Some Obesity-Related Bacteria

fat_bug_bwThe introduction for this and related posts can be found in our post, “The Microbiome – More Dots to Connect.” One professional who uses the dot-connecting simile is Dr. Noel T. Mueller, who is all in favor of diversity among the populations of bacteria by which we are inhabited. He says:

Many studies in adult populations have found that greater bacterial diversity of the gut is associated with protection from various diseases, including those that are autoimmune in nature, like asthma, and those that are metabolic in nature, like obesity.

Yesterday, Childhood Obesity News mentioned the KOALA cohort study, the longitudinal data collection effort whose information can be used to support or inspire many other studies. Through KOALA, it has been discovered that the bacteria M. smithii is found in great concentrations in obese young people. Other research found that Bacteroides fragilis is also “associated with greater childhood weight gain.” But associated how?

Meet Bacteroides Fragilis

This species of intestinal flora (or, more properly, fauna) belongs to the phylum Bacteroidetes, as Claudia Wallis explains in Scientific American:

An early hint that gut microbes might play a role in obesity came from studies comparing intestinal bacteria in obese and lean individuals…Lean individuals, for example, tended to have a wider variety of Bacteroidetes, a large tribe of microbes that specialize in breaking down bulky plant starches and fibers into shorter molecules that the body can use as a source of energy.

Bacteroidetes is the second largest subgroup of species that occupy human digestive tracts. Inside the phylum Bacteroidetes is the genus Bacteroides, which have the reputation of being opportunistic pathogens. But if there’s one thing the research shows, it’s that any given bug might play various roles, and turn into a hero or a villain depending on factors we haven’t begun to understand.

Another thing that seems evident to researchers is that some of our bugs produce neurochemicals and use them to communicate with the nervous system and probably the brain itself. One such species is Bacteroides fragilis. Sarkis Mazmanian, a microbiologist at the California Institute of Technology, is very familiar with it:

A company he co-founded, Symbiotix Biotherapies, is developing a complex sugar called PSA, which is associated with Bacteroides fragilis, into treatments for intestinal disease and multiple sclerosis.

Recently, Dr. Mazmanian and co-author Elaine Hsiao published a report on another characteristic of Bacteroides fragilis in the journal Cell. The New York Times said:

Their research found that mice exhibiting abnormal communication and repetitive behaviors, like obsessively burying marbles, were mollified when they were given one of two strains of the bacterium Bacteroides fragilis.

That quotation connects two dots. Where have we heard about repetitive and obsessive behavior? When discussing the question of exactly what kind of addiction drives people to overeat. And the other dot of course is the microbiome, or at least one member of it. If people have enough Bacteroides fragilis in their systems, can they be “mollified” out of repetitive behaviors like compulsive eating? Can this particular bug help people escape from obesity?

The Inflammation Dot

Does this connect with yet another dot? At least one study has provided “evidence toward an inverse relationship between adiposity and Bacteroides proportions in the gut.” Obese mice and humans have a lower proportion of Bacteroides, and lean ones have a higher proportion of Bacteroides. If there is a cause-and-effect relationship, which way does it flow? If we find out, how can that knowledge benefit us in the struggle to end childhood obesity? Here is another bit of the puzzle.

Inflammation is associated with a higher proportion of Bacteroides, and inflammation is also associated with obesity. This seems not to fit with the previous paragraph, and other studies also deliver mixed messages. Some evidence hints strongly that balance is paramount, and we do not have a clue about what the perfectly balanced proportions are for anything in the microbiome. It is also an area where synergy can perform some amazing feats. This branch of scientific inquiry is in its infancy. Only about a quarter of the residents of our guts have even been cultured. It’s the Wild West in there, and anything could happen.

Your responses and feedback are welcome!

Source: “The Gut Microbiome and Childhood Obesity: Connecting the Dots.”, June 2015
Source: “How Gut Bacteria Help Make Us Fat and Thin,”, 06/01/14
Source: “Can the Bacteria in Your Gut Explain Your Mood?,”, 06/23/15
Source: “Bacteroide composition in the gut,”, undated
Image by Ben Piddington

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About Dr. Robert A. Pretlow

Dr. Robert A. Pretlow is a pediatrician and childhood obesity specialist. He has been researching and spreading awareness on the childhood obesity epidemic in the US for more than a decade.
You can contact Dr. Pretlow at:


Dr. Pretlow’s invited presentation at the American Society of Animal Science 2020 Conference
What’s Causing Obesity in Companion Animals and What Can We Do About It

Dr. Pretlow’s invited presentation at the World Obesity Federation 2019 Conference:
Food/Eating Addiction and the Displacement Mechanism

Dr. Pretlow’s Multi-Center Clinical Trial Kick-off Speech 2018:
Obesity: Tackling the Root Cause

Dr. Pretlow’s 2017 Workshop on
Treatment of Obesity Using the Addiction Model

Dr. Pretlow’s invited presentation for
TEC and UNC 2016

Dr. Pretlow’s invited presentation at the 2015 Obesity Summit in London, UK.

Dr. Pretlow’s invited keynote at the 2014 European Childhood Obesity Group Congress in Salzburg, Austria.

Dr. Pretlow’s presentation at the 2013 European Congress on Obesity in Liverpool, UK.

Dr. Pretlow’s presentation at the 2011 International Conference on Childhood Obesity in Lisbon, Portugal.

Dr. Pretlow’s presentation at the 2010 Uniting Against Childhood Obesity Conference in Houston, TX.

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