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GASTROINTESTINAL

Which Bacteria in the Gut are Crucial For Fat Absorption?

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Although the vast majority of research on the gut microbiome has focused on bacteria in the large intestine, a new study — one of a few to concentrate on microbes in the upper gastrointestinal tract — shows how the typical calorie-dense western diet can induce expansion of microbes that promote the digestion and absorption of high-fat foods.

Several studies have shown that these bacteria can multiply within 24 to 48 hours in the small bowel in response to consumption of high-fat foods. The findings from this work suggest that these microbes facilitate production and secretion of digestive enzymes into the small bowel.

Those digestive enzymes break down dietary fat, enabling the rapid absorption of calorie-dense foods. Concurrently, the microbes release bioactive compounds. These compounds stimulate the absorptive cells in the intestine to package and transport fat for absorption. Over time, the steady presence of these microbes can lead to over-nutrition and obesity.

“These bacteria are part of an orchestrated series of events that make lipid absorption more efficient,” said the study’s senior author, Eugene B. Chang, MD, the Martin Boyer Professor of Medicine and director of the NIH Digestive Diseases Research Core Center at the University of Chicago Medicine. “Few people have focused on the microbiome of the small intestine, but this is where most vitamins and other micronutrients are digested and absorbed.”

“Our study is one of the first to show that specific small-bowel microbes directly regulate both digestion and absorption of lipids,” he added. “This could have significant clinical applications, especially for the prevention and treatment of obesity and cardiovascular disease.”

The goals of the study, published April 11, 2018 in the journal Cell Host and Microbe, were to find out if microbes were required for digestion and absorption of fats, to begin to learn which microbes were involved, and to assess the role of diet-induced microbes on the digestion and uptake of fats.

The study involved mice that were germ-free, bred in isolated chambers and harboring no intestinal bacteria, and mice that were “specific pathogen free (SPF),” meaning healthy but harboring common non-disease causing microbes.

The germ-free mice, even when fed a high-fat diet, were unable to digest or absorb fatty foods. They did not gain weight. Instead, they had elevated lipid levels in their stool.

SPF mice that received a high-fat diet did gain weight. This diet quickly boosted the abundance of certain microbes in the small intestine, including microbes from the Clostridiaceae and Peptostreptococcaceae families. A member of Clostridiaceae was found to specifically impact fat absorption. The abundance of other bacterial families decreased on a high-fat diet including Bifidobacteriacaea and Bacteriodacaea, which are commonly associated with leanness.

When germ-free mice were subsequently introduced to microbes that contribute to fat digestion, they quickly gained the ability to absorb lipids.

“Our study found that, at least in mice, a high-fat diet can profoundly alter the microbial make-up of the small intestine,” Chang said. “Certain dietary pressures, such as calorie-dense foods, attract specific bacterial strains into the small intestine. These microbes are then able to allow the host to digest this high-fat diet and absorb fats. That can even impact extra-intestinal organs such as the pancreas.”

“This work has important implications in developing approaches to combat obesity,” the authors conclude. This includes decreasing the abundance or activity of certain microbes that promote fat absorption, or increasing the abundance of microbes that may inhibit fat uptake.

“I would say the most important takeaway overall is the concept that what we eat — our diet on a daily basis — has a profound impact on the abundance and the type of bacteria we harbor in our gut,” said Kristina Martinez-Guryn, PhD, lead author of the study, and now an assistant professor at Midwestern University in Downers Grove, IL. “These microbes directly influence our metabolism and our propensity to gain weight on certain diets.”

Although this study was very preliminary, she added, “our results suggest that maybe we could use pre- or probiotics or even develop post-biotics (bacterial-derived compounds or metabolites) to enhance nutrient uptake for people with malabsorption disorders, such as Crohn’s disease, or we could test novel ways to decrease obesity.”

