Bowdoin Science Journal

Ericah Folden

Infant Development and The Social Sharing of Microbes

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The human digestive tract, which includes the mouth, esophagus, stomach, intestines, anus, and rectum, contains a highly diverse and complex community of microorganisms, known collectively as the gut microbiome. The gut microbiome forms during birth and rapidly evolves throughout infancy. This phase of development is absolutely crucial for the health of the immune system, metabolism, and long-term health outcomes, as it plays a large role in reducing the risk of allergies, diabetes, and many bowel and cardiovascular diseases (Borrego-Ruiz and Borrego, 2025). While it may be tempting to think that the sharing of microbes among infants is a bad thing, this is largely not the case. The microbes that are transmitted in a nursery and other similar environments are mainly non-pathogenic, meaning that they do not cause disease. Many of these microbes are actually beneficial for health, aiding in digestion, immune system development, and more. Typically, the development of the gut microbiome has been thought to be through vertical transmission, where microbes are passed from the mother or other close, immediate family contact, to infants. However, a recent study from Ricci et al. (2026) published in Nature suggests that gut microbiome development through simple vertical transmission serves as an incomplete model. Instead, they suggest that another key process is at play: horizontal transmission. This kind of transmission is the sharing of microbes between unrelated people. In infants, this often occurs in communal environments like nurseries.

Researchers Ricci and colleagues sought to investigate if infants obtained microbes just from their families, or if they acquired them from their peers, especially in a daycare setting. In the study, Ricci et al. analyzed how social interactions influence the human gut microbiome in infants by studying 43 infants in the UK between the ages of four and fifteen months, as well as their parents, siblings, caregivers, and even household pets. These authors analyzed DNA sequences obtained from samples of the stool of these infants over time to gather data about which microbes were present in their gut microbiomes. They analyzed several timepoints, including before and after nursery attendance began. This study design allowed the researchers to effectively track the transmission of microbes between individuals. The DNA sequencing approach taken by this group offered the advantages of more accurate, strain-level identification of microbes and allowed tracking of where certain microbes originated, because they analyzed entire DNA sequences. The alternative to this approach is culturing stool samples, which offers opportunity for additional testing but a severe limitation in that most microbes can not be cultured in a lab (Ricci et al., 2026).

Looking specifically in a daycare setting, the researchers discovered that infants experienced a significant increase in sharing of microbes, indicating that a nursery setting can play an important role in shaping the gut microbiome. This increase in transmission was seen at a much higher rate compared to settings within family-only units, where sharing was relatively stable. These findings indicate that horizontal transmission can be just as important as vertical transmission when it comes to developing and increasing biodiversity of the infant gut microbiome.

But how are microbes transmitted between infants in a nursery environment? It all comes down to proximity and activities. Researchers found increased microbial transmission between infants who spent more time together or had frequent physical contact compared to those that didn’t. Additionally, activities such as playing with the same toys and touching the same surfaces further facilitated microbe exchange. These findings show that exchange of microbes between infants in a nursery is not random, but rather a process that follows a pattern of behavior exhibited by the infants.

On top of understanding that microbial transmission occurs, the study highlighted the individual differences between infants and how they were affected by the conditions in the nursery. The researchers found that infants with siblings had more diverse microbiomes from the start of the study compared to those without, likely due to more microbe exposure at home from siblings. In turn, these infants acquired less new microbe strains, suggesting that previous microbe exposure can influence later development of the microbiome. This highlights the importance of being exposed to a diverse range of microbes as an infant, tying into the hygiene hypothesis, which states that without an early, diverse exposure to microbes, children are more likely to develop allergies and autoimmune diseases (Sironi and Clerici, 2010). Infants who had previously taken antibiotics, and therefore had a lack of diversity in their gut microbiome, experienced a recovery from being in a nursery environment. While the infants did recover fully, they recovered with a different composition of microbes. Through exposure to new strains around them, infants regained microbial diversity, highlighting the adaptability of the infant gut microbiome in the face of disruption. The findings of the study illustrate how the social environments of infants can be crucial in developing a gut microbiome that supports a healthy body.

This study also has its limitations. The number of nurseries was quite small, three to be exact, and researchers did not address the long-term presence of these microbes and their impact on health in the future. As such, because the infants were from a similar region, these findings may not be applicable to other populations who have different environmental and socioeconomic differences. Additionally, possible confounding variables such as diet and broader environmental conditions were not studied. Future research is needed to account for these limitations and progress the body of knowledge on infant horizontal transmission, as well as the microbial composition of infants who do not get placed in daycare or other social settings.

Overall, Ricci et al. (2026) challenge our current understanding of how the human gut microbiome develops. By looking beyond the process of vertical transmission, we learn that the microbiome is also influenced by social interaction, behaviors, and shared environments. Specifically, nursery settings are centers of important microbial exchanges, where infants are constantly exposed to new microbes. The results of this study emphasize that infant social interactions can biologically shape long-term health through development of the gut microbiome.

 

Works Cited

Borrego-Ruiz A, Borrego JJ. 2025. Early-life gut microbiome development and its potential long-term impact on health outcomes. Microbiome Res Rep. 4(2). doi:10.20517/mrr.2024.78. [accessed 2026 May 3]. https://www.oaepublish.com/articles/mrr.2024.78.

