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Cannabis Changes the Oral Bacteria of Smokers, Study Finds

By Leo Bear-McGuinness

Published: Dec 09, 2021   
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Cannabis smoke can change the kind of bacteria present in a person’s mouth, according to a new study.

Published in EBioMedicine, the study compared the saliva of chronic cannabis smokers against the saliva of non-smokers. The researchers found that the smokers had more species of Streptococcus and Actinomyces bacteria and fewer species of Neisseria bacteria than the non-smokers.

The presence of Actinomyces bacteria was a particular concern to researchers, as species of Actinomyces have been associated with the likelihood of developing Alzheimer’s disease.

To test how these microbes may affect the health of smokers, the research team also inoculated several mice with the bacteria, studied their behaviors, and dissected their brain tissue.

Bacteria-vaporizing vapor

After the stomach, the human mouth houses the most diverse community of microbes found in the body. Over 700 species of bacteria can typically be found living on a person’s tongue, teeth, and oral tissue.

Several studies have already demonstrated that cigarette smoke can upset this fragile ecosystem, but much less is known about the effects of cannabis smoke.

To address this oversight, a research team from the Medical University of South Carolina recruited 16 cannabis smokers and 27 non-smokers for their study. The 16 smokers were quizzed on their history of cannabis use, and all subjects were asked to provide saliva samples (after washing their mouths to remove food debris). These samples were then genetically sequenced to reveal the number and type of bacteria species present in the subjects’ mouths.

“We found that cannabis smoking was associated with decreased oral microbial diversity compared to those in the non-smoking control group,” the researchers wrote in their paper.

Proteobacteria and Neisseria species, in particular, were far less present in the saliva of cannabis smokers compared to the saliva of non-smokers. Actinomyces, Veillonella, Megasphaera, and Streptococcus bacteria, on the other hand, were found to be more abundant in the smokers’ mouths.

Interestingly, the scientists found that a smoker's level of oral Actinomyces meyeri bacteria was associated with the age they first consumed cannabis. The younger they were when they first started smoking, the more A. meyeri bacteria were found in their mouth.

After multiple comparisons, the researchers concluded that, compared to the non-smokers, the cannabis smokers had less bacteria from 16 certain species and more bacteria from 20 other species.

Mice microbes

To better understand how these changes in oral bacteria could affect a person’s health, the South Carolina researchers inoculated mice with either Actinomyces or Neisseria bacteria species, to recreate the oral conditions of cannabis smokers and non-smokers, respectively.

“The most exciting aspect of our work is the link between the oral microbiome and brain health,” Wei Jiang, an associate professor at Medical University of South Carolina and head of the study, said in a statement.

“To prove this link, we went beyond the typical associative studies in humans and conducted a causal study in mice.”

The mice’s movements and behaviors were recorded. The animals were later euthanized, and their brains dissected for study.

Jiang and her colleagues observed that the mice inoculated with A. meyeri bacteria (the “cannabis smoker” group) were much slower and moved less than the mice treated with Neisseria elongate (the “non-smoker” group).

High levels of macrophages (a type of white blood cell) and a protein called β-amyloid were also found in the brains of mice inoculated with A. meyeri bacteria. Both factors have been associated with the development of neurodegenerative diseases, such as Alzheimer’s and dementia.

However, while many macrophages were found in the mice brains, the cells hadn’t led to the kind of brain inflammation associated with such neurodegenerative diseases and brain dysfunction.

“That’s a puzzle,” Jiang remarked. “In general, people think that if you activate macrophages and they migrate to the brain, they produce pro-inflammatory signals that result in brain inflammation and eventually brain dysfunction.”

“However, brain dysfunction can also occur without the conventional inflammatory signals.”

Jiang and her team now plan to carry out more studies to investigate how cannabis use and A. meyeri bacteria affect brain health.


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