GASTROINTESTINAL

Viruses In Blood Lead To Digestive Problems

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While studying viruses best known for infecting the brain, researchers at Washington University School of Medicine in St. Louis stumbled upon clues to a conundrum involving a completely different part of the anatomy: the bowel, and why some people possibly develop digestive problems seemingly out of the blue.

The researchers found that viruses such as West Nile and Zika that target the nervous system in the brain and spinal cord also can kill neurons in the guts of mice, disrupting bowel movement and causing intestinal blockages. Other viruses that infect neurons also may cause the same symptoms, the researchers said.

The findings, published Oct. 4 in the journal Cell, potentially could explain why some people experience recurrent, unpredictable bouts of abdominal pain and constipation — and perhaps point to a new strategy for preventing such conditions.

“There are a number of people who are otherwise healthy who suddenly develop bowel motility problems, and we don’t understand why,” said Thaddeus S. Stappenbeck, MD, PhD, the Conan Professor of Laboratory and Genomic Medicine and the study’s co-senior author.

“But now we believe that one explanation could be that you can get a viral infection that results in your immune cells killing infected neurons in your gut. That might be why all of a sudden you can’t move things along any more.”

Postdoctoral researcher and first author James White, PhD, was studying mice infected with West Nile virus, a mosquito-borne virus that causes inflammation in the brain, when he noticed something peculiar. The intestines of some of the infected mice were packed with waste higher up and empty farther down, as if they had a blockage.

“We actually noticed this long ago, but we ignored it because it wasn’t the focus of our research at the time,” said West Nile expert Michael S. Diamond, MD, PhD, the Herbert S. Gasser Professor of Medicine and the paper’s co-senior author.

“But Jim White dug in. He wanted to figure out why this was happening.”

White, Diamond, Stappenbeck and colleagues including Robert Hueckeroth, MD, PhD, of the University of Pennsylvania, found that not only West Nile virus but its cousins Zika, Powassan and Kunjin viruses — all of which target the nervous system like West Nile — caused the intestines to expand and slowed down transit through the gut. In contrast, chikungunya virus, an unrelated virus that does not target neurons, failed to cause bowel dysfunction.

Further investigation showed that West Nile virus, when injected into a mouse’s foot, travels through the bloodstream and infects neurons in the intestinal wall. These neurons coordinate muscle contractions to move waste smoothly through the gut. Once infected, the neurons attract the attention of immune cells, which attack the viruses — and kill the neurons in the process.

“Any virus that has a propensity to target neurons could cause this kind of damage,” said Diamond, who is also a professor of molecular microbiology and of pathology and immunology.

“West Nile and related viruses are not very common in the U.S. But there are many other viruses that are more widespread, such as enteroviruses and herpesviruses, that also may be able to target specific neurons in the wall of the intestine and injure them.”

If that’s the case, such widespread viruses may provide a new target in the prevention or treatment of painful digestive issues. Having chronic gut motility problems is a miserable experience, and while the condition can be managed, it can’t be cured or prevented.

“Many of the viruses that might target the gut nervous system cause mild, self-limiting infections, and there’s never been reason to develop a vaccine for them,” Diamond said.

“But if you knew that some particular viruses were causing this serious and common problem, you might be more apt to try to develop a vaccine.”

The infected mice’s digestive tracts gradually recovered over an eight-week time span. But when the researchers challenged the mice with an unrelated virus or an immune stimulant, the bowel problems promptly returned. This pattern echoed the one seen in people, who cycle through bouts of gastrointestinal distress and recovery. The flare-ups often are triggered by stress or illness, but they also can occur for no apparent reason.

“It’s amazing that the nervous system of the gut is able to recover and re-establish near normal motility, even after taking a pretty big hit and losing a lot of cells,” said Stappenbeck, who is also a professor of developmental biology.

“But then, it’s really just barely functioning normally, and when you add any stress, it malfunctions again.”

Previous studies have linked bowel motility to changes in the microbiome – the community of bacteria, viruses and fungi that live in the gut.

