Published in Hormones and Behavior the study used a mouse model to demonstrate that natural microbial exposure influences brain development not only immediately after birth but may also begin shaping it in the womb. Mice were selected due to their close biological and behavioral similarities to humans and because no alternative models exist to study the role of microbes in brain development.

The findings are particularly significant given that modern obstetric practices such as peripartum antibiotic use and Cesarean delivery can disrupt maternal microbial communities. In the United States 40% of women receive antibiotics around childbirth and one-third of births are Cesarean.

At birth a newborn is colonized by microbes as it passes through the birth canal coinciding with critical brain development events. We aimed to investigate how the arrival of these microbes might influence brain development said Alexandra Castillo Ruiz lead author and assistant professor in the MSU Department of Psychology. The team focused on the paraventricular nucleus of the hypothalamus (PVN) a brain region that regulates stress blood pressure water balance and social behavior. Previous studies had shown that germ-free mice raised without microbes experienced increased neuron death in the PVN during early development. The new research sought to determine whether this heightened cell death affected long-term neuron numbers and whether the effects were triggered by microbial exposure at birth or began earlier in the womb through maternal microbial signals.

To investigate the researchers employed a cross-fostering approach placing germ-free newborn mice with microbe-containing mothers and comparing them to control groups. Remarkably examination of the brains just three days after birth revealed that all mice gestated by germ-free mothers had fewer neurons in the paraventricular nucleus of the hypothalamus (PVN), regardless of whether they were exposed to microbes after birth. Additionally germ-free adult mice showed a persistent reduction in PVN neuron numbers. Our study demonstrates that microbes play a crucial role in shaping a brain region essential for regulating body functions and social behavior, and that these effects begin in the womb through signals from maternal microbes said Castillo-Ruiz. She emphasized the broader significance noting Rather than viewing microbes as harmful we should recognize them as vital partners in early development—they are helping build our brains from the very beginning.

Source: https://msutoday.msu.edu/news/2025/08/msu-study-finds-tiny-microbes-shape-brain-development