Microbiological contaminants of marijuana

J. M. McPartland

Vermont Alternative Medicine, 53 Washington Street, Middlebury, VT 05753, USA.

McPartland, J.M., 1994. Microbiological contaminants of marijuana. Journal of the International Hemp Association 1: 41-44.

Use of marijuana as a medicament is on the rise. Many medical marijuana users have a suppressed immune system, owing to their disease or treatment. Herbal marijuana, whether field grown or hydroponically cultivated, contains many microorganisms. Many of these organisms may pose a threat to immunosuppressed individuals. The microflora of marijuana is well described in the literature. Similarly, the microflora that cause opportunistic infections in AIDS patients is well documented. These separate literatures are correlated with commentary, and methods for detecting and eliminating microbial contaminants are discussed.


Marijuana is classified as a Schedule I drug "having no medical use, with a high potential for abuse" by the US Drug Enforcement Administration. Nevertheless, a handful of US citizens are permitted use of government-supplied "medical marijuana" for a variety of ailments (Bowersox 1992). Additionally, a large clinical trial utilizing marijuana in AIDS patients has been approved in San Francisco (Doblin 1994).

Doblin and Kleiman (1991) document the widespread illicit use of marijuana for medical purposes. Nearly half of the oncologists polled by Doblin and Kleiman recommended that their patients experiment with marijuana as a medicine. Many of these patients, however, are immuno-suppressed, making them very susceptible to opportunistic bacteria and fungi. Since hundreds of bacteria and fungi are associated with the cultivation of marijuana (McPartland 1991), possible microbiological hazards need to be assessed.

A majority of the organisms associated with marijuana are strictly plant pathogens. They cannot infect healthy humans. But a subset of opportunistic plant pathogens are associated with "post-harvest" or "storage" decay of marijuana. These organisms may infect immuno-compromised individuals and become opportunistic human pathogens. Additionally, many opportunist organisms on plants produce dangerous toxins and/or elicit allergenic reactions when inhaled.

Lastly, a handful of strictly human pathogens have been isolated from marijuana, as contaminants. These agents are highly infectious, and some also exude toxins.

AIDS and opportunistic infections

People with AIDS are immunosuppressed, and very susceptible to infections. According to Bossche et al. (1990), the most common infectious agents include viruses (cytomegalovirus, hepatitis A & B, herpes simplex), bacteria (Mycobacterium spp., Salmonella spp.), fungi (Cryptococcus neoformans, Coccidiodies immitis, Histoplasma capsu-atum, Candida spp.), and protozoans (Pneumocystis carinii, Toxoplasma gondii, Cryptosporidium sp.).

Bossche et al. (1990) also describe rare infections in AIDS patients caused by opportunistic fungi — Aspergillus fumigatus, Penicillium marneffei, Fusarium proliferatum, Alternaria alternata, Mucor sp., Rhizopus sp., Curvularia sp., and Bipolaris sp.

Most of these organisms infect people via the respiratory tract, or by oral-mucosal contact with contaminated material. Inhaling a marijuana cigarette contaminated with any of these organisms poses a risk.

Microflora of marijuana

Viruses and protozoans

Cates & Warren (1975) associated marijuana use with an epidemic of hepatitis B in Germany, but did not isolate the virus from plant material. An outbreak of Hepatitis A in Washington state was linked to consumption of Mexican marijuana, fertilized with human excrement (Alexander 1987). No protozoans are reported from marijuana in the literature.


Several bacteria species cited as present on marijuana are human pathogens, and produce toxins: Taylor et al. (1982) isolated Salmonella muenchen from illicit marijuana. Ungerlerder et al. (1982) cultivated Klebsiella pneumoniae, Enterobacter cloacae, E. agglomerans, group D Streptococcus, and Bacillus sp. from marijuana supplied by the National Institute on Drug Abuse (NIDA). Kurup et al. (1983) isolated several thermophilic actinomycetes from marijuana cigarettes, including Thermoactinomyces candidus, T. vulgaris, and Micropolyspora faeni. These organisms, while not infectious, cause allergic pneumonitis in hypersensitive individuals.

