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Total Yeast and Mold Tests Cannot Account for Biocontrol Agents in Cannabis Samples, Study Finds

By Alexander Beadle

Published: Jul 06, 2022   
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Biocontrol agents have been touted as a more sustainable alternative to the harsh chemical pesticides that are traditionally used in agriculture.

Many of these agents are non-toxic, non-pathogenic microbial strains that are applied to crops with the hope that they will naturally outcompete similar harmful pathogenic microbes in that niche. Often, they are simply modified versions of target pathogens that have been genetically altered to halt the production of toxin.

These agents are attractive to farmers on account of their reduced chemical use. Many agricultural markets that utilize extraction techniques also prefer to make use of biocontrol agents as they tend to be eliminated naturally from products going through the extraction process, unlike chemical pesticides and fungicides, which often become more concentrated.

But according to a new preprint study, authored by scientists from Medicinal Genomics and the Arizona-based testing laboratory Green Scientific Labs, the cannabis testing industry may be poorly prepared to handle the use of such biocontrol agents.

Biocontrol agents could be causing failed cannabis tests

Concerns were first raised by testing labs in Arizona, after they noticed an unusual pattern in the state’s microbial test results.

“[Green Scientific Labs] had been noticing a slightly higher Aspergillus fail rate down in Arizona,” Kevin McKernan, founder and chief scientific officer of Medicinal Genomics, told Analytical Cannabis.

“Why is it more frequent in Arizona than anywhere else? One of the growers that they work with down there said, ‘Well, maybe it is this biocontrol agent that’s used’,” McKernan continued. “If you look in the literature, there’s a lot of testing going on with this biocontrol agent known as AF36 that is in frequent use in Arizona; it’s used in the cotton and in the nut industries.”

AF36 is a biocontrol agent based on the Aspergillus flavus fungus. The agent contains a single mutation in the alfC gene that makes it unable to produce toxins, but otherwise it is genetically identical to Aspergillus flavus.

Aspergillus species are a known risk for cannabis crops on account of their causing aspergillosis, and so are routinely tested for in cannabis testing facilities. This unexpected source of Aspergillus-type microbes in the environment is something that cultivators need to be aware of, the preprint authors warn.

The trouble with current tests

Total yeast and mold (TYM) and total aerobic count (TAC) tests are two of the main methods used for microbial testing in cannabis, but the use of these methods is currently the subject of much debate within the cannabis testing industry.

“While measuring the amount of yeast and mold is important for the cannabis industry, more emphasis needs to be placed on standardized methods for this analysis and a singular procedure should be selected,” Jini Curry, laboratory director at Modern Canna Labs, recently wrote in an article for Analytical Cannabis.

“It is our lab’s suggestion that the industry consider moving away from including TYM as part of the standard cannabis testing panels and instead, test for additional yeast or mold species that commonly grow on cannabis and are dangerous to humans,” Curry said.

Indeed, the broad net that is cast by TYM and TAC tests could mean that the states that require these tests are inadvertently banning the use of biocontrol agents as these tests are not specific enough to distinguish between a pathogen and its safer biocontrol form.

“[This preprint] is a reminder to everyone in the field that we have these total aerobic count tests and these total yeast and mold tests, and they’re really large catch-all tests that, when they’re put in place in certain states, limit your ability to use really any biocontrol agent,” McKernan explained. “Because they’re just counting colonies, and so anything that’s a microbe gets you penalized.”

Discerning between biotoxins and biocontrol agents

This vulnerability to biocontrol agents is not a problem that is exclusive to TYM and TAC tests; the qPCR tests that are commonly in use are also likely to run into problems. While qPCR tests are a form of genetic testing, generally these tests do not use the allele-specfic qPCR assays needed to assess relevant point mutations.

“Most of the cannabis testing kits out there targeting Aspergillus probably aren’t sensitive to this subtle, single variant difference because the rest of the genome is pretty much the same,” McKernan said.

“We looked at our [current] kits, and of course, ours aren’t going to be able to discern the difference. But we’re in the process of now validating one test that can split the difference between these two.”

In the case of AF36, this difference is a largely an inconsequential one, McKernan says. While AF36 does not produce the toxin like Aspergillus flavus, exposure to AF36 can still cause lung problems. But having a test that can tell the difference may still be helpful for cultivators.

“I don’t know if the regulators are necessarily going to want to differentiate [AF36 and Aspergillus flavus], because both of them can still give you a lung infection,” McKernan said. “But a grower might still want to know, if I’m failing for Aspergillus, what’s the reason? Is it because there’s a farm next door that’s spraying this stuff? Or is it because I’ve got to clean up elsewhere in my facility?”

As the desire for greener cultivation practices pushes more industries towards the use of biocontrol agents, it is important that the cannabis industry is prepared to adapt. For other non-Aspergillus-based biocontrol agents, the development of such tests could help with the wider adoption of biocontrol agents both within and without the cannabis industry.


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