“What we need to explore now is how this story connects to everything else we know about gut motility,” Stappenbeck said.

“What effect does damage to the gut nervous system have on the microbiome? We would love to connect those dots.”

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GASTROINTESTINAL

Could The Microbiome Of Our Gut, Hold The Secret To Living Longer?

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You are what you eat. Or so the saying goes. Science now tells us that we are what the bacteria living in our intestinal tract eat and this could have an influence on how well we age. Building on this, McGill University scientists fed fruit flies with a combination of probiotics and an herbal supplement called Triphala that was able to prolong the flies’ longevity by 60 % and protect them against chronic diseases associated with aging.

The study, published in Scientific Reports, adds to a growing body of evidence of the influence that gut bacteria can have on health. The researchers incorporated a symbiotic — made of probiotics with a polyphenol-rich supplement — into the diet of fruit flies.

The flies fed with the synbiotic lived up to 66 days old — 26 days more than the ones without the supplement. They also showed reduced traits of aging, such as mounting insulin resistance, inflammation and oxidative stress.

“Probiotics dramatically change the architecture of the gut microbiota, not only in its composition but also in respect to how the foods that we eat are metabolized,” says Satya Prakash, professor of biomedical engineering in McGill’s Faculty of Medicine and senior author of the study.

“This allows a single probiotic formulation to simultaneously act on several biochemical signaling pathways to elicit broad beneficial physiological effects, and explains why the single formulation we present in this paper has such a dramatic effect on so many different markers.”

The fruit fly is remarkably similar to mammals with about 70 % similarity in terms of their biochemical pathways, making it a good indicator of what would happen in humans, adds Prakash.

“The effects in humans would likely not be as dramatic, but our results definitely suggest that a diet specifically incorporating Triphala along with these probiotics will promote a long and healthy life.”

The authors also say that the findings can be explained by the “gut-brain axis,” a bidirectional communication system between microorganisms residing in the gastrointestinal tract — the microbiota — and the brain. In the past few years, studies have shown the gut-brain axis to be involved in neuropathological changes and a variety of conditions such as irritable bowel syndrome, neurodegeneration and even depression. Few studies, however, have successfully designed gut microbiota-modulating therapeutics having effects as potent or broad as the formulation presented in the new study.

Learning from traditional medicine

The herbal supplement used in the study, Triphala, is a formulation made from amalaki, bibhitaki and haritaki, fruits used as medicinal plants in Ayurveda, a form of traditional Indian medicine.

Susan Westfall, a former PhD student at McGill and lead author of the study, says the idea of combining Triphala and probiotics comes from her long-standing interest in studying natural products derived from traditional Indian medicine and their impact on neurodegenerative diseases.

“At the onset of this study, we were hopeful that combining Triphala with probiotics would be at least a little better than their individual components in terms of physiological benefit, but we did not imagine how successful this formulation would be,” says Westfall, who is now a postdoctoral fellow at the Icahn School of Medicine at Mount Sinai in New York, USA.

The new study, which includes data filed in a US provisional patent through a company cofounded by the authors, has the potential to impact the field of the microbiome, probiotics and human health.

Considering the broad physiological effects of this formulation shown in the fruit fly, Prakash hopes their formulation could have interesting applications in a number of human disorders such as diabetes, obesity, neurodegeneration, chronic inflammation, depression, irritable bowel syndrome and even cancer.

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GASTROINTESTINAL

Ingestible ‘Bacteria On A Chip’ Could Help Diagnose Disease

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MIT researchers have built an ingestible sensor equipped with genetically engineered bacteria that can diagnose bleeding in the stomach or other gastrointestinal problems.

This “bacteria-on-a-chip” approach combines sensors made from living cells with ultra-low-power electronics that convert the bacterial response into a wireless signal that can be read by a smartphone.

“By combining engineered biological sensors together with low-power wireless electronics, we can detect biological signals in the body and in near real-time, enabling new diagnostic capabilities for human health applications,” says Timothy Lu, an MIT associate professor of electrical engineering and computer science and of biological engineering.

