This research illustrates how little we currently know about gut bacteria.But it did show the importance of diet. From Science Daily:
Trillions of bacteria live in each person's digestive tract. Scientists believe that some of these bacteria help digest food and stave off harmful infections, but their role in human health is not well understood.
To help shed light on the role of these bacteria, a team of researchers led by MIT associate professor Eric Alm recently tracked fluctuations in the bacterial populations of two research subjects over a full year. The findings, described in the July 25 issue of the journal Genome Biology, suggest that while these populations are fairly stable, they undergo daily fluctuations in response to changes in diet and other factors...."To a large extent, the main factor we found that explained a lot of that variance was the diet."
There are a few thousand strains of bacteria that can inhabit the human gut, but only a few hundred of those are found in any given individual, Alm says. For one year, the two subjects in the study collected daily stool samples so bacterial populations could be measured. They also used an iPhone app to track lifestyle factors such as diet, sleep, mood, and exercise, generating a huge amount of data.
Analysis of this data revealed that dietary changes could produce daily variations in the populations of different strains of bacteria. For example, an increase in fiber correlated with a boost in the populations of Bifidobacteria, Roseburia, and Eubacterium rectale. Four strains -- including Faecalibacterium prausnitzii, which has been implicated in protecting against inflammatory bowel disease -- were correlated with eating citrus.
During the study, each of the two subjects experienced an event that dramatically altered the gut microbiome. Subject B experienced food poisoning caused by Salmonella, and Subject A traveled to a developing nation, where he experienced diarrheal illness for two weeks.
During Subject B's infection, Salmonella leapt from 10 percent of the gut microbiome to nearly 30 percent. At the same time, populations of bacteria from the phylum Firmicutes, believed to be beneficial to human health, nearly disappeared. After the subject recovered, Firmicutes rebounded to about 40 percent of the total microbiome, but most of the strains were different from those originally present.
Subject A also exhibited severe disruptions to his microbiome during his trip, but once he returned to the United States, it returned to normal. Unlike Subject B's recovery from food poisoning, Subject A's populations returned to their original composition.