Storage molds associated with marijuana. Left to right: Rhizopus nigrans, Mucor hiemalis, Penicillin chrysogenum, P. italicum, Aspergillus flavus, A. fumigatus, A. niger. Top row: sporangia and conidial heads cross-sectioned to reveal internal structures, with M. hiemalis demonstrating spore release after sporangium dehiscence (400 x). Bottom row: natural habit of these fungi (25 x). From Bush Doctor (1987).


In the popular press, Margolis and Clorfene (1975) and DuToit (1980) describe molds colonizing marijuana in the United States and South Africa, respectively. Surprisingly, both accounts report the mold enhancing psychotropic effects of marijuana. These publications initiated a fad among marijuana users, who exposed their marijuana to molds in the hope of increasing the potency of their crop.

The "black weblike fungus" described by Margolis & Clorfene (1975) suggests an Aspergillus species. Chusid et al. (1975) cite A. fumigatus causing pneumonitis in a patient who buried his marijuana in the ground for "aging." Llamas et al. (1978) recovered A. fumigatus from marijuana owned by a patient suffering allergic bronchopulmonary aspergillosis. Kagen (1981) isolated several Aspergillus species from moldy marijuana, including A. fumigatus, A. niger and A. flavus. Babu et al. (1977) cultured A. niger, A. flavus, A. tamarii, A. sulphureus, and A. repens from seeds in marijuana. Schwartz (1985) isolated A. niger from the sinuses of a marijuana smoker who suffered severe headaches. Llewellyn & O'Rear (1977) demonstrated aflatoxin production in marijuana contaminated with A. flavus and A. parasiticus.

Penicillium species have been isolated from marijuana cigarettes by Kagen et al. (1983) and Kurup et al. (1983). Babu et al. (1977) cultured P. chrysogenum from Cannabis seeds. Bush Doctor (1993) reports isolating P. italicum from marijuana stored with an orange peel at 0o C.

Mucor species have been recovered from marijuana by Kagen et al. (1983) and Kurup et al. (1983). A related zygomycete, Rhizopus stolonifer, was isolated from damp marijuana by Bush Doctor (1993).

Babu et al. (1977) cultured several other fungi from Cannabis seeds that, according to Bossche et al. (1990), may cause opportunistic infections in AIDS patients. These include Alternaria alternata and Curvularia lunata. McPartland (1983, 1991, 1992) recovered these fungi from Cannabis in the field, plus a Fusarium spp. All these fungi produce toxins. Ramírez (1990) reports four policeman contracting histoplasmosis, caused by Histoplasma capsulatum, after destroying a Cannabis field in Puerto Rico.

Detecting contaminants in marijuana

Cultivators and consumers of illicit marijuana identify microbiological contaminants by several crude, but seemingly effective, screening techniques (Bush Doctor 1993). Their methods mimic those used by tobacco growers (Lucas 1975).

Visual screen

Contaminated marijuana often darkens in color and exhibits changes in surface texture. Strands of white to pale-gray fungal hyphae become visible in moldy material. Exposed to light, the fungi sporulate, providing a "fuzzy" appearance. Generally, Rhizopus and Mucor species produce grey-black spores, Penicillium conidia are light blue-green, and Aspergillus conidia are dark green-black.

Olfactory screen

Uncontaminated marijuana produces a minty or aromatic odor. Once infested, the smell changes to a "stale" or "musty" odor. Bush Doctor (1993) reports P. italicum producing a lavender or lilac odor, and A. flavus "smells like a locker room."

Temperature screen

Contaminated marijuana undergoing rapid decay may feel warm to touch.

Fluorescence screen

Bush Doctor (1993) screened marijuana for aflatoxin-producing A. flavus with a UV-A lamp. Contaminated material fluoresces a light green color. This screening method has not been tested by other researchers (Llewellyn & O'Rear 1977).


If any of the above screening tests is positive, the suspect marijuana is discarded, without identifying the contaminant. If identification is desired, the marijuana is either cultured or bioassayed. Kurup et al. (1983) and Kagen et al. (1983) provide the description of culture methods used to identify microbiological contaminants.