Ricci L, Heidrich V, Punčochář M, Armanini F, Ciciani M, Nabinejad A, Fazaeli F, Piperni E, Servais C, Pinto F, et al. 2026. Baby-to-baby strain transmission shapes the developing gut microbiome. Nature. 651(8104):191–200. doi:10.1038/s41586-025-09983-z. [accessed 2026 May 3]. https://www.nature.com/articles/s41586-025-09983-z.

Sironi M, Clerici M. 2010. The hygiene hypothesis: an evolutionary perspective. Microbes and Infection. 12(6):421–427. doi:10.1016/j.micinf.2010.02.002. [accessed 2026 May 3]. https://linkinghub.elsevier.com/retrieve/pii/S128645791000050X.



The Science of When You Exercise

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People often think the most important aspect of how exercise affects your overall health is how hard you work, how much weight you can lift, or how far you can run. However, two recent studies have uncovered another factor that might be just as important for maximizing health benefits – when you exercise. Multiple studies have looked at the impact of how when you exercise affects your body. For example, one study looked at the impact of exercise timing in mice, focusing on the growth of muscle tissue, while another study looked at a large population of people and how their exercise habits affected sleep quality. Together, these studies show that when exercise takes place matters more than most people believe.

The first study, done by Liu et al. and published in Nature Communications, looked at how timing of exercise in mice affected long-term health (Liu et al., 2025). Mice, like people, have a circadian rhythm, which is a 24-hour internal clock in the body that regulates and affects energy, metabolism, and sleep. Muscles in the body also have internal clocks, which decide when to burn fat or sugar.

In the study, Liu et al. had two groups of mice run at a low intensity and low volume on treadmills at different times of day: one group exercised before sleep and the other exercised right after waking up. Training lasted for several months, and researchers measured the mice’s body weight throughout the study and measured the mice’s strength, endurance, and blood sugar before and after the study was conducted, all of which are indicators of long-term exercise results. The results of the study were quite clear. Mice who exercised before sleep showed increased physical and metabolic improvements after the period of consistent exercise, meaning they gained less fat, had more endurance, and showed better blood sugar control. The group of mice that exercised after waking saw less improvement in these areas (Liu et al., 2025).

The second study, done by Leota et al. and also published in Nature Communications, tracked the health data of over 14,000 human participants using fitness wearables over four million nights of sleep (Leota et al., 2025). The researchers wanted to see whether exercising in the evening, before bedtime, affected sleep quality.

The researchers found that the later and harder people worked out, the more their sleep was affected. When people exercised four or more hours before going to bed, their sleep was normal, regardless of the intensity of the workout. When people exercised two to four hours before going to bed, they took a longer time to fall asleep and slept less. When people exercised two hours or less before going to bed, especially at a high intensity, sleep noticeably got worse. Some took up to over an hour longer to fall asleep, slept about 40 minutes less overall, and had a higher heart rate throughout the night (Leota et al., 2025).

Although the mice study found that exercise before bed improved overall health, the human study found that the closer exercise got to bedtime, the worse sleep became, which is also known to negatively impact recovery and overall health. While these studies may seem contradictory, they actually align upon consideration of the factor of exercise intensity. High-intensity training in the evening negatively affects sleep, while low/moderate-intensity exercise in the evening is beneficial for muscle growth and recovery without impacting sleep.

Although both studies were different, they arrived at the same key conclusion: that the body works best when its internal cycles, like its circadian rhythm, are not disrupted. Exercise, such as heavy lifting or sprinting, activates the body’s sympathetic nervous system, which is the part of the nervous system responsible for the “fight or flight” response. Sleep, along with recovery, lowered heart rate, and relaxation, is activated by the parasympathetic nervous system, otherwise known as the “rest and digest” state. While the activation of the sympathetic nervous system is good for exercise and performance, it is not good when the body needs to sleep. Instead of letting the body settle down, activation of the sympathetic nervous system keeps your body revved up, lessening sleep time and quality, and therefore overall recovery and future performance.

Because of the busyness of daily life, it’s not always possible to perfectly time every workout. Evening workouts are often unavoidable due to the realities of many people’s daily schedules. However, the combination of results from these studies shows that evening workouts aren’t automatically bad for overall health. In fact, they can even improve the benefits of exercise as long as their intensities are adjusted according to their relation to bedtime. If working out in the evening more than four hours before bedtime, high-intensity exercise can take place without risk of impacting sleep quality and physical health. If working out four hours or less before bedtime, it is better to opt for lower-intensity exercise, which will allow you to sleep better and recover more quickly. In the end, both studies show that being slightly more intentional about when and how hard you train can make a real difference in your sleep, recovery, and overall performance.

 

Works Cited

Liu, J., Xiao, F., Choubey, A., Kumar S, U., Wang, Y., Hong, S., Yang, T., Otlu, H. G., Oturmaz, E. S., Loro, E., Sun, Y., Saha, P., Khurana, T. S., Chen, L., Hou, X., & Sun, Z. (2025). Muscle Rev-erb controls time-dependent adaptations to chronic exercise in mice. Nature Communications, 16(1), 5708. https://doi.org/10.1038/s41467-025-60520-y

Leota, J., Presby, D. M., Le, F., Czeisler, M. É., Mascaro, L., Capodilupo, E. R., Wiley, J. F., Drummond, S. P. A., Rajaratnam, S. M. W., & Facer-Childs, E. R. (2025). Dose-response relationship between evening exercise and sleep. Nature Communications, 16(1), 3297. https://doi.org/10.1038/s41467-025-58271-x