A Short Guide to the Microbiological Testing of Cannabis Plants

By Jodi McDonald
Published: August 22, 2017 | Last updated: April 27, 2022 11:47:30
Key Takeaways

While there are a host of benefits that lab testing your cannabis can provide, one of the most important is looking for microbial contamination because it is not something that can be detected easily.

Labs providing test services to the cannabis industry look at two big-picture things: what is in the product (potency) and what is not in the product (contamination). A single sample sent to a lab for testing is divided up into the required number of sub-samples and processed specifically for each test.


Often the testing happens at the same time in different areas of the lab. There is a wealth of information that comes from the testing cycle of a single product and it becomes more valuable as more batches of products are tested, allowing comparison between different batches.

In this article, we will look at the importance of microbial contamination testing in cannabis.


When a company prepares a drug product, natural health product, or medical device, one of the first things they must decide is how it will be delivered or administered to the patient.

Speaking microbiologically, there are two choices: it is either sterile and is delivered directly to the patient—for example, through injection—or it is an aseptic preparation and it will be delivered to the patient through a less direct means—for example, through an oral administration. When we consider cannabis, it falls into the second group.

As the name implies, microorganisms are microscopic and thus not visible by the naked eye. The testing of this contamination category is important for that very reason. The lab can use the way the organisms like to grow in order to visualize and count how many are present.


All these potential sources of contamination can be easily controlled by procedural controls like operator hygiene practices and post production care of the material.”

For products that are prepared aseptically, the goal is to prepare it without adding extra microbial organisms. However, it is not required to be free from bacteria, yeast, and mold (that would mean it was sterile). There are allowable limits for the number of microorganisms in an aseptic preparation, and the tests that determine how many are present are the Total Aerobic count and the Total Yeast and Mold counts.


In addition to total numbers of organisms, there are limits to the number of pathogens that can be found in cannabis. The Oregon Health Authority advises that products with levels of E. coli or salmonella species present in concentrations higher than 100 colony-forming units per gram of product not be allowed to go to market. The limits that California and Colorado have outlined are more stringent, requiring “none detectable in one gram” of product for either organism.

These tests combined form a clear picture of the total number of organisms that could be isolated in a particular preparation. From this data, a company can monitor production, raw materials, training of staff, and packaging controls.

If a product begins to trend upward or even have a count well above what is expected or allowed, an investigation would include exploring these factors. All these potential sources of contamination can be easily controlled by procedural controls like operator hygiene practices (hand washing) and post production care of the material (keeping it dry).

The sources of microbiological contamination can be broken into two broad categories. The first is production and the second is staff that touches the product. During production (or growing), microbial contamination can come from water and nutrient sources (think of manure or compost, which is loaded with microbial activity), as well as the air in the facility (or outdoors, for those lucky enough to grow outside).

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Once the crop has been harvested, microorganisms can be introduced by the individuals who are in contact with the material—including transfer from the operators’ hands, coughing, and sneezing. Each method of introduction into the product represents a unique category of potential pathogens. The water and nutrient stocks are likely to introduce contamination found in fecal material. The operator may introduce contamination that would represent pathogens from the respiratory system, the gastrointestinal system, the skin, and from fecal material.

Limiting the number of microbial organisms in cannabis is important for three basic reasons. The first is certainly the most important: patient safety. Without limits or testing requirements for microbiological contamination, patients are at risk for infections.

The second reason is also concerns patient safety: exposure to toxins can directly impact the health of a patient or can create an immune crisis for the patient even when there are no viable microorganisms present. The final reason is directly related to production control: it is an indication that the steps in the process for creating the product are not clearly defined, controlled, or monitored.

Cannabis naturally has antibacterial activity. When dried well, stored carefully, and processed using heat, it is generally a safe product when considering microbial contamination. All these factors, while good news for the cannabis user, do not eliminate contamination of products. Microorganisms are tenacious and resistant to harsh conditions.

Couple this with an inability for the average human to detect them with the naked eye, and lab testing of microorganisms becomes an important piece in guaranteeing the safe supply of cannabis products.


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Written by Jodi McDonald

Profile Picture of Jodi McDonald

Jodi McDonald is the president and founder of Keystone Labs and is a medical microbiologist, inventor and super science-geek. She has 20 years’ experience in the quality control, regulatory, and quality assurance environment for the pharmaceutical industry. Jodi believes testing is essential for taking control of your personal health.

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