If you pick up any probiotic from a pharmacy shelf, the label will offer almost nothing useful. It lists the species name, possibly a strain code in tiny print, and promises general benefits for the gut.
Underlying all of this is the assumption that a good bacterium is a good bacterium, and that if it belongs to the right family, it will do its job.
A team of researchers from South Korea decided to test this assumption directly by comparing dozens of strains across all the parameters that determine probiotic effectiveness. The differences they found were more striking than expected, and nothing like what the label would suggest. The study results were published in the journal Frontiers in Microbiology.
The bacteria that make up most probiotics belong to a large group of lactic acid bacteria—the same microbes that ferment milk into cheese and sour vegetables into pickles. They have coexisted with us in food and in our gut for a very long time, carrying an impeccable safety record.
Researchers at Korea University in Seoul collected 27 such bacterial isolates from two sources: the human oral cavity and locally produced cheese. Inseo Kim, working alongside Hyun Park from the Department of Biotechnology, led the effort to determine what these microbes are and what they can do.
Genetic identification classified all 27 samples into five species. One of them, Lactobacillus rhamnosus, dominated the oral samples, while the cheese contained a mix of the other four species. Naming them was easy. The harder part was deciding whether any of them could survive the journey through the human body and earn the reputation they so often carry.
A probiotic is useless if it dies on the way to the stomach. Stomach acid is strong enough to dissolve most living organisms. Bile entering the small intestine adds another challenge. To be considered beneficial, a microbe must withstand both and remain alive.
The team submerged each strain in acid with a harsh pH of four and then waited two hours. The results clearly split the group. Two species, L. rhamnosus and Lactobacillus reuteri, tolerated the acid well: about 98 percent of their cells remained alive afterward.
Others were almost completely destroyed, though a few individual specimens held on here and there despite the odds. This study clearly shows that acid resistance is not a trait shared by an entire species. Within a single species, one strain survived the acid exposure while its relatives did not.
A bottle may list the correct name, but the microbe could still be present unchanged and never reach the intestine.
Survival is only the first step. To be beneficial, a probiotic must attach to the intestinal lining and hold on long enough to establish itself. If it gets washed away, no trace remains.
The researchers grew a layer of human intestinal cells in the lab and measured how firmly each strain could attach to them. It turned out that one strain was a clear winner.
The L. reuteri strain attached at about 80 percent, far ahead of all other strains in the lineup, which mostly ranged from half to two-thirds. One strain did not attach at all.
Genome analysis provided a clue. One gene, present in extra copies in the most adhesive strain, appears to help bacteria remodel their outer surface—the part that directly contacts the intestinal lining. The researchers refrained from making direct claims about whether this gene is the direct cause of the strong grip or merely a contributing factor.
Fighting harmful microbes is one of the main promises listed on any probiotic label. The team tested each strain against three problematic bacteria, including Escherichia coli and Staphylococcus aureus, as well as common yeast.
L. rhamnosus showed the strongest antimicrobial activity. It suppressed all three pathogens. Others were more selective, killing one pathogen while leaving the others largely untouched. This pattern did not follow any clear rules, and each strain fought its own battle.
After examining the DNA, the researchers found that the strongest microbe fighter had two separate gene clusters responsible for producing antimicrobial compounds, while its weaker relatives had only one cluster or none at all.
These genetic differences matched exactly what was observed in the petri dish, though the team notes that antimicrobial activity could also be due to acids or other byproducts they did not isolate.
The biggest surprise came when comparing strains of the same species. Before this study, which compared such a large number of isolates in a single set of tests, it was believed that knowing the species of a microbe provided most of the necessary information. That is not the case.
Two strains with identical species names and genomes that looked nearly identical on paper behaved differently in the lab. One tolerated acid, and the other did not.
The lack of obvious gaps in their DNA does not explain this, pointing to differences in how genes are turned on and off, rather than which genes are present, as a more likely explanation. For the first time, across so many parallel tests, it was shown that the species name on a label is only a rough guide. Significant differences exist just one level down, at the individual strain level.
We now know that two probiotics with the same species name can differ dramatically in the traits that determine whether they will benefit you at all.
Acid resistance, intestinal adhesion, and the ability to fight microbes vary from strain to strain, and the genes underlying these differences are beginning to become clear.
This knowledge allows manufacturers and doctors to evaluate probiotics more precisely. Instead of relying on the species name, they can identify specific strains that survive, attach, and fight infections.
They can then create products based on those that pass the test. The same genetic markers could become a checklist for identifying the most promising strains before a single capsule ever reaches a store shelf.
For those who have taken probiotics on faith, this lesson is both sobering and useful. The benefits of probiotics vary widely, and a new generation of supplements may finally be able to tell you exactly what kind of probiotic you are getting.
About the Author
