The latest study from the Lawrence Berkeley National Laboratory, UC San Francisco, and Nanjing Medical University shows that exposure to thirdhand smoke in early life is associated with long-lasting changes to our microbiomes—the microbes that help keep us healthy.  

By Leta Dickinson

July 2, 2021

The microbiome, or the unique assortment of bacteria, fungi, and other microscopic life that lives on surfaces and living things, is getting more and more attention. The more we learn about the microbiome, the more we realize how important these tiny microbes are to human health. Humans first come into contact with microbes during and shortly after birth when they inherit microbes from their mother through exposure in the birth canal and breast milk. From that point on, everything we touch and eat can affect the microbes within and on us. A child’s early years are most important in establishing a stable and balanced microbiome to support human health.

Many environmental factors can change the microbiome our bodies develop. Thirdhand Smoke Research Consortium members from Lawrence Berkeley National Laboratory and UC San Francisco, with colleagues from Nanjing Medical University, set out to explore the impact of thirdhand smoke exposure on our microbiomes by modeling thirdhand smoke exposure in mice at various stages of life. They found that mice exposed to thirdhand smoke just after birth displayed the most significant changes in their microbiomes. This stage of development in mice is comparable to human infants in their first year of life, suggesting that humans in this period of life may be the most sensitive to thirdhand smoke exposure.

The study separated mice into three distinct life stages: infant mice just after birth to 3 weeks old, pubescent mice between 4 and 7 weeks old, and young adult mice between 9 and 12 weeks old. Some of the mice were kept in enclosures that contained thirdhand smoke contaminated pieces of cloth; others were kept in enclosures that did not contain contaminated cloth. After three weeks of living in these enclosures, microbiome samples were taken from a few mice and analyzed for the types and numbers of microbes in their bodies.

The researchers found that the microbiomes of mice in different stages of life responded differently to the thirdhand smoke exposure. In the youngest mice, there were significant changes in the microbiomes of the exposed mice, when compared to the unexposed mice. The levels of microbes in the exposed infant mice decreased in 7 out of the 9 microbe types that were affected by exposure. In the older mice (the prepubescent and the adult), there were no significant changes in the microbiomes of the exposed mice when compared to the unexposed.

Dr. Jian-Hua Mao emphasizes that this study focused on long-lasting changes to the microbiome due to thirdhand smoke exposure and how the results they observed in mice might translate to humans. “A lot of studies show that for humans, from birth to around 3 years old is a very critical window for the microbiome establishment in your body,” Dr. Mao says. “If you make some microbiome changes, they probably remain persistent—[the microbiome] will never go back. Later, in puberty and as an adult, if there are some changes, you can recover from that.”

This means that the microbiomes of the older mice were more or less the same regardless of their thirdhand smoke exposure suggesting the microbe environment in adults is more developed and resilient. The infant mice, however, were still establishing their microbiomes, meaning any changes in response to thirdhand smoke may not be reversed.  

The microbiome is a community of many different types of organisms that is constantly changing. Exposure to the same toxin can produce different results under different circumstances. In this study, the researchers observed that one family of bacteria, called Anaeroplasmataceae, showed three different responses to thirdhand smoke exposure: The number of Anaeroplasmataceae bacteria increased in infant mice, remained the same in pubescent mice, and decreased in the adult mice.

“The microbiome dynamically responds, especially early in life, to environmental exposure,” Dr. Antoine Snijders says. “It could be that interactions between bacteria are different later in life. It’s complex interaction. We’ve observed these changes, but we don’t exactly know how or what the mechanism is.”

This study, while not on humans, helps identify what life stage thirdhand smoke microbiome studies should focus on—infancy and early life. There are a few previous studies on infant and child microbiome changes associated with thirdhand smoke, and these have studied some of the same bacterial families. While it is clear that thirdhand smoke alters the microbiome, there still remain questions about how exactly these changes translate to health and disease.

Click here to read the research study.