Some probiotics could feed, rather than fend off, infections

Probiotics aren’t universally gut-friendly, a study in mice suggests. At least one type of usually beneficial gut bacteria may invite an unwanted guest.

A single dose of Lactobacillus acidophilus bacteria, a popular probiotic component found naturally in milk and yogurt, can make it easier for Clostridioides difficile to take hold in the guts of mice that were recently treated with antibiotics, researchers report July 21 in mBio. By contrast, another strain of probiotic microbe called Lactobacillus gasseri Lg-36 helped mice resist infection by C. difficile, a pathogen that can cause severe and sometimes life-threatening diarrheal disease after taking antibiotics.

Both Lactobacillus microbes were chosen for the study because of their known beneficial effects on gut health, says Matthew Foley, a microbiologist at North Carolina State University in Raleigh. He and colleagues expected the bacteria to have subtle differences in their ability to fend off the pathogen but were surprised to find that “they were essentially diametrically opposed.”

L. gasseri Lg-36 has extra copies of genes for making antibacterial peptides that could directly fight C. difficile, the researchers discovered. Not all L. gasseri strains have equal numbers of antibacterial peptide genes, showing that the exact strain matters, says N.C. State microbiologist Rodolphe Barrangou. 

L. gasseri Lg-36 also encouraged the growth of Muribaculaceae bacteria. Muribaculaceae scavenge for the same types of food that C. difficile prefers, effectively starving the competition. By contrast, L. acidophilus may inadvertently feed the pathogen by breaking down proteins into amino acids C. difficile needs to grow.

Those indirect effects lasted long after L. gasseri Lg-36 passed through the mice’s guts, the researchers found. That finding that even temporary residents of the gut can leave a lasting mark is underappreciated, Barrangou says.

The team hasn’t yet tested blends of multiple species of Lactobacillus that resemble probiotics people may buy. And the mouse results may not be directly applicable to humans, the researchers say. Strains and species listed on probiotic labels may behave differently depending on whether a person is healthy or ill, the exact blend of bacteria in the probiotic, and the microbes already in the person’s gut, Barrangou says. “Not all probiotics are the same, not all diseases are the same…. Not all outcomes are the same.”