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WIRE: Researchers find problems with common culture-based cannabis testing methods

WOBURN, MA – Medicinal Genomics (MGC) and Courtagen Life Sciences announced the publication of a new research study that identifies several significant shortcomings of traditional culture-based safety testing methods for yeast and mold in the Cannabis industry.

The report, titled “Metagenomic analysis of medicinal Cannabis samples; pathogenic bacteria, toxigenic fungi, and beneficial microbes grow in culture-based yeast and mold tests,” by Kevin McKernan and a team of scientists from MGC and Courtagen, was recently published in the peer-reviewed journal F1000Research.

The report compares 15 Cannabis samples that previously failed microbial testing with culture-based methods from seven different cultivators. The samples were all tested using the PathogINDICAtor™ qPCR microbial detection platform, a DNA-based method, from MGC and were also re-tested using both commercial culture-based methods from 3M and BioMerieux to ensure accuracy and reproducibility of the findings. All of the samples and cultures were analyzed using next-generation sequencing before and after culture to definitively determine the viable microbial species present.

“Accurate testing to distinguish the differences between beneficial, commensal and symbiotic microbiota from pathogenic bacteria and fungi found in medicinal Cannabis should be of utmost importance to healthcare providers as they consider the benefits and safety of this form of treatment to their patients,” said Ethan Russo, M.D., medical director of PHYTECS.

The study shows that culture-based methods do not accurately measure the level of microbial contamination on Cannabis, potentially exposing patients with weakened immune systems to dangerous pathogens.

“We cannot allow the use of contaminated medical cannabis products.  Many patients using this medicine are quite ill with cancer, epilepsy and other serious conditions,” said Bonni Goldstein, M.D., medical director of Canna-Centers. “The most rigorous testing methods should be employed so that physicians like myself can confidently recommend cannabis to those who will benefit from it.”

Some of the culture-based methods have been borrowed from food testing protocols, and may not take into consideration the significant differences between Cannabis and other food products. In particular, the following conditions have been found:

  • Over 60 percent of the DNA from microbes growing on yeast and mold culture media was bacterial (both harmful and beneficial) calling into question whether such false positives could induce overuse of dangerous fungicides like Myclobutanil (also known commercially as Eagle-20), which is safe to eat but generates hydrogen cyanide at smoking temperatures.
  • Multiple beneficial microbes such as Bacillus and Trichoderma were found to grow on culture-based platforms thus discouraging the use of pesticide-free practices to thwart pathogenic growth.
  • Aspergillus—a fungus like yeast or mold, and the predominant microbe associated with clinical harm concerning Cannabis—grows poorly in culture medium and is under-reported by these platforms, leading to false negative failures and potential harm to immuno-compromised patients.
  • The types and amounts of fungal and bacterial growth found at the end of the culture-based method’s incubation period do not match what was actually on the sample prior to incubation, leading to both false-negative and false-positive failures.
  • It is well known that over 95 percent of microbes are not culturable, highlighting the failure of culture-based methods to address viability.
  • Viable Clostridium Botulinum, the bacteria responsible for botulism poisoning, was detected in over 60 percent of the samples and was only viable in non-standard anaerobic growth methods.
  • The study generated over 262 microbiomes and 30 gigabases of sequence with greater than 99 percent reproducibility between duplicate samples and a specificity of over 99 percent for discrimination between fungi and bacteria in the PathogINDICAtor Total Yeast and Mold Detection Assay.

“This study comes at a critical time as medicinal and recreational Cannabis use is already legal or newly legalized in 30 states (including the District of Columbia) and demonstrates weaknesses in the most common safety-testing methods employed today,” said Kevin McKernan, chief scientific officer of Medicinal Genomics. “We believe that the use of the more accurate genetics-based technology could help reduce both risks to patient safety and help protect medicinal Cannabis producers from liability and extra costs.”

False-negative failures can put medical Cannabis patients at risk and expose producers to massive liability when their medical Cannabis contains microbial contaminants, such as Aspergillus, that went undetected. Furthermore, false-positive failures can result in huge costs to Cannabis producers who may have to scrap large amounts of plants that were not actually contaminated with dangerous organisms. These false positives may also encourage increased and misguided fungicide use. The use of more accurate DNA-based technologies, such as PathogINDICAtor Total Yeast and Mold Detection Assay, could help reduce these risks.

About Medicinal Genomics Corporation

Medicinal Genomics Corporation applies state-of-the-art life science technology to Cannabis plant genetics. Our products help growers, dispensaries, and safety testing laboratories characterize and understand the quality of medicinal Cannabis. Medicinal Genomics utilizes a highly sophisticated Next Generation Sequencing laboratory, bioinformatics system, and DNA based technologies to deliver unmatched technical solutions to decipher the genetic code and quality level of medicinal Cannabis. Medicinal Genomics is a wholly owned subsidiary of Courtagen Life Sciences, Inc.

To learn more, please visit or follow us on Facebook, Twitter, and Instagram using the hashtags #MGC and #MedicinalGenomics.

About Courtagen Life Sciences, Inc.

Courtagen Life Sciences, Inc., located in Woburn, Mass., is a CLIA/CAP certified molecular information company focused on the diagnosis of a range of neurological disorders. Courtagen operates a highly sophisticated Next Generation DNA Sequencing, bioinformatics, and clinical interpretation business. Courtagen’s work helps physicians elucidate the linkages between the genotypes and phenotypes of various neurological diseases. For more information, please visit

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