Tag Archives: Quantitative Polymerase chain reaction


How Do You Know You’re Right? qPCR vs. Plating

By Dr. Sherman Hom

Cannabis testing to detect microbial contamination is complicated. It may not be rocket science, but it is life science, which means it’s a moving target, or at least, it should be, as we acquire more and more information about how the world we live in works. We are lucky to be able to carry out that examination in ever increasing detail. For instance, the science of genomics1 was born over 80 years ago, and just twenty years ago, genetics was still a black box. We’ve made tremendous progress since those early days, but we still have a long way to go, to be sure.

Much of that progress is due to our ability to build more accurate tools, a technological ladder, if you will, that raises our awareness, expertise, and knowledge to new levels. When a new process or technology appears, we compare it against accepted practice to create a new paradigm and make the necessary adjustments. But people have to be willing to change. In the cannabis industry, rapid change is a constant, first because that is the nature of a nascent industry, and second because in the absence of some universal and unimpeachable standard, it’s difficult to know who’s right. Especially when the old, reliable reference method (i.e. plating, which is basically growing microorganisms on the surface of a nutritional medium) is deeply flawed in its application to cannabis testing vs. molecular methods (i.e., quantitative polymerase chain reaction, or qPCR for short).

Dr. Sherman Hom, Director of Regulatory Affairs at Medicinal Genomics

Plating systems have been used faithfully for close to 130 years in the food industry, and has performed reasonably well.2 But cannabis isn’t food and can’t be tested as if it were. In fact, plating methods have a host of major disadvantages that only show up when they’re used to detect cannabis pathogens. They are, in no particular order:

  1. A single plating system can’t enumerate a group of microorganisms and/or detect specific bacterial and fungal pathogens. This is further complicated by the fact that better than 98% of the microbes in the world do not form colonies.3 And there is no ONE UNIVERSAL bacterial or fungal SELECTIVE agar plate that will allow the growth of all bacteria or all fungal strains. For example, the 5 genus species of fungal strains implicated in powderly mildew DO NOT plate at all.
  2. Cannabinoids, which can represent 10-30% of a cannabis flower’s weight, have been shown to have antibacterial activity.4 Antibiotics inhibit the growth of bacteria and in some cases kill it altogether. Salmonella species & shiga toxin producing coli (STEC) bacteria, in particular, are very sensitive to antibiotics, which leads to either a false negative result or lower total counts on plates vs. qPCR methods.
  3. Plating methods cannot detect bacterial and fungal endophytes that live a part or all of their life cycle inside a cannabis plant.5,6 Examples of endophytes are the Aspergillus pathogens (A. flavus, A. fumigatus, A. niger, and A. terreus). Methods to break open the plant cells to access these endophytes to prepare them for plating methods also lyse these microbial cells, thereby killing endophytic cells in the process. That’s why these endophytes will never form colonies, which leads to either false negative results or lower total counts on plates vs. qPCR methods.
  4. Selective plating media for molds, such as Dichloran Rose-Bengal Chloramphenicol (DRBC) actually reduces mold growth—especially Aspergillus—by as much as 5-fold.This delivers false negative results for this dangerous human pathogen. In other words, although the DRBC medium is typically used to reduce bacteria; it comes at the cost of missing 5-fold more yeast and molds than Potato Dextrose Agar (PDA) + Chloramphenicol or molecular methods. These observations were derived from study results of the AOAC emergency response validation.7
  5. Finally, we’ve recently identified four bacterial species, which are human pathogens associated with cannabis that do not grow at the plating system incubation temperature typically used.8 They are Aeromonas hydrophila, Pantoea agglomerans, Yersinia enterocolitica, and Rahnella aquatilis. This lowers total counts on plates qPCR methods.

So why is plating still so popular? Better yet, why is it still the recommended method for many state regulators? Beats me. But I can hazard a couple of guesses.

A yeast and mold plate test

First, research on cannabis has been restricted for the better part of the last 70 years, and it’s impossible to construct a body of scientific knowledge by keeping everyone in the dark. Ten years ago, as one of the first government-employed scientists to study cannabis, I was tapped to start the first cannabis testing lab at the New Jersey Dept. of Health and we had to build it from ground zero. Nobody knew anything about cannabis then.

Second, because of a shortage of cannabis-trained experts, members of many regulatory bodies come from the food industry—where they’ve used plating almost exclusively. So, when it comes time to draft cannabis microbial testing regulations, plating is the default method. After all, it worked for them before and they’re comfortable with recommending it for their state’s cannabis regulations.

Finally, there’s a certain amount of discomfort in not being right. Going into this completely new area—remember, the legal cannabis industry didn’t even exist 10 years ago—we human beings like to have a little certainty to fall back on. The trouble is, falling back on what we did before stifles badly needed progress. This is a case where, if you’re comfortable with your old methods and you’re sure of your results, you’re probably wrong.

