Scientists don't actually know how the bacteria in yogurt and other fermented foods help humans digest food easier. But a recent study hit on a possible explanation. Turns out, some probiotics seem to be capable of altering gene expression in our native gut flora.
Jeffrey Gordon, a microbiologist at Washington University in St. Louis, and his team gave a commercially-available probiotic yogurt containing five strains of bacteria to healthy adult volunteers and administered the same five strains to mice that harbored a subset of genetically-characterized human gut microbes. The yogurt bacteria did not significantly alter population structure in any of the entrenched gut microbes, in humans or mice—a result that is not surprising, according to Mills. "To assume that you could eat a yogurt and numerically challenge what's in your gut is kind of like dumping a gallon of Kool-Aid in your swimming pool and expecting it to change color," he said.
But RNA sequencing of the human gut microbes in the mice revealed that the probiotic bacteria changed the expression of gut microbe genes encoding key metabolic enzymes, such as those involved in the catabolism of sugars called xylooligosaccharides, which are found in many fruits and vegetables. Mass spectrometry of metabolites in urine, which result from the ramped up metabolic processes in the probiotic-fed mice, confirmed the alterations, and when the researchers ran similar analyses on gut microbes from the human yogurt eaters, they found upregulation of the same genes.
This study won't be the final word on the subject of how probiotics work. It needs to be replicated and, even then, there are still a lot of questions that need answering. But it does represent an interesting peek inside an interaction between our bodies and other life forms. Cool stuff!