In the new study, appearing in the May 24 online edition of Science, the researchers created sensors that respond to heme, a component of blood, and showed that they work in pigs. They also designed sensors that can respond to a molecule that is a marker of inflammation.

Lu and Anantha Chandrakasan, dean of MIT’s School of Engineering and the Vannevar Bush Professor of Electrical Engineering and Computer Science, are the senior authors of the study. The lead authors are graduate student Mark Mimee and former MIT postdoc Phillip Nadeau.

Wireless communication

In the past decade, synthetic biologists have made great strides in engineering bacteria to respond to stimuli such as environmental pollutants or markers of disease. These bacteria can be designed to produce outputs such as light when they detect the target stimulus, but specialized lab equipment is usually required to measure this response.

To make these bacteria more useful for real-world applications, the MIT team decided to combine them with an electronic chip that could translate the bacterial response into a wireless signal.

“Our idea was to package bacterial cells inside a device,” Nadeau says. “The cells would be trapped and go along for the ride as the device passes through the stomach.”

For their initial demonstration, the researchers focused on bleeding in the GI tract. They engineered a probiotic strain of E. coli to express a genetic circuit that causes the bacteria to emit light when they encounter heme.

They placed the bacteria into four wells on their custom-designed sensor, covered by a semipermeable membrane that allows small molecules from the surrounding environment to diffuse through. Underneath each well is a phototransistor that can measure the amount of light produced by the bacterial cells and relay the information to a microprocessor that sends a wireless signal to a nearby computer or smartphone. The researchers also built an Android app that can be used to analyze the data.

The sensor, which is a cylinder about 1.5 inches long, requires about 13 microwatts of power. The researchers equipped the sensor with a 2.7-volt battery, which they estimate could power the device for about 1.5 months of continuous use. They say it could also be powered by a voltaic cell sustained by acidic fluids in the stomach, using technology that Nadeau and Chandrakasan have previously developed.

“The focus of this work is on system design and integration to combine the power of bacterial sensing with ultra-low-power circuits to realize important health sensing applications,” Chandrakasan says.

Diagnosing disease

The researchers tested the ingestible sensor in pigs and showed that it could correctly determine whether any blood was present in the stomach. They anticipate that this type of sensor could be either deployed for one-time use or designed to remain the digestive tract for several days or weeks, sending continuous signals.

Currently, if patients are suspected to be bleeding from a gastric ulcer, they have to undergo an endoscopy to diagnose the problem, which often requires the patient to be sedated.

“The goal with this sensor is that you would be able to circumvent an unnecessary procedure by just ingesting the capsule, and within a relatively short period of time you would know whether or not there was a bleeding event,” Mimee says.

To help move the technology toward patient use, the researchers plan to reduce the size of the sensor and to study how long the bacteria cells can survive in the digestive tract. They also hope to develop sensors for gastrointestinal conditions other than bleeding.

In the Science paper, the researchers adapted previously described sensors for two other molecules, which they have not yet tested in animals. One of the sensors detects a sulfur-containing ion called thiosulfate, which is linked to inflammation and could be used to monitor patients with Crohn’s disease or other inflammatory conditions. The other detects a bacterial signaling molecule called AHL, which can serve as a marker for gastrointestinal infections because different types of bacteria produce slightly different versions of the molecule.

“Most of the work we did in the paper was related to blood, but conceivably you could engineer bacteria to sense anything and produce light in response to that,” Mimee says. “Anyone who is trying to engineer bacteria to sense a molecule related to disease could slot it into one of those wells, and it would be ready to go.”

The researchers say the sensors could also be designed to carry multiple strains of bacteria, allowing them to diagnose a variety of conditions.

“Right now, we have four detection sites, but if you could extend it to 16 or 256, then you could have multiple different types of cells and be able to read them all out in parallel, enabling more high-throughput screening,” Nadeau says.

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