The bacteria in our gut are estimated to weight up to 2 kg.
Most of us are aware that the bacteria in our gut play an important role in digestion. When the stomach and small intestine are unable to digest certain foods we eat, gut microbes jump in to offer a helping hand, ensuring we get the nutrients we need.
In addition, gut bacteria are known to aid the production of certain vitamins - such as vitamins B and K - and play a major role in immune function.
But increasingly, researchers are working to find out more about how gut bacteria - particularly the bacteria that is unique to us individually - influence our health and risk of disease.
Perhaps most studied is how gut microbiota affects an individual's risk of obesity and other metabolic conditions. In November 2014, for example, Medical News Today reported on a study claiming our genetic makeup shapes what type of bacteria reside in our gut, which may affect our weight.
In this Spotlight, we take a look at obesity and some of the other - perhaps surprising - health conditions that may be driven by our gut microbiota.
The development of gut microbiota
Belief has long held that the development of gut microbiota does not start until birth, with the gastrointestinal tract of a fetus considered to be a sterile environment.
According to Gut Microbiota Worldwatch - an information service created by the Gut Microbiota and Health Section of the European Society for Neurogastroenterology & Motility, a member of the United European Gastroenterology (UEG) - the digestive tract of a newborn is rapidly colonized with micro-organisms from the mother and the surrounding environment.
An infant's gut microbiota, for example, can be influenced by breastfeeding. Gut Microbiota Worldwatch explain that the gut of breastfed babies primarily consists of Bifidobacteria - considered a "friendly" bacteria that benefits the gut - while formula fed babies are likely to have less of these bacteria.
However, some studies have challenged the belief that the fetus is a sterile environment, suggesting that the development of gut microbiota begins before birth.
A 2008 study published in the journal Research in Microbiology identified bacteria, including Enterococcus and Staphylococcus, in the early feces of baby mice - known as the meconium - indicating the bacteria were transferred to the fetus from the mother's gut during pregnancy.
In this study, a group of pregnant mice was also inoculated with the bacterium Enterococcus fecium, which was isolated from human breast milk. The baby mice were delivered by Cesarean section 1 day before the predicted labor date, and their meconium was tested. The researchers identified E. fecium in their feces, but no trace was found in the meconium of a control group.
"Based on the sum of evidence, it is time to overturn the sterile womb paradigm and recognize the unborn child is first colonized in the womb," Seth Bordenstein, a biologist at Vanderbilt University in Nashville, TN, told The Scientist last year.
The more diverse our gut bacteria, the better
While the debate over whether infants are born with gut bacteria continues, it seems scientists are in agreement about one thing: from birth until old age, our gut bacteria are constantly evolving.
As mentioned previously, two thirds of the gut microbiome is unique to each person, and what makes this unique is the food we eat, the air we breathe and other environmental factors. Some studies have even suggested the makeup of the gut microbiome is influenced by genes.
But how does this unique gut bacteria affect our health? This is a question that researchers have become increasingly interested in answering.
Past research has suggested that a broader diversity of bacteria in gut is better for human health. A recent study reported by MNT, for example, found that infants with less diverse gut bacteria at the age of 3 months were more likely to be sensitized to specific foods - including egg, milk and peanut - by the age of 1 year, indicating that lack of gut bacteria diversity in early life may be a driver for food allergies.
But the implications of a low-diversity gut microbiome do not stop there. You may be surprised to learn how lack of or overpopulation of specific bacteria may impact your health.
More and more studies are looking at the association between the gut microbiome and weight gain, with some scientists suggesting the makeup of bacteria in the gut may influence an individual's susceptibility to weight gain.
Earlier in the article, we mentioned a 2014 study that claims our genes may determine what bacteria live in our gut, and that these bacteria may influence how heavy we are.
One study identified a specific strain of gut bacteria that may influence our weight.
In detail, the study - conducted by researchers from Cornell University in Ithaca, NY, and King's College London in the UK - found that a certain strain of bacteria - Christensenellaceae minuta - was more common in people with a low body weight, and that the presence of this particular strain is highly influenced by genes.
What is more, introducing this bacteria to the guts of mice caused the animals to gain less weight, indicating the bacteria may reduce or prevent obesity.
"Our findings show that specific groups of microbes living in our gut could be protective against obesity - and that their abundance is influenced by our genes," said study author Prof. Tim Spector of King's College London. "The human microbiome represents an exciting new target for dietary changes and treatments aimed at combating obesity."
In 2012, another study published in the Journal of Proteome Research suggested that a lack of bacteria in the large intestine may drive obesity by slowing down the activity of brown fat, which protects against weight gain when stimulated by burning calories and white fat.
A more recent study provides further evidence that gut bacteria may influence weight gain. A case report published in the journal Open Forum Infectious Diseases revealed how a women who underwent fecal microbiota transplantation (FMT) using an overweight donor rapidly became obese herself following the procedure.
While the jury is still out on whether gut bacteria are directly associated with obesity, it is an area that certainly warrants further investigation.