Marijuana can also be tested with bioassay kits, to detect specific organisms. These assays utilize ELISA (enzyme linked immunoabsorbent assay) biotechnology. They are more sensitive than cultures and much quicker, but also much more expensive. Positive identification of microbiological toxins may require gas chromatography (Rood et al. 1988)

Eliminating contaminants

Kurup et al. (1983) demonstrated that spores of Aspergillus fumigatus and Mucor species survive in smoke drawn from pyrolyzed marijuana cigarettes. Also, some microbiological toxins are not degraded by combustion (Llewellyn & O'Rear 1977).

Moody et al. (1982) evaluated waterpipes for smoking Aspergillus- contaminated marijuana. They found only a 15% reduction in transmission of fungal spores.

Levitz and Diamond (1991) suggested baking marijuana in home ovens at 150o C, for five minutes before smoking. Oven treatment killed conidia of A. fumigatus, A. flavus and A. niger, and did not degrade the active component of marijuana, tetrahydrocannabinol (THC).

Ungerlerder et al. (1982) gas-sterilized marijuana with a mix of 12% ethylene oxide and 88% dichlorodifluoromethane, at 8.5-10 psi, for 4.5-5 hours. They report no loss of THC from fumigation. But residues remain in air pockets of fumigated material, posing another health hazard. They also sterilized marijuana with high-dose Cobalt 60 irradiation (20,000 Gray Units or less), again without THC reduction.

None of these methods, however, degrade microbial antigens. Sensitized patients may still develop bronchospasm after smoking treated material. Nor do these methods decompose microbial toxins.

The best method of eliminating microbial antigens and toxins is by exclusion. This requires careful attention by both cultivators and consumers. Proper cultivation techniques should eliminate human pathogens. Careful cultivation also excludes most opportunistic plant pathogens. Particular attention is required at harvest, to avoid damaging marijuana before it completely dries. Flue-cured marijuana contains fewer opportunistic fungi than air or sweat-cured crops. Sweat-cured Cannabis (common in illicit marijuana from Colombia) maintains a "tradition" of Aspergillus contamination (Bush Doctor 1993).

Bush Doctor (1975) states that infestation by opportunistic fungi cannot occur in plant material below 15% moisture content (MC). Properly dried marijuana contains about 10% MC (material below 10% MC becomes excessively brittle). The dried, unmanicured marijuana, in small aliquots, can then be vacuum-sealed in plastic pouches. This packaging differs from the marijuana currently supplied by NIDA. NIDA supplies marijuana as manicured, pre-rolled cigarettes, in tin cans (Chait and Pierri 1989).

Opening sealed pouches of marijuana reexposes the material to contamination. Consumers must prevent marijuana from resorbing moisture above 15% MC. Storage at freezer temperatures will not protect marijuana above 15% MC. Placing a lemon or orange peel in stored marijuana (to impart a "pleasant bouquet," according to Frank and Rosenthal, 1978) is discouraged, because the peel may introduce Penicillium or Aspergillus spp. into the stored marijuana. Contaminated material should be discarded.


Many publications have described a variety of microorganisms resident on marijuana. Nevertheless, most researchers remain unaware of these hazards. Schwartz (1987) wonders if the source of a young patient's Curvularia lunata sinusitis could be from contaminated marijuana. Brummund et al. (1987) reply that this is possible, but no Curvularia species have been cited in the Cannabis literature. However, this was indeed reported by Babu et al. (1977). More recently, another Schwartz (1992) states, "I am unaware of any report of non-Aspergillus fungal contamination of marijuana cigarettes." The publications cited herein, provide such reports.

Carefully cultivated and harvested marijuana harbors a minimum of hazardous microorganisms. For added protection, material must be screened for contamination before it is packaged for use as medical marijuana. Since opportunistic infections pose the greatest danger to immunosuppressed consumers, marijuana should be sterilized, preferably by gamma irradiation. Lastly, consumers must be given careful instructions to ensure their marijuana does not become contaminated prior to use.