Certain gut microorganisms can boost human health, but it is unclear how diet could be har- nessed to easily manipulate the composition of gut microbes to boost the levels of desired bacteria. Writing in Cell, Patnode et al. 1 present a useful approach for assessing interactions between human gut microbes and the dietary fibre that sustains their existence. Dietary fibre is promoted as part of a healthy diet worldwide. Many people, however, do not achieve their recommended fibre intake because they consume insufficient fruit, veg- etables and cereals. Inadequate fibre intake is associated with common conditions includ- ing obesity, diabetes and cancer 2 . Yet under- standing the mechanisms that link fibre-rich food to good health is challenging. Dietary fibre encompasses a wide range of complex molecules, most of which are present in plant cells; among them are carbohydrate molecules called glycans, which are resistant to digestion by human enzymes. As a consequence, some ingested fibre is excreted unchanged in faeces, whereas most is metabolized by gut microbes. These microbes have a diverse and extremely complex metabolic capacity. Bacteria that express different enzymes for metaboliz- ing fibre can survive and grow using a range of foods. Some bacterial species might compete with each other for the same food source, which could lower the abundance of species that compete less successfully. How might gut microbes be manipulated through human dietary intervention? For example, the concept of using prebiotics — compounds that affect gut microbes, thereby benefiting the human host — has been pro- posed. One such idea is to use particular fibre sources that provide food for the desired gut microbes 3,4 . However, determining whether dietary fibre can promote health in this way requires a sophisticated understanding of the interactions that occur when the com- plex community of gut microbes encounters a source of fibre. Previous work 5 had indicated that trans- ferring the gut microbes of human twins who have contrasting body masses (obese and lean) into mice induced a corresponding difference in the animals’ body masses. However, when some of the obese mice were housed with the lean mice, they had less adipose fat than did obese animals that were not co-housed with lean mice — and this weight-loss effect correlated with the transfer of Bacteroides bacterial species from the lean mice to the obese mice 5 . High consumption of fibre-rich plant foods was required for this adipose-fat reduction to occur 5 . However, the types of fibre responsible for this effect, and how these interact with specific gut microorganisms, was unknown. Patnode and colleagues now reveal how particular types of glycan can drive com- petition between different Bacteroides species resident in the human gut. Patnode et al. studied mice that lacked their normal microbes, and instead harboured 15 strains of gut-dwelling bacteria from a lean human who had an obese twin. The authors fed the mice different combinations of fibre sources as part of their diet. Analys- ing faecal samples enabled the researchers to track how the diets affected the relative abundance of each bacterial species in the animals’ gut. This approach pinpointed, for example, a dose–response effect of pea fibre on the relative abundance of Bacteroides thetaiotaomicron in the bacterial popula- tion, as well as a pronounced effect of certain Microbiology Food for thought about manipulating gut bacteria Nathalie M. Delzenne & Laure B. Bindels Knowing how dietary fibre nourishes gut microorganisms might suggest ways to boost health-promoting bacteria. A method developed to pinpoint bacteria that consume particular types of dietary fibre could advance such efforts. 15 bacterial strains that dwell in the human gut Food particles B. vulgatus B. cellulosilyticus B. ovatus Arabinan in pea fibre Competition Arabinoxylan 14 bacterial strains (excluding B. cellulosilyticus) Metabolic flexibility a b Figure 1 | Investigating how human gut-dwelling bacteria metabolize dietary fibre. a, Patnode et al. 1 gave mice that lacked their natural gut microbes a set of 15 bacterial strains that dwell in the human gut, including the species Bacteroides cellulosilyticus, Bacteroides ovatus and Bacteroides vulgatus. The authors developed a method for tracking fibre digestion. They generated magnetic beads coated with a fibre of interest, and fed these beads (termed food particles) to the animals. Applying a magnetic field enabled the recovery of food particles and assessment of the extent of fibre degradation. The animals received food particles that included some coated with pea fibre that is rich in the molecule arabinan, and some coated with the molecule arabinoxylan. B. vulgatus and B. cellulosilyticus competed to degrade the arabinan, B. cellulosilyticus degraded arabinoxylan, and B. ovatus degraded other molecules (not shown). b, When the experiment was repeated without B. cellulosilyticus, B. ovatus demonstrated metabolic flexibility, by switching to degrade arabinoxylan. B. ovatus degraded less arabinoxylan than did B. cellulosilyticus. Nature | 1 News & views https://doi.org/10.1038/d41586-019-03704-z ©2019SpringerNatureLimited.Allrightsreserved.