Infants exposed to higher levels of nicotine that lingers on surfaces and in the air had different gut bacteria than babies with lower exposure, a study found.

Researchers examined bacterial assemblages in stool collected from infants born into households that were smoke-free or into households that included a tobacco smoker. The study, published April 16 in Environmental Research, suggests that exposure to thirdhand smoke can reshape the gut microbiomes of these tiny patients, with yet unknown consequences for long-term health.

"There's a lot of interest in the human microbiome as a major contributor to immune functioning, among other things," said Thomas Northrup, lead author of the study and an associate professor of family and community medicine at McGovern Medical School at UTHealth. "And we've known for a long time that tobacco smoke and tobacco toxicants are antimicrobial and antifungal."

Research in adults shows that smoking can alter bacterial communities in the gut, mouth and airways, and other studies have found that secondhand smoke and thirdhand smoke can affect the gut microbiome of children. But less is known about the effects of hand-me-down smoke on infants' developing microbial assemblages.

Unlike secondhand smoke, which describes the fumes from the burning end of a cigarette and smokers' exhalations, thirdhand smoke is the residue and particles from cigarettes that cling to surfaces, become trapped in clothes and carpets, and build up in house dust. Even if parents smoke only outdoors, children can be exposed to these toxic particles as they are transferred from hands and clothes.

As a first glimpse of how thirdhand smoke could affect newborns' microbiomes, Northrup and his colleagues examined infants admitted to a metropolitan neonatal intensive care unit, or NICU, shortly after birth. In total, the study included 43 mother-infant pairs: 11 from nonsmoking households, and 32 from households where at least one household member smoked cigarettes. The babies were studied before they were discharged from the NICU and were never exposed to secondhand smoke.

To quantify thirdhand smoke exposure, the team measured nicotine residue on furniture in the NICU, and levels of a nicotine byproduct in infant urine. For the gut microbiome analysis, the researchers analyzed the bacteria found in poop scraped from a dirty diaper.

According to Northrup, the main finding was that babies who lived with smokers had lower gut bacterial diversity than babies from smoke-free homes. 

Looking closer at different bacteria that made up the microbiome, the researchers found that the proportions of seven out of eight bacterial groups were different between infants from households with smokers and infants from households without smokers. And the level of bacteria was associated with other measures of nicotine exposure. 

Most notably, infants from tobacco-using households had lower levels of a bacterial genus called Bifidobacterium, which includes a species called Bifidobacterium infantis, which Northrup said is especially important during infancy to promote overall health.

"There are even some cool studies that show the more an infant's gut is colonized by Bifidobacterium infantis, the better their vaccine response is," he added.

Differences in the microbiome could have adverse effects on the developing infant, but Northrup said that more research is needed to evaluate how thirdhand smoke impacts health.

"I think [this study] adds to the growing literature about the number of different factors that can potentially disrupt gut microbiome development, and it suggests we need to devote additional resources to examining the long-term impact of the antimicrobial activity contributed by tobacco and tobacco byproducts," he said.

Only one poop sample was collected from each baby, but in future work, the researchers will follow infants for up to six months and collect multiple stool samples to understand how the gut microbiome evolves over time. To better isolate the influence of thirdhand smoke, Northrup said the researchers will also more comprehensively measure other variables that are known to influence gut bacteria. 

"We know that air pollution and exposure to metals and elements can have a negative impact on some types of taxa present in the gut," he explained. "We know that living with a furry pet in your home, for example, can alter your gut microbiome compared to infants or children who don't have a furry pet in the home."

The study, "Thirdhand smoke associations with the gut microbiomes of infants admitted to a neonatal intensive care unit: An observational study," published April 16 in Environmental Research, was authored by Thomas F. Northrup, Angela L. Stotts, Robert Suchting, Amir M. Khan, Charles Green, Michelle R. Klawans and Mary Johnson, The University of Texas Health Science Center at Houston (UTHealth), McGovern Medical School; Georg E. Matt, Eunha Hoh, Penelope J. E. Quintana and Melbourne F. Hovell, San Diego State University; Neal Benowitz and Peyton Jacob, University of California, San Francisco; and Christopher J. Stewart, Newcastle University.