Probiotics are often praised for their gut health benefits, but new research suggests that not all strains are equally helpful—and some might even do more harm than good.

A recent study published on July 21 in mBio reveals that Lactobacillus acidophilus, a common probiotic found in yogurt and milk, may actually increase the risk of Clostridioides difficile (C. difficile) infection in mice. When administered after antibiotic treatment, a single dose of L. acidophilus made it easier for the dangerous gut pathogen to colonize the intestines.

In contrast, another probiotic strain, Lactobacillus gasseri Lg-36, showed promising protective effects. Mice given this strain were better able to resist C. difficile infection—a serious and potentially deadly diarrheal illness that often occurs after antibiotic use.

The study underscores the complexity of the gut microbiome and the need for targeted probiotic therapies. While some strains may bolster the body’s defenses, others might inadvertently compromise gut health, especially in vulnerable individuals recovering from antibiotic treatments.

Experts caution that more human studies are needed, but the findings emphasize that probiotics should not be seen as universally beneficial. Strain specificity and context matter—especially when it comes to preventing infection.