So let’s accept the fact that we’re all in this uncharted territory together. We don’t yet know everything about cannabis we need to know, but we do know some things, and we already have some pretty good tools, based on real science, that happen to work really well. Let’s use them to help light our way.


  1. J. Weissenbach. The rise of genomics. Comptes Rendu Biologies, 339 (7-8), 231-239 (2016).
  2. R. Koch. 1882. Die Aetiologie der Tuberculose.  Berliner Klinische Wochenschrift, 19, 221-230 (1882)
  3. W. Wade. Unculturable bacteria—the uncharacterized organisms that cause oral infections. Journal of the Royal Society of Medicine, 95(2), 91-93 (2002).
  4. J.A. Karas, L.J.M. Wong, O.K.A. Paulin, A. C. Mazeh, M.H. Hussein, J. Li, and T. Vekov. Antibiotics, 9(7), 406 (2020).
  5. M. Taghinasab and S. Jabaji, Cannabis microbiome and the role of endophytes in modulating the production of secondary metabolites: an overview. Microorganisms 2020, 8, 355, 1-16 (2020).
  6. P. Kusari, S. Kusari, M. Spiteller and O. Kayser, Endophytic fungi harbored in Cannabis sativa L.: diversity and potential as biocontrol agents against host plant-specific phytopathogens. Fungal Diversity 60, 137–151 (2013).
  7. K. McKernan, Y. Helbert, L. Kane, N. Houde, L. Zhang, S. McLaughlin, Whole genome sequencing of colonies derived from cannabis flowers & the impact of media selection on benchmarking total yeast & mold detection toolshttps://f1000research.com/articles/10-624 (2021).
  8. K. McKernan, Y. Helbert, L. Kane, L. Zhang, N. Houde, A. Bennett, J. Silva, H. Ebling, and S. McLaughlin, Pathogenic Enterobacteriaceae require multiple culture temperatures for detection in Cannabis sativa L. OSF Preprints, https://osf.io/j3msk/, (2022)

Cannabis Scientists and Labs Can Help with National COVID-19 Research Volunteer Database

By Aaron Green
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Harvard Neuroscientist, Michael Wells, and a team of volunteer scientists from endCoronavirus.org have created and stocked a national database of scientists and researchers ready, willing and able to help with response efforts for COVID-19. At the time of this writing, more than 8,000 scientists have registered from all 50+ states.

It all started with a Tweet on March 18th. 

“I really wanted an outlet for me, like someone like me, to be able to help out in this fight,” Wells said in a Harvard Crimson interview. “I knew I was, by far, not the only one who felt this way. And so what happened was, on the walk home from work that day from the lab, I thought, ‘Hey, I should try to organize something here in Boston so I could potentially be a part of a group that makes themselves available to health department officials or county officials.’”

Volunteers are made up of a mix of laboratory scientists, data scientists, software engineers, medical writers, CEOs and epidemiologists – from academic research institutes, national labs and private industry. Many state and local government agencies and organizations have already accessed the list for reference, including FEMA.

PCR testing is used in a wide variety of applications
Image: Peggy Greb, USDA

Members of the cannabis industry can help to combat COVID-19. “The cannabis industry relies on specialized laboratories that routinely perform qPCR-based microbial tests,” says Wells. “As a result, these labs have basic skill sets and facilities required to participate in community COVID-19 testing.” Quantitative Polymerase chain reaction (qPCR), is a common technique for determining if there are microbial contaminants in flower, concentrates and infused products.

Some cannabis industry leaders have already taken to the call. “With the trend in legalization, the cannabis industry has built an excess testing capacity in anticipation of an increase in volumes,” says David Winternheimer, PhD, CEO of Pacific Star Labs, a Los Angeles-based cannabis research organization with an ISO-accredited testing laboratory. “As an essential industry, cannabis companies are open to helping the wider population in a crisis like this, and testing could easily be adopted in labs with excess microbial testing capacity.”

Michael Wells and his band of volunteers are asking to help get the word out to other scientists who would like to sign-up at https://covid19sci.org and for anyone to help share the database link with any relevant person in government or health services. “Right now, it is all hands on deck. We need every lab, facility, and pair of skilled hands to be deployed in this fight against the most dangerous pathogen our species has experienced at this scale in our lifetimes.”


endCoronavirus.org is a volunteer organization with over 6,000 members built and maintained by the New England Complex Systems Institute (NECSI) and its collaborators. The group specializes in networks, agent-based modeling, multi-scale analysis and complex systems and provides expert information on how to stop COVID-19.

The COVID-19 National Scientist Volunteer Database is a database of over 8,000 scientists from all 50 states, DC, Puerto Rico, and Guam who are eager to volunteer our time, expertise, equipment, and consumables to help you respond to the COVID-19 crisis. They have aggregated our contact information, locations, and skills sets into this easy to use centralized database. Their members include experts in scientific testing, bioinformatics, and data management, as well as key contacts willing to donate lab space and testing supplies.