Starch rich diets can influence the gut-microbiome and subsequently behavior

The microbiome’s role in modulating the gut-brain axis has been well-supported by a large body of evidence.  Many experiments in the past have demonstrated this in preclinical models by administering probiotics with specific bacterial strains or by fecal microbiome transplant in rodent models, which were then associated with changes in behavior.  Diet has also been implicated in these modulations, as food intake can influence species diversity and composition.  Low-digestible carbohydrates, or resistant starch, have received attention as being beneficial toward health, as these components are not digested but rather fermented by resident microbiota to produce an array of beneficial metabolites.  In a recent study, researchers from Texas Tech University investigated whether a diet rich in resistant starches were also associated with changes in behavior.

48 mice were randomly assigned to 3 different treatment groups, with each group either fed normal corn starch diet, a resistance starch rich diet, or an octenyl-succinate diet for 6 weeks.  The animals were monitored for weight, were subject to robust behavioral tests, and fecal samples were examined for microbiota composition.  The animals on the resistant starch diet exhibited similar weight gains as compared to the normal corn starch diet, and the octenyl-succinate group demonstrated lower weight gain.  Fecal microbiota analysis revealed diet correspondence to specific diet, and that resistant starch diet groups displayed increases in Verrucomicrobia and Actinobacteria as compared to octenyl-succinate and normal corn starch group, respectively.  In all groups, mice displayed significant anxiety-like-behavior in an elevated plus maze, and in open-field tests the mice fed resistance starch rich and octenyl-succinate diet mice exhibited high-anxiety-like behaviors. 

This data again supports that diet manipulation can have marked influence on behavior, and that starch rich diets could perhaps induce undesirable behavioral effect via modulation of the gut-brain axis.  This could be an important drawback to the beneficial components provided for microbial fermentation.  

Exposure to the cold affects our microbiome and modulates our energy homeostasis

When exposed to cold temperatures, you begin shivering to compensate for the sudden temperature decrease.  Biologically, exposure to cold environment can induce brown adipose tissue differentiation and catabolism of lipids to produce heat.  The gastrointestinal tract is an important site for the regulation of our energy metabolism, and as we know, harbors a diverse population of gut bacteria.  A team of researchers sought to investigate what role the microbiome of mice plays in induction of physiological changes to cold exposure.

The researchers exposed different groups of mice to cold temperatures for 31 days, and the initial observation was attenuation of energy expenditure and white fat.  Importantly, food intake was consistent throughout the trial, pointing to a temperature-driven effect.  Fecal collection and post-mortem intestinal analysis revealed marked shifts in microbiota compositions of cold-exposed mice as compared to those kept at room temperature.  Notably, families within Actinobacteria, Verrucomicrobia, and Tenericutes were less abundant in the cold sample mice, and Akkermansia muciniphila species were observed to be significantly decreased in the cold mice.  The researchers also transplanted microbiota from cold-group mice to naïve, germ-free mice.  Upon cold temperature exposure, germ-free mice demonstrated heightened insulin sensitivity, browning of white fat, increased energy expenditure, and white fat loss, indicative of a tolerance to the cold.  Prolonged cold exposure in both cold-exposed and cold-transplanted mice also induced adaptive biological mechanisms in intestinal villi and microvilli to maximize caloric uptake, ultimately attenuating weight loss.  This increase in intestinal absorption was observed concomitant to altered intestinal gene expression, as genes that promote tissue remodeling where upregulated while apoptotic genes were suppressed.  In a final experiment, the aforementioned effect was reduced when Akkermansia muciniphila (which were originally observed to decrease in cold-induced mice) were transplanted to cold-exposed mice, supporting a microbiota-driven relationship.

This was a very interesting study that sheds lights on organismal adaptive mechanisms during energy scarcity brought on by factors such as cold temperatures.  Again we observe heavy involvement of the microbiome in these processes, a finding that calls for more investigation.  

The microbiome of the international space station

Characterization of the microbial composition of the International Space Station (ISS) is a topic that currently interests the National Aeronautics and Space Administration (NASA). The ISS is an interesting environment because it is a built environment that experiences constant human contact, microgravity and space radiation. Understanding the ISS microbial community would help with help and safety concerns as well as proper maintenance of the ISS. Scientists across the United States combined their efforts to properly characterize the microbial community of the ISS, and compared it to cleanrooms on Earth. The results were published by Microbiome.

