Gut microbiome depletion promotes healthier brown fat and reduces obesity in mice

The white and brown turkey meat from a Thanksgiving dinner

An interesting article from Switzerland was published last week in Nature Medicine.  The scientists reported on a new connection between the gut microbiome and metabolic syndrome (i.e. insulin sensitivity, obesity, etc.)  Whereas most papers observe microbiome disruption and depletion is associated with obesity, this paper describes a different phenomenon: that mice with depleted microbiomes are metabolically healthier than their untouched microbiome counterparts.  As part of the basis for the paper it is important to understand that mammals have two types of fat, brown fat and white fat.  Brown fat is associated with exercise, insulin sensitivity, and health, and white fat is associated with insulin resistance and diabetes.  Brown fat can actually repopulate white fat in a process called browning, and this transition is healthy.  

In the study, the scientists started with either normal mice, germ free mice, or mice that had antibiotics administered to them. They challenged each group of mice with glucose, and noted that antibiotic administration led to improved insulin sensitivity.  When they investigated where the glucose was going, they discovered that it was uptaken by white adipose tissue under the skin.  Then, they compared the normal mice and antibiotic mice, and observed that the antibiotic mice actually had smaller volumes of fat after the glucose uptake.  Interestingly, the fat cells in the germ free and antibiotic mice were smaller and more dense, whereas the normal mice had fewer, larger cells.  The researchers then confirmed that browning of fat was occurring in the germ free and antibiotic mice.  Finally, when the scientists transplanted the microbiome of normal mice into the germ free mice a reversal of many the above described characteristics occurred.  In these mice the fat stopped browning, insulin resistance decreased, and the mice gained weight.

The scientists were able to attribute some of the above phenomena to the release of specific cytokines (molecules that regulate the immune system).  This paper, then, adds to the wealth of research that describes the complex but critical interaction between the gut microbiome, the immune system, and metabolic syndrome.  Although the relationships between these things is yet to be fully understood, this paper may at least change the way you think about the dark and white meat during Thanksgiving dinner this Thursday.

The gut-vascular barrier’s role in regulating bacterial systemic dissemination

Microbiota that populate the gut do not have access to lymphatics, the liver, or other peripheral tissues in healthy individuals.  The gut-vascular barrier (GVB) plays an essential role in limiting bacteria access to systemic circulation in addition to protecting against pathogenic invasion.  However, some pathogenic bacteria can reach these tissues and induce a systemic immune response, but how this is exactly occurs is unknown.  Researchers from Italy sought to address this by exploring how GVB disruption lends itself to bacterial-pathogenesis in liver and lymphatic tissue sites. 

Two groups of mice were analyzed, one healthy control group and one group infected with Salmonella, a bacteria selected to study the pathogenic effects on the GVB.  Healthy control mice were first analyzed using fluorescent dextran polymer dyes at different sizes to assess the leakage of the dye into the intestines for visualization and characterization of the GVB.  After injection, intestinal blood vessels were analyzed using two-photon microscopy, and it was observed that an endothelial barrier could discriminate particles that are functionally similar according to size.  Specifically, dextran polymers 4 kilodaltons (kD) in size could enter the blood stream freely, while dextran polymers 70 kD in size could not.  The same experiment was then repeated in mice who were administered Salmonella prior to dextran, and interestingly the 70 kD polymers diffused freely into the blood stream, suggesting that the Salmonella disrupted the GVB.  Exploring deeper, the experimenters characterized the endothelial cell types that formed this tight-junction barrier, as it was found that plasma-lemma vesicle-associated protein-1 (PV1) was up-regulated in blood 6 hours after being exposed to Salmonella.  Furthermore, this finding was associated with systemic damage present in other organs. 

In another experiment using the dextran dye, the researchers determined that the dye reached the liver in mice infected with Salmonella after 60 minutes following injection, as it was concluded that the Salmonella traveled to the liver via portal vein circulation as opposed to lymphatic transport (where it would be present in the spleen).  Lastly, it was determined that Salmonella could discretely regulate B-catenin signaling by blocking these signals from being released in vascular endothelial cells. 

These findings highlight important mechanistic features of a physiological construct potentially implicated in many pathogenic diseases.  In the context of liver damage and celiac disease, these findings could link the GVB as drivers of pathogenesis as the GVB is disrupted in celiac patients.  Disruptions to the GVB deserve more investigation as this physiological barrier could be implicated in many pathogenic diseases.  

Effects of antibiotics on gut microbiome and glucose metabolism

The gut bacteria in the human gut plays an important role in glucose (sugar) metabolism. Alterations to the gut microbiome have been linked to obesity and Type-2 diabetes, as a result of inability to breakdown sugar. Scientists wanted to see the effect of removing as much bacteria as possible using antibiotics and what the effect was on human health after eating meals and published their results in PLOS ONE.

The study design included twelve healthy male volunteers, with a mean age of 23.4 years and no glucose intolerances. The participants were tested 5 separate days (day 0, 4,8, 42, and 180) during the study. Between days 0 and 4, the participants consumed a 4-day 30 drug antibiotic mix, made of meropenem, vancomycin, and gentamicin. On days 0, 4, and 42 the males participated in a liquid meal test, in which blood samples were taken 30, 15, and 0 minutes before and 15, 30, 45, 60, 75, 90,120, 150, 180, 210, and 240 minutes after ingestion of a specially concocted liquid meal. At 270 minutes, the participants were given a solid food meal. On days 8 and 180, blood pressure and sample was taken. The day before each of the 5 visits, the participants collected a fecal sample.

