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Rethinking Microbial Screening

Rethinking Microbial Screening

In the current method of microbial screening, the current microbial bioburden limits for total aerobic bacteria and total yeast and mold on cannabis in Nevada are too restrictive for those growers whose use of beneficial microbes is in compliance with the Nevada Department of Agriculture’s current acceptable list of pesticides.

Therefore, the use of certain nutrient products containing State-approved beneficial microbes here in Nevada in cannabis cultivation, has the potential to fail future crops due to resulting microbial counts over and above the State-mandated tolerance levels set at 100,000 colony-forming units per gram (CFU/g) for total aerobic count (TAC) and 10,000 CFU/g for total yeast & mold (TYM) on cannabis. TAC enumerates all bacteria, including those intended to act as “beneficial microbes,” so if your cannabis tests over 100,000 (CFU/g) for flower, the sample fails. Because the State of Nevada was the first MMJ-legal State to comprehensively screen for microbial contamination, this issue had not come to light until now.


DigiPath Labs is collaborating with another company, Medicinal GenomicsTM, who is working on a clever solution to the dilemma. The approach is genomic based and involves the depletion of those DNA molecules associated with the specified list of beneficial microbes currently allowed in the State of Nevada but could be customized to each MMJ legal state. Those excluded beneficial microbial DNA molecules would be captured out of solution and removed prior to qPCR reactions with existing primers for all of the other TAC bacterial DNA molecules and likewise for TYM. This ingenious DNA depletion method is proven and was originally developed as the foundation for exome (protein coding region) capture systems when total genome sequencing is out of the question due to cost and time. The depletion step has the advantage that it’s easy to add more probes given the likelihood that the acceptable beneficial microbe list may change or the depletion step can be omitted to reveal whether or not beneficial microbes are present on the sample.


The current list of acceptable beneficial microbes for use as biopesticides in Nevada include:

Bacteria                                                          Fungi    

Bacillus amyloquefaciens                               Beauveria bassiana

  1. licheniformis                                            Metarhizium anisopliae
  2. megaterium                                             Paeciloyces fumosoroseus 
  3. pumilius                                                   Trichoerma harzianum
  4. subtilis                                                      T. reesei
  5. thuringiensis                                           T. virens
  6. Chromobacterium substsugae
  7. Streptomyces griseoviridis
  8. lydicus


The adoption of this proposed genomic approach toward microbial screening of cannabis would most certainly enable a safer pest management approach with the use of beneficial microbes rather than the use of unapproved pesticides and once again would place Nevada at the cutting edge of applying the best science to the production of quality-assured cannabis.

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