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HomeHealth articlesirritable bowel syndromeWhat Is the Link Between Irritable Bowel Syndrome and Bacterial Overgrowth?

Breaking the Link-Irritable Bowel Syndrome and Bacterial Overgrowth

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A surplus of bacteria in the intestine causes irritable bowel syndrome (IBS). Read the article below to know more about it.

Written by

Dr. Gayathri. N

Medically reviewed by

Dr. Ghulam Fareed

Published At January 25, 2023
Reviewed AtJanuary 19, 2024

What Is Irritable Bowel Syndrome?

Irritable bowel syndrome constitutes a group of symptoms that affects the digestive system. The onset of symptoms should occur at least six months before diagnosis, but many patients suffer long-term symptoms related to this disorder.

Historically, intestinal bowel syndrome was associated with stress and anxiety. The cause of this syndrome was once thought to be psychogenic in origin but is now considered to be multifactorial. One reason for this change is the presence of gut bacteria which includes the small intestinal bacterial overgrowth causing the symptoms of irritable bowel syndrome. Several clinical trials targeting the gut microbes have proven that this condition originates from the change in gut microbial flora and not because of the psychological changes caused in the brain.

What Are the Symptoms of Irritable Bowel Syndrome?

The symptoms of intestinal bowel syndrome include:

  • Abdominal pain or cramps usually occur in the lower half of the abdomen.

  • Bloating.

  • Hard bowel movements.

  • Diarrhea.

  • Constipation.

  • Excess formation of gas.

  • Mucus (a thin watery fluid) in the poop.

  • These symptoms flare up in women during their periods.

How Is This Condition Diagnosed?

1. There is no definitive diagnosis of irritable bowel syndrome.

2. The doctor will do a complete physical examination.

3. Medical examination.

4. Ruling out all other conditions.

5. Rome criteria include pain in the belly and discomfort for at least one day a week in the last three months, which should occur with at least two of the other symptoms, which include:

  • Pain and discomfort due to defecation.

  • A change in the frequency of defecation.

  • Change in the consistency of the stool.

  • Irritable bowel syndrome is a common gastrointestinal disorder. It occurs with a series of symptoms, such as abdominal pain, bloating, and loose motion. Patients with small intestinal bacterial overgrowth also experience the same symptoms.

  • The patients with bacterial overgrowth have an increase in the count of bacterial colonies, which will be greater than or equal to ten lakh units per milliliter. The main difference between the two is that small intestinal bacterial overgrowth occurs without the predisposing anatomical factors that occur in the case of irritable bowel syndrome. Thus some researchers are interested in finding the link between the two, which was useful in the research since earlier, there was an understanding that psychological stress caused the peptic ulcer. Scientists found out that it was caused by a bacterium called Helicobacter pylori.

  • The gastrointestinal tract is considered the most heavily colonized organ, with more than 70 percent of the microbes residing in the gut composed of 500 to 1000 bacterial species. The small intestine consists of gram-positive (they are the group of bacteria that gives a positive result in gram staining, which is a test to identify bacteria based on their cell wall and aerobic bacteria (they grow in the presence of oxygen). In contrast, the large intestine is composed of gram-negative anaerobic bacteria (they grow in the absence of oxygen).

  • Normal gut flora has its benefits, which include fermentation of undigested food, absorption of sodium chloride and water, synthesis of vitamin K, and protection against other pathogens by a barrier effect. Despite all this, gram-negative bacteria, which are supposed to grow in anaerobic conditions, sometimes tend to invade the small intestine and grow in the wrong place, which can also result in bacterial overgrowth.

How Do These Bacteria Occur?

The body maintains the balance of bacterial flora in the gut through chemical and mechanical functions. Some chemicals also control the growth of bacteria, which include:

  • The acid is secreted in the stomach.

  • The enzymes of the bile.

  • Mucin production by intestinal mucosal epithelial cells inhibits pathogenic bacteria.

If these chemicals get disrupted, then it causes the bacteria to overgrow. Emptying food contents from the small intestine to the large intestine is another important mechanism that inhibits bacterial overgrowth. If this mechanism is impaired, the bacteria will have more time to breed, and the bacteria from the large intestine will invade and enter the small intestine.

Numerous pathogens have shown an increase in their count in bacterial overgrowth and intestinal bowel syndrome, including Enterococcus, Escherichia coli, and Klebsiella.

How Do These Bacteria Affect the Body?

Bacteria in the small intestine digest carbohydrates and convert them into gas and short-chain fatty acids. As a result, more bacteria produce more gas which can often trigger diarrhea. These bacteria also consume proteins, vitamin B12, and bile salts that help digest fats. All these lead to poor absorption of nutrients and poor digestion of fats, eventually leading to malnutrition in the long run. Over time, these vitamin and mineral deficiencies cause long-lasting damage to the bones and the nervous system.

How Is Bacterial Overgrowth Diagnosed?

The standard diagnostic technique used in diagnosing small intestinal bacterial overgrowth is the presence of greater than 1000 colony-forming units per milliliter.

Breath Test: This test detects the presence of hydrogen and methane gas. These gasses are not produced by the human body but by microbes. These gasses diffuse through the walls of the stomach and enter the circulatory system, where it is excreted in the breath and can be used for direct measurement of small intestinal bacterial overgrowth.

What Are Some Factors Associated With Intestinal Bowel Syndrome and Bacterial Overgrowth?

  • Many patients with intestinal bowel syndrome could be taking proton pump inhibitors, which reduce the acid secretion in the stomach to control indigestion, which could influence the growth of bacteria which is one of the predisposing factors for small intestinal bacterial overgrowth to occur in patients with irritable bowel syndrome.

  • Low hemoglobin levels are also associated with bacterial overgrowth in irritable bowel syndrome patients.

What Are the Treatment Options for Bacterial Overgrowth in Patients With Irritable Bowel Syndrome?

Therapeutic management of the microbial flora in the gut with antibiotics and probiotics has been on the rise.

  • Antibiotics: When selecting antibiotics, one must consider the broad spectrum, including aerobes and anaerobes. Rifaximin can be used. It is a semi-synthetic, nonabsorbable antimicrobial agent against aerobic and anaerobic bacteria.

  • Probiotics: They are live microorganisms made from the human body, which, when administered in sufficient quantities, may reduce the symptoms of intestinal bowel syndrome. Probiotics can also be used to enhance the effect of antibiotics. One study showed that using probiotics along with Rifaximin reduced the symptoms of small intestinal bacterial overgrowth. However, more studies are still needed to evaluate probiotics' efficacy in patients with irritable bowel disease.

  • Dietary Management of the Microbial Flora of the Gut: A diet rich in complex carbohydrates favors the growth of fewer bacteria than a diet rich in proteins and fats. A vegetarian diet and diet rich in fiber led to high production of short-chain fatty acids that inhibit potentially invasive bacteria like E.coli.


The recent discovery of small intestinal bacterial overgrowth involvement in irritable bowel disease has helped in viewing the disease from a different perspective since the cause was thought to be psychological. The main difference between the above two conditions is that bacterial overgrowth has no anatomical disturbances as a predisposing factor for the overgrowth. Thus with advancements in science and technology, more diagnostic methods will be introduced in the upcoming years, which would help better understand the differences between the two conditions.

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Dr. Ghulam Fareed
Dr. Ghulam Fareed

Medical Gastroenterology


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