Microbes in the gut can greatly affect host physiology and disease. They can locally influence the development of immune cells and distally affect systems by metabolizing dietary components and releasing factors into the circulation. Several studies have suggested that diet profoundly influences the composition of the bacterial community in the gut, termed the microbiota. Altered composition of the microbiota is found in obese individuals, and germ-free mice that lack gut microbes are protected from diet-induced obesity and insulin resistance.

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To assess how these microbes interact with diets and metabolic disease, Robert Caesar, Fredrik Bäckhed and colleagues at the University of Gothenburg (Cell Metab. doi:10.1016/j.cmet.2015.07.026; published online 27 August 2015) fed mice diets that were rich in either lard (saturated lipids) or fish oil (polyunsaturated lipids). As was expected, mice that were fed lard gained much weight and developed insulin resistance, whereas mice that were fed fish oil did not develop metabolic disease.

Each diet profoundly changed the composition of the microbiota. Bilophila spp. bacteria were more prevalent in mice that were fed lard, which is consistent with prior reports that describe the presence of similar bacteria in humans fed a diet rich in saturated fats.

To experimentally test whether changes in a microbiota directly contribute to disease, researchers can transfer the microbiota from a donor into a recipient mouse that lacks a microbiota. In this study, recipients were treated with antibiotics to deplete the microbes in their gut; they were then colonized with the microbiota from mice fed either lard or fish oil. All recipient mice were subsequently fed the lard diet. Over 3 weeks, mice that received microbiota from mice that were fed fish oil gained less weight and had less inflammation in fat tissue than other recipient mice. This suggests that their donated microbiota conferred protection against a diet rich in saturated fats.

In both donors and colonized recipients of this beneficial microbiota, the bacteria Akkermansia muciniphila was notably abundant. A. muciniphila has previously been shown to contribute to protection against diet-induced obesity, and these findings support the investigation of new probiotics or prebiotics to treat metabolic disease. However, in a press release, Fredrik Bäckhed clarifies that “further investigations will be needed to determine if these bacteria can be used as probiotic strain and, in that case, how it should be combined with diet to optimize health outcomes.”