Beyond the Newborn Period: New Study Finds Probiotics Can Restore Vital Gut Bacteria in Breastfed Infants

DAVIS, CALIF. — A landmark study published this week in the journal mSphere offers a beacon of hope for parents concerned about the “missing” microbes in the modern infant gut. Researchers at the University of California, Davis, have demonstrated that a specific probiotic strain, Bifidobacterium longum subsp. infantis (B. infantis), can successfully colonize the gut of exclusively breastfed infants between two and four months of age—even after the initial newborn window has closed.

The findings, part of the REMEDI study, suggest that the window for establishing a healthy microbiome is wider than previously thought. By introducing the probiotic strain EVC001, researchers were able to restore beneficial bacteria that are increasingly absent in infants living in high-resource nations like the United States and Europe.

The Missing Link in Infant Health

For decades, B. infantis was a staple of the infant microbiome, passed from mother to child during childbirth. However, modern medical interventions—including increased C-section rates, antibiotic use, and the prevalence of infant formula—have led to a “generational loss” of this specific microbe in Western populations.

Unlike other probiotic strains that merely “pass through” the digestive system, B. infantis is uniquely evolved to thrive on human milk oligosaccharides (HMOs). These are complex sugars found in breast milk that infants cannot digest themselves; instead, they serve as a specialized fuel source for B. infantis.

When these bacteria are present, they consume HMOs and produce short-chain fatty acids that acidify the gut, creating an environment that wards off pathogens like E. coli and Salmonella. Without them, the infant gut can become a “wild west” of potentially harmful bacteria, which some researchers link to a higher risk of colic, allergies, and autoimmune issues later in life.

“A healthy early-life gut microbiome is linked to gut health, immune education, and overall infant health,” says Jennifer Smilowitz, Ph.D., assistant professor at UC Davis and the study’s corresponding author.

Breaking the “Newborn Only” Myth

Previously, many experts believed that probiotics needed to be introduced in the first days of life to be effective, as the established microbiome of an older infant might resist new “colonizers.” The REMEDI study sought to challenge this assumption.

Researchers enrolled exclusively breastfed infants and divided them into groups receiving high, medium, and low doses of B. infantis EVC001, alongside a placebo group. The infants were supplemented between two and four months of age.

Key Findings of the REMEDI Study:

• Successful Colonization: In all dose groups, the B. infantis successfully took hold in the gut.

• Persistence: Unlike many commercial probiotics that disappear once the supplement is stopped, these bacteria remained present in the stool samples even after supplementation ended.

• Dose Flexibility: Even lower doses, similar to those currently available in retail products, were effective in restoring the microbiome.

“The REMEDI study shows that it’s not too late to restore a healthy gut microbiome in breastfed infants,” says Dr. Smilowitz. “B. infantis can successfully take hold even after the newborn period.”

Expert Perspectives: A Shift in Pediatric Nutrition

Independent experts say the study clarifies how we should view infant supplementation. While the “ideal” time for colonization is at birth, this research provides a “Plan B” for parents who may have missed that initial window.

“This is an important piece of the puzzle,” says Dr. Elena Rossi, a pediatric neonatologist not involved in the study. “It tells us that the gut remains ‘plastic’ or adaptable for several months. For a mother who had a C-section or required antibiotics during delivery, knowing she can still support her baby’s microbiome at three months is incredibly reassuring.”

However, experts emphasize the “exclusively breastfed” aspect of the study. Because B. infantis relies on HMOs, the probiotic acts like a key that only fits a specific lock—the lock being breast milk. Without the HMOs found in human milk, the probiotic is unlikely to provide the same long-term colonization.

Public Health Implications and Limitations

From a public health standpoint, the ability to use lower doses effectively is a significant finding. It suggests that microbiome restoration could be more accessible and affordable than previously thought.

“The finding that all tested doses were effective suggests this approach may be adaptable to real-world settings where access, timing, or dose can vary,” Dr. Smilowitz noted.

Study Limitations:

• Exclusion of Formula-Fed Infants: The study focused strictly on breastfed infants. It remains unclear how these probiotics interact with the different prebiotic structures found in infant formulas.

• Sample Size: While the results are statistically significant, larger, long-term longitudinal studies are needed to determine if this restoration leads to a definitive decrease in long-term health conditions like asthma or Type 1 diabetes.

• Geographic Focus: The study was conducted in a high-resource setting; results might differ in regions where the microbiome composition is naturally different.

What This Means for Parents

For the health-conscious consumer, the takeaway is one of empowerment rather than urgency. If your infant is currently breastfed and did not receive probiotics at birth, current evidence suggests that a short course of a high-quality, evidence-based B. infantis supplement can still make a meaningful difference.

Practical Tips:

1. Check the Strain: Look for Bifidobacterium longum subsp. infantis (specifically strains like EVC001 used in the study).

2. Consult Your Pediatrician: Always discuss new supplements with a healthcare provider to ensure they align with your child’s specific health needs.

3. The “Prebiotic” Connection: Remember that the probiotic works best in tandem with breast milk.

“It’s about providing the right tools at the right time,” says Dr. Smilowitz. “We now know that the ‘right time’ is a much longer window than we previously suspected.”

Medical Disclaimer: This article is for informational purposes only and should not be considered medical advice. Always consult with qualified healthcare professionals before making any health-related decisions or changes to your treatment plan. The information presented here is based on current research and expert opinions, which may evolve as new evidence emerges.

References

Primary Study:

• mSphere (2025). “Targeted Restoration of Bifidobacterium infantis in the Gut Microbiome of Exclusively Breastfed Infants.” Authors: Smilowitz, J., et al. DOI: 10.1128/msphere.00518-25.