It has been suggested that there is a relationship between dietary patterns, gut microbiota and the development of obesity; one of the leading theories is based on altered permeability of the gut barrier as a key factor in triggering obesity. However, the vast majority of research about the role of gut microbial communities in obesity has focused on bacteria, and little is known regarding whether fungi could also be involved.
A new study, led by Dr. Cheryl Gale from the Department of Paediatrics at University of Minnesota in Minneapolis (Minnesota, USA), has found that high-fat diets can alter how bacteria and fungi communicate in mouse intestines, contributing to obesity.
The researchers investigated the effects of a high-fat diet (consisting of 60% calories from fat) compared to standard chow (containing 18% calories from fat) on both bacterial and fungal community structures in mice using the internal transcribed spacer region 2 (ITS2) of fungal ribosomal deoxyribonucleic acid (rDNA) and the 16S ribosomal ribonucleic acid (rRNA) genes of bacteria.
First of all, the study provides evidence that ingestion of a high-fat diet is associated with weight gain and metabolic markers associated with obesity (such as insulin resistance) together with shifts in the bacterial and fungal microbiomes of mice. However, a high-fat diet caused similar magnitudes of change in overall bacterial and fungal microbiome and mycobiome, respectively, structures.
The relative abundances of 19 bacterial (genus-level) and 6 fungal (genus- or species-level) taxa were significantly different between the two diets. Firmicutes bacteria increased, while Bacteroidetes bacteria decreased in abundance in response to high-fat diet, which is in agreement with previous studies (here; here). Regarding fungi, the abundances of the Alternaria, Saccharomyces, Septoriella, and Tilletiopsis genera; Saccharomyces cerevisiae; and Tilletiopsis washingtonensis were higher in mice fed standard chow. Candida albicans was found in small amounts in the faeces of both groups of mice.
Furthermore, the researchers found that strong and complex interkingdom structural and functional positive and negative relationships exist between gut bacteria and fungi and these are perturbed by a high-fat diet. Although the authors could not find a direct connection between the composition of gut fungi and weight gain, they hypothesized that changes in relationships between bacteria and fungi triggered by a high-fat diet might be involved in the development of obesity.
Finally, the researchers sought to determine links between fungi and metabolic potential of gut bacterial communities. Of a total of 558 potential Kyoto Encyclopedia of Genes and Genomes functional modules, the researchers determined that the relative abundances of 46 modules were significantly different between the high-fat- and standard-diet-fed mice. Specifically, functional modules related to carbohydrate and lipid metabolism, energy metabolism and nucleotide and amino acid metabolism were decreased in mice fed the high-fat diet compared to standard chow. Although each dietary group showed different correlation patterns, there were no significant associations between fungal taxa and functional phenotypes.
To sum up, this is the first study in mice showing a role for fungal and bacterial gut community interactions in the development of obesity. Further studies that start looking at the mycobiome in addition to the bacterial microbiome will depict a better picture regarding the role of gut fungal communities in host metabolic health.
Heisel T, Montassier E, Johnson A, et al. High-fat diet changes fungal microbiomes and interkingdom relationships in the murine gut. mSphere. 2017; 2(5):e00351-17. doi: 10.1128/mSphereñ00351-17.