In recent years, scientists have increasingly investigated the link between gut bacteria and cancer.
In a 2013 study published in The Journal of Cancer Research, US researchers claimed to discover specific bacteria in the intestines - Lactobacillus johnsonii - that may play a role in the development of lymphoma, a cancer of the white blood cells.
Another 2013 study conducted by UK researchers found that a common gut bacteria called Helicobacter pylori may cause stomach cancer and duodenal ulcers by deactivating a part of the immune system involved in regulating inflammation.
And in 2014, MNT reported on research from the Icahn School of Medicine at Mount Sinai, New York, NY, in which investigators associated a specific combination of gut bacteria with the development of colorectal cancer.
For this study, the team gave antibiotics to mice that possessed gene mutations known to cause colorectal polyps, which can develop into cancer. The antibiotics were administered to interfere with the gut bacteria of the mice. The researchers found that these mice did not develop polyps, suggesting that gut microbes may be involved in their development.
But as well as being linked to cancer development, research has found that gut bacteria may be important for improving the effectiveness of cancer treatment.
In 2013, a study by researchers from the National Cancer Institute found that immunotherapy and chemotherapy were less effective in mice lacking gut bacteria, with such treatments working significantly better in mice with a normal gut microbiome.
Similar results were found in another 2013 study by French researchers. An antitumor drug - cyclophosphamide - was found to be much less effective in mice with limited gut bacteria, compared with mice with normal gut bacteria.
"Both studies are very exciting in that they show initial links between gut microbiota and [response to] therapies," Harvard University's Peter Turnbaugh told The Scientist. "The papers underscore the importance [of] microbes in shaping not just our initial predisposition to disease, but also our recovery from it."
Not many of us are likely to think about how gut bacteria affect the mental state, but they actually play a very important role.
According to the American Psychological Association (APA), gut bacteria produce an array of neurochemicals that the brain uses for the regulation of physiological and mental processes, including memory, learning and mood. In fact, 95% of the body's supply of serotonin is produced by gut bacteria, according to the APA.
Since gut bacteria produce many of the neurochemicals responsible for regulating mental processes, it is no surprise that researchers have linked gut bacteria to mental health.
In 2014, for example, a study published in the journal Psychopharmacology found that prebiotics - carbohydrates that boost healthy bacteria in the gut - may be effective for reducing stress and anxiety.
For the study, 45 healthy adults were randomized to receive the prebiotic or a placebo once a day for 3 weeks. All participants were then exposed to both negative and positive stimuli.
The team found that the participants who received the prebiotic were less likely to pay attention to the negative stimuli than those who received the placebo - suggesting lower anxiety in negative situations. They also had lower levels of the "stress hormone" cortisol.
"Time and time again, we hear from patients that they never felt depressed or anxious until they started experiencing problems with their gut," said lead study author Dr. Kirsten Tillisch, the study's lead author. "Our study shows that the gut-brain connection is a two-way street."
Autism is estimated to affect 1 in 68 children in the US. While studies have associated environmental factors - such as pollution - and genetics as potential causes of the disorder, researchers are increasingly looking at the role of gut bacteria in its development.
In 2013, a study by researchers from Arizona State University found that children with autism possessed lower levels of three types of gut bacteria - Prevotella, Coprococcus and Veillonellaceae - compared with children free of the condition.
A more recent study from the team found that concentrations of specific chemicals produced by gut bacteria - called metabolites - in fecal samples of children with autism differed to the concentrations found in the fecal samples of children without the disorder.
This led the researchers to hypothesize that gut microbes alter the metabolites associated with communication between the gut and the brain, which interferes with brain function.
Further strengthening the association between gut bacteria and autism is a 2013 study published in Cell that found the bacterium Bacteroides fragilis reduced autism-like symptoms in mice.
Can we alter our gut bacteria?
Since the gut microbiome is influenced by the food we eat and the environment around us, it makes sense that there are ways to make it healthier.
According to the Mayo Clinic, a healthy diet can encourage the presence of good gut bacteria. They note that consuming fermented foods - such as miso and sauerkraut - increases the level of fermenting bacteria in the gut. In addition, fruits and vegetables contain fibers and sugars that can boost the health of gut bacteria.
Exercise may also be key to improving gut bacteria diversity, according to a study reported by MNT in June 2014.
The study, published in the journal Gut, compared the gut bacteria of 40 professional rugby players with that of two control groups. They found that the rugby players had much higher levels of Akkermansiaceae in their gut - a bacterium that has been associated with reduced risk of obesity.
In an editorial linked to the study, Dr. Georgina Hold, of the Institute of Medical Sciences at Aberdeen University in Scotland, noted that a better understanding of ways to improve health via gut bacteria is crucial:
"As life expectancy continues to increase, it is important that we understand how best to maintain good health. Never has this been more relevant than in respect of our resident microbiota. Understanding the complex relationship among what we choose to eat, activity levels and gut microbiota richness is essential.
Developing new ways to manipulate the beneficial properties of our microbiota by finding ways to integrate health-promoting properties into modern living should be the goal."