagenomic analysis of medicinal Cannabis samples”, compared two widely used approaches: traditional culture-based testing and quantitative polymerase chain reaction (qPCR). The researchers examined 15 cannabis samples from seven indoor growing facilities in the northeastern United States. They also used DNA sequencing to characterize organisms detected before and after culture.

Traditional culture methods involve placing a sample on a growth medium and counting the colonies that develop. These tests are relatively accessible, provide evidence that organisms are capable of growing under the test conditions, and remain useful for estimating viable microbial loads. Commercial systems can simplify testing for broad categories such as total yeast and mold.

The main drawback is that many microorganisms do not grow well on commonly used media or under standard laboratory conditions. The choice of medium, incubation time, temperature, and other laboratory conditions can therefore affect the result. In the 2016 study, the microbial composition of samples changed substantially after incubation, suggesting that culture-based tests captured only a subset of the organisms present.

qPCR takes a different approach. It detects and amplifies targeted genetic sequences, allowing laboratories to look for specific microorganisms or broader groups of bacteria and fungi. Results can often be produced within hours rather than the one to three days required for many culture-based tests, making qPCR attractive for products that must be released quickly.

However, a positive qPCR result does not necessarily show that a living organism is present. DNA can remain detectable after a microbe has died, and the presence of a gene does not by itself demonstrate that a toxin is being produced. qPCR assays are also limited by the targets they are designed to recognize: organisms outside those targets may go undetected.

The study found that qPCR identified microbial signals that some culture systems missed, while sequencing revealed bacterial and fungal taxa associated with potential health concerns. The findings raised important questions about whether broad colony counts alone provide a complete picture of contamination. They did not, however, establish that every detected organism was viable, infectious, or producing toxins. The small, selectively assembled sample set also means the results should not be treated as representative of the cannabis industry as a whole.

For these reasons, culture and molecular testing are better viewed as complementary tools than as competing replacements. Culture can help determine whether organisms are viable and can support enumeration, while qPCR and sequencing can detect organisms that are difficult to grow and provide more specific identification. In some situations, laboratories may need enrichment steps or additional methods to distinguish DNA from living contamination.

Standards and validation remain important because a method that performs well in food or environmental testing may behave differently in cannabis flower. AOAC International’s guidance for validating microbiological methods for cannabis products addresses the need to evaluate performance in the actual product matrix. A broader review of fungal and mycotoxin contamination in cannabis and hemp flowers likewise cautions that molecular results should be interpreted alongside methods capable of assessing viability and toxin risk.

The practical lesson is not that qPCR is automatically more accurate than culture, but that each method measures something different. Effective cannabis safety programs should match the test to the hazard, validate it for the product being examined, and interpret results in light of viability, toxin production, sampling, and the limitations of the assay.