         Samples were collected from ISS high-efficiently particulate arrestance (HEPA, vacuum cleaner bag components from the ISS, and two cleanrooms at the Jet Propulsion Laboratory (JPL) in Pasadena, CA. Cleanrooms are closed rooms with little human traffic and filtered air. Bacterial and fungal samples were cultured and sequenced using next generation sequencing techniques in order to determine identities. Sixteen fungal strains were isolated from the ISS samples compared to the three strains from JPL samples, with most strains being associated with the phylum Ascomycota. Bacterial samples from the ISS were dominated by Actinobacteria, Bacilli, and Clostridia, while samples from the JPL were dominated by Alphaproteobacteria and Gammaproteobacteria. On a genus level, the two sample environments were completely distinct as well.

         This study shows that the International Space Station has a very distinct microbial community that must be monitored. As we know that the microbiome is so influential on health, it is important that the ISS bacteria are characterized in order to ensure the health and safety of those on board. This is just another important example that the microbiome has a great influence on humans, even from out in space.         

The gut microbiome shift during pregnancy is related to the mother’s secretor status

Fucose chemical structure

An estimated 20% of women of European descent are not able to produce mucous that have fucose sugars attached to the ends of their mucin molecules.  These women are called ‘non-secretors’, as opposed to ‘secretors’ who can fucosylate their mucins.  This rather peculiar genetic anomaly is not appreciated until it is looked at under the lens of the microbiome.  Many of the microbiota in the gut feed off the host’s mucins for energy, and the lack of fucose is a major factor in dictating which communities can survive in their guts.  During pregnancy the mother’s gut microbiota undergoes a dramatic shift, although what variables are important in determining this shift remain unknown.  Last week though, researchers from Finland showed that secretor status was an important indicator in how a women’s gut microbiome shifts during pregnancy.  They published their results in PLoS ONE.

The researchers sampled the gut microbiome of 71 women throughout their pregnancy, and compared it to the secretor status, as determined by genetic testing.  In the first trimester of pregnancy each women, secretors and non-secretors alike, had similar diversities in their gut microbiota.  However, by the third trimester the non-secretor’s gut microbiomes were much lower than their secretor counterparts.  When the scientists measured specific phyla, they observed an increase in the abundance of Actinobacteria in the secreting women, and an increase In the abundance of Proteobacter in the non-secretors.

The changes in gut microbiota in these women may be very important to the microbiome of the infant that is born to them.  As an infant passes through the birth canal he or she is exposed to the mothers’ vaginal and gut microbiota, and these bacteria serve as the initial populations that seed the infants’ own guts.  In addition, some of these specific bacterial populations, such as Proteobacter, are implicated in diseases like IBD.  If these bacteria persist in the mother after birth they may explain the onset or increased risk of some of these diseases.

Sex, body mass index, and dietary fiber correlated with microbiome composition

On last week’s podcast, we talked with Erica and Justin Sonnenburg about how the food we eat, and specifically dietary fiber, is important for “feeding” our microbiomes. All of the variables that influence microbiome composition are not fully understood, however research is continually being conducted to better understand what factors affect the microbiome.  To this end, a team of scientists from New York University School of Medicine set out to find how sex, body mass index (BMI), and dietary fiber intake impact the microbiome.

The scientists analyzed fecal samples from 82 individuals, 51 men and 31 women. They found that the women had different microbiome composition than the men, specifically a lower abundance of Bacteroidetes. They also found that BMI impacted microbiome diversity, specifically in women. Overweight and obese women had less diverse gut bacteria than normal weight women and women with a higher BMI also had less Bacteroidetes in their guts compared to the normal weight women.

The scientists also found that various sources of dietary fiber differentially impacted the microbiome of subjects.  Fiber intake from fruits and vegetables resulted in higher levels of Clostridia and fiber intake from beans was associated with greater abundance of Actinobacteria. It is possible that dietary fiber is influencing the microbiome by reducing gut transit time and lowering the pH. It is also possible that it is influencing systemic levels of estrogen, which could alter microbiome composition.

As the microbiome continues to be implicated in diseases, the ability to identify variables that affect the microbiome are important and can potentially be used for altering microbiota composition to prevent or possibly treat disease.