Gamma irradiation, widely used to reduce microbial contamination in medicinal and recreational cannabis, does not fully eliminate toxic fungi or their chemical residues, according to a new study from McGill University. The findings suggest that current sterilization and testing practices may underestimate contamination risks, particularly for immunocompromised users.
The study examined dried cannabis buds subjected to gamma irradiation, alongside non-irradiated material and commercially available products from licensed producers. Gamma irradiation is intended to inactivate microorganisms by damaging DNA and RNA and is commonly applied to meet regulatory microbial limits. While the treatment substantially reduced overall microbial load, the researchers found that it did not achieve complete sterilization.
Using a combination of microbiological and analytical approaches, the team detected viable spores from mycotoxigenic fungi, fragments of fungal DNA, and residual mycotoxins in irradiated samples. The persistence of these contaminants is notable, the authors point out, given that most cannabis is consumed by smoking or vaping, which may deliver spores or toxins directly to lung tissue.
To assess contamination, the researchers applied three complementary methods. Culture-based assays were used to identify viable fungi and bacteria; molecular techniques, including PCR and quantitative PCR (qPCR), targeted fungal DNA and genes associated with mycotoxin biosynthesis; and antibody-based enzyme-linked immunosorbent assays (ELISA) were used to quantify toxins such as aflatoxins and ochratoxins.
Culture-based testing showed that irradiation eliminated detectable bacteria but not all fungi, with mycotoxigenic genera including Aspergillus, Penicillium, and Fusarium persisting after treatment. Molecular analyses revealed that mycotoxin-related genes remained detectable after irradiation, indicating that DNA damage did not equate to complete degradation. ELISA measurements confirmed the presence of residual mycotoxins in irradiated samples and in products that had passed routine regulatory testing.
The authors report that ELISA, when used alone, may lack the sensitivity needed to detect trace residues following irradiation, and that similar limitations can apply to mass spectrometry when used in isolation. Combining culture-based, molecular, and immunological methods provided a more complete assessment of contamination than any single approach.
“A single spore can cause disease, so we had to go beyond the ELISA limit to see,” said Saji George, senior author of the study. “To the general population, this may not have much significance, but immunocompromised people will be at much higher risk.”
The researchers emphasize that preventing contamination during cultivation and processing may be more effective than relying on post-harvest sterilization alone. “We need stricter safety standards, especially for medicinal cannabis,” George said.
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