Antibiotic treatment did not seem to cause any serious or unexpected adverse effects. Between day 0 and day 180, an increase of mean body weight was found at only 1.3 kg and the corresponding change in BMI was 0.3. No changes in average blood pressure was observed, nor were any health complaints or symptoms.

As for the gut microbiome, after the 4 days of antibiotics, total anaerobic bacterial count decreased from 8.5 log10CFU/g to 6.2 log10 CFU/g. Enertococci, coliforms and bifidobacteria decreased significantly as well. On day 8 of the study, the abundance of aerobic bacteria had actually significantly increased passed the concentration number before antibiotics were consumed. By day 180 of the study, however, bacterial counts returned to the same baseline level from before antibiotic consumption. Overall, no significant effect on glucose consumption or metabolism was observed in this study as a result of changed gut bacteria composition. 

Short-chain fatty acids and their effect on dendritic cells

Short-chain fatty acids (SCFA) are metabolites produced from microbiota fermentation.  SCFAs have been subject to extensive investigation in attempts to delineate the pathways and mechanisms that underlie health outcomes from a species-host relationship.  Many have investigated the role of SCFAs in intestinal pathways or intestinal dysbiosis-driven disease (e.g. liver disease), but little is known as to how these metabolites interact with biological components of the immune system and blood stream.  Specifically, it is important to learn about this as some immune cells, such as dendritic cells (DC), patrol the blood stream to sense and respond to bacterial metabolites and present these pathogens to other immune cells in the lymph nodes.  To investigate further, a European conglomerate of researchers examined the interaction between SCFAs and DCs at the molecular level.

PCR was first performed on human-derived DCs to characterize protein expression patterns of SCFA receptors on these cells, as SCFAs are postulated to be ligands for G-coupled protein receptors (supported by the PCR).  Next, still using human-derived DCs the researchers determined that individual SCFAs were shown to have different effects, more so on mature DCs.  Butyrate and propionate in particular strongly modulated gene expression in both immature and mature human DCs.  The researchers next conducted an ingenuity pathway analysis based on differential gene expression which determined that propionate and butyrate modulate leukocyte (white blood cell) trafficking.  On top of this, SCFA significantly tempered release of an array of pro-inflammatory cytokines.  Lastly, butyrate and propionate were shown to inhibit the expression of lipopolysaccharide-induced cytokines to support a strong anti-inflammatory effect.

Together, these results suggest that metabolites derived from microbiota fermentation and metabolism and differentially modulate inflammatory response by way of dendritic cell interaction.  These findings illustrate another key component to the host-species relationship, and provide more evidence as to the scale and important of a healthy synergistic relationship between host and microbiome as the interactions are involved in disease-prone pathways that require careful molecular regulation (e.g. inflammation and immunity).  This in vitro study was a good first step in this investigation.  

Sepsis-like syndrome in a patient after Fecal Microbiota Transplant

Clostridium difficile infection causes pain and diarrhea, is sometimes fatal, and normally occurs after a course of antibiotics leaves the gut in a state of dysbiosis where the C. diff can thrive.  Doctors normally prescribe antibiotics to cure this infection, but this can sometimes exacerbate the problem, making the gut even more prone to infection.  Fecal microbiota transplants (FMT) are the most successful therapy to treat the condition and have been seen to be successful in as many as 95% of treatments. 

A group of doctors in California chronicled the story of a 56-year-old woman who suffered from C. diff after she took a 10-day course of amoxicillin after she became sick with bacterial sinusitis. She went to the doctor after getting very sick and reporting 8-10 bowel movements per day. She was then prescribed various other antibiotic regimes that did not improve her condition over several days and a stool analysis found that she had C. diff.

While admitted at the hospital she was prescribed more antibiotics including metronidazole, vancomycin, and fidaxomicin however this only exacerbated her problems. Finally, her husband was identified as a potential stool donor and on Day 15 she underwent an FMT.

Six hours after the FMT, the woman developed sepsis-like syndrome and had a fever, tachycardia, and hypotension. After the woman was transferred to ICU, it was decided that no further antibiotics would be initiated as this could prevent the FMT from being effective and she did not clinically appear to be severely ill despite her vitals.  The following morning she was recovering and her vital signs normalized. Three days later, she was discharged and six weeks later, her stool frequency had reduced to 2-3 times per day and there was no C. diff recurrence.

Why was it that, this woman suffered from a condition that looked like sepsis after the FMT? The hypotheses included that it could have been a result of another pathogen derived from the donated stool. Second, it could have been a compilation from the procedure such as a perforated colon. Third, the FMT may have been unsuccessful resulting in untreated infection after the cessation of antibiotics the day prior. And finally, it could have been a result of the representation of an immune response as a result of a new gut microbiota.

While this was only an example of one patient and they did not discover the reason for her sepsis-like symptoms, this was an example of the harm that an FMT can cause. The authors state that well designed, executed, and interpreted clinical trials should be conducted if FMTs are to be used for higher risk/benefit conditions.

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.