Genes are important, but diet is more important, in determining the abundance of bacterial species contained in a person’s gut flora, according to a new study by scientists at the University of California, San Francisco (UCSF). This mouse study explores this balance between innate and acquired, and the findings are published in the Dec. 18, 2014, issue of Cell Host and Microbe. Gut bacteria greatly outnumber our own cells, and the microbial species that normally inhabit our bodies throughout our lives can influence our health by affecting our metabolism and even our behavior. However, the composition of the gut flora varies greatly between individuals and changes over time. What has been unclear until now is whether these changes are driven more by differences in a person’s invariant genetic background or are influenced by different diets and other environmental factors. Peter Turnbaugh, first author of the paper and associate professor of microbiology and immunology at UCSF, notes, “In healthy adults, the same microbial strains and species can colonize the gut for many years, while the relative abundance – in other words, the community structure – of each member yet is highly dynamic.” Plastic microbiome Turnbaugh said, “These new findings emphasize that unlike mammalian genomes – which are relatively constant – the microbial genomes that make up the intestinal flora are relatively plastic.” According to Turnbaugh, this holds promise for the treatment of groups with unhealthy gut flora. Turnbaugh put it this way: “Maybe one day we can design some kind of diet that forms a therapeutic gut genome. The good news is that the microbes that respond to a given diet may be similar for many people’s microbiomes, suggesting that we may not need to tailor different interventions for each individual. ” Turnbaugh’s group found that shifting mice to a high-glycemic, high-fat diet can remodel gut microbial community abundance to a new, stable composition over a three-day period in a reproducible manner that does not depend on genetic differences between individual mice. These findings, are consistent with a recent human study by Turnbaugh and colleagues in which gut microbes were rapidly and reproducibly altered when 10 participants consumed a vegan or animal-based diet. Dietary changes, that study showed, can exert powerful effects on gut flora in a matter of days. But this new study – based on data from hundreds of mice with a clear genetic background – shows that diet can trump genetic variation in the host over a period of days to months. Flora changes rapidly with diet When Turnbaugh was conducting his thesis research at the University of Washington with Jeffrey Gordon, he began to strongly suspect that diet might be the most important factor in changing gut flora. genes) were more similar. In this new study, Turnbaugh’s experimental group, selected five mice from five different groups of inbred mice, each of which was genetically identical. In addition, they studied four strains of mice that lacked genes related to immunity and obesity. More than 200 “distant” strains of mice were added – breeding that produced offspring with rich, undetectable genetic diversity. The researchers fed the mice different diets, switching between a high-fat, high-sugar diet (containing 14.8 percent protein, 44.6 percent fat and 40.6 percent carbohydrate) and a low-fat, plant-based diet (containing 22.2 percent protein, 16 percent fat and 61.7 percent carbohydrate). Regardless of the genetic background of the mice, fecal analysis showed that a high-fat, high-sugar diet increased the abundance of members of the thick-walled phylum Bacillus and decreased the abundance of members of the phylum Bacillus. Turnbaugh said, “In the past we’ve shown that altered gut flora in response to a high-fat, high-sugar diet can lead to obesity, and we’ve been very interested in how the metabolic activity of these organisms is affected by diet.” ”Do changes in the microbial community come directly from changes in the nutritional environment within the gastrointestinal tract? Or is it indirectly due to the effect of altered diet on host physiology (an effect that is consistent across genotypes)? remains a fascinating area for future research.” Another surprising finding was that when the researchers returned the mice to their original diet, the changes in microbial abundance were largely – but not completely – reversed. Gut microbes are influenced by past dietary imprinting as well as by the current diet. Repeated dietary changes suggest that most gut flora changes are reversible,” Turnbaugh said. But we also found some bacterial species whose abundance depended on prior dietary imprinting.”