Small intestinal bacterial overgrowth: An unwanted guest : Nursing made Incredibly Easy

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Feature: NCPD Connection

Small intestinal bacterial overgrowth

An unwanted guest

Perkins, Amanda DNP, RN

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Nursing Made Incredibly Easy! 20(2):p 22-30, March/April 2022. | DOI: 10.1097/01.NME.0000816544.68933.c6


Small intestinal bacterial overgrowth (SIBO), also referred to as blind loop syndrome, develops when the intestinal transit time is slowed, allowing for the growth of bacteria in the small intestine.1 A variety of complications are associated with SIBO, with the worst possible outcome being death. Let's take a closer look at this disorder, including its causes, risk factors, signs and symptoms, diagnosis, complications, and treatment.

Breaking it down

The number of bacterial cells in the gastrointestinal (GI) tract is approximately 10 times higher than the number of cells in the body.2 Although the bacterial count in the GI tract is high, normally it's low in the small intestine because of gastric secretions and food moving rapidly through this portion of the digestive tract.1,3 The lower portion of the GI tract, specifically the colon, has the highest microbial count and the upper GI tract has the lowest.4 In fact, more than 70% of the microbes in the body are found in the colon.5

The bacteria typically found in the small intestine are Gram-positive and aerobic; in the large intestine, the bacteria tend to be Gram-negative and anaerobic.2,5 Although a variety of bacteria can be present in the small intestine when a patient has SIBO, the most common are Streptococcus, Escherichia coli, Lactobacillus, and Bacteroides.3 These are bacteria typically found in the large intestine, not the small intestine.

Before moving on, it's important to understand the role of the small intestine in the digestion and absorption of nutrients and the role of the large intestine in the absorption of water. Problems with the small intestine, such as SIBO, can lead to difficulty with nutrient absorption and malnutrition. Intestinal contents moving too fast, such as occurs with diarrhea, can lead to dehydration because less water is absorbed by the large intestine. Understanding the role of both the small and large intestine will help you better understand how SIBO affects patients.


SIBO may develop because of conditions that slow intestinal transit time, including structural problems, such as adhesions and diverticulosis; medical conditions, such as pancreatitis, Crohn disease, radiation enteritis, diabetic enteropathy, scleroderma, and celiac disease; or as a complication of abdominal surgery.1,2

When a patient has pancreatitis, the resulting decrease in pancreatic secretions can alter the biochemical clearance of bacteria.2 In the small intestine, pancreatic secretions work in conjunction with enzymes to aid digestion. An alteration in this process may lead to increased bacteria in the small intestine.

Crohn disease is an autoimmune inflammatory bowel disease that can be found anywhere in the GI tract, although it's most common in the small intestine at the junction where the small and large intestine meet.6 This disease can lead to obstruction from narrowing that develops because of chronic inflammation, which slows intestinal transit time and increases the risk of SIBO.6 Enteritis, an inflammation of the intestines that can cause scar tissue development, may also slow intestinal transit time.6

Associated with both constipation and diarrhea, diabetic enteropathy is a dysfunction of the large intestine that results in nerve dysfunction in the GI tract.7 Although constipation can increase the risk of SIBO, the diarrhea associated with diabetic enteropathy affects motility of the small intestine and is commonly associated with the disorder.

Scleroderma is an autoimmune disease that results in the replacement of normal tissue with fibrous tissue. When this happens in the GI tract, it can cause constipation and increase the risk of SIBO. Celiac disease is an autoimmune disorder in which a person's body begins to attack the small intestine after the consumption of gluten. The changes that occur with celiac disease put patients at risk for SIBO.

SIBO has also been associated with chronic opioid use and proton-pump inhibitors (PPIs).2,4 Chronic opioid use can cause constipation, leading to an increased risk of developing SIBO. It's believed that PPIs may increase the risk of SIBO because of decreased gastric acid, which aids in the destruction of bacteria in the small intestine.4 PPIs also lead to hypochloremia, which alters the biochemical clearance of bacteria.2 Short-term use of PPIs isn't typically associated with SIBO; the risk increases with extended use.2

Risk factors

SIBO is seen more commonly in women, as an individual ages, and in individuals with irritable bowel syndrome (IBS).5

Many of the risk factors associated with SIBO mirror its causes. The following increase the risk of SIBO:1,4

  • small intestinal dysmotility
  • obesity
  • gastric surgery
  • structural defects in the small intestine
  • fistula in the GI tract
  • Crohn disease
  • history of radiation to the abdomen
  • diabetes mellitus
  • diverticulosis
  • adhesions.

More research is needed on the association between obesity and SIBO in adults; the correlation between obesity and SIBO in children isn't well defined at this time. A 2020 study conducted by Esposito and colleagues did show an association between excess weight and SIBO in children.8 Having an awareness of risk factors will help you identify patients at risk and improve the nursing assessment for these patients.

Signs and symptoms

Some patients with SIBO are asymptomatic, whereas others may experience a variety of symptoms, such as abdominal pain and diarrhea.8 The following are common signs and symptoms associated with this disorder:1,4,8

  • anorexia
  • abdominal pain
  • nausea
  • belching
  • bloating
  • flatulence
  • discomfort after eating
  • diarrhea
  • weight loss
  • indigestion
  • malnutrition.

Many of the signs and symptoms associated with SIBO match those seen with IBS and other GI disorders.5 When a patient has SIBO, bacterial fermentation of the diet produces hydrogen, methane, and carbon dioxide. The production of these gases may be associated with distension, flatulence, abdominal pain, and bloating. Methane gases are associated with slowed intestinal transit time and, in many cases, constipation. These patients may report bloating and abdominal pain.9 Understanding that constipation may be associated with SIBO will help you identify affected patients.

Be aware that approximately 40% to 45% of patients treated for SIBO will have recurrent symptoms.10 The recurrence of SIBO is seen more frequently in older patients, after an appendectomy, and in those taking PPIs.3 Monitor the patient with SIBO for associated symptoms before and after treatment and report abnormal findings to the healthcare provider.


Performed during an endoscopy where fluid can be removed from the small intestine and tested for bacterial growth, small intestine aspirate and culture is the gold standard diagnostic tool for SIBO.1 However, it isn't without limitations. This procedure is invasive, has a potential for oropharyngeal flora contamination of the sample, and detects only proximal SIBO.2,4 Additionally, anaerobic bacteria may not grow if air is used during the endoscopy; using nitrogen or carbon dioxide during the procedure removes this risk.5

A commonly used diagnostic tool is breath testing to measure the amount of hydrogen or methane a person breathes out after drinking a carbohydrate substrate (see Principles of the hydrogen breath test).1,4 When the carbohydrate substrate is exposed to bacteria, hydrogen and methane are produced and then absorbed from the GI tract into the patient's bloodstream and exhaled through the lungs.4 If hydrogen or methane levels increase after drinking the carbohydrate substrate, it may indicate SIBO.1

During breath testing, glucose, lactulose, or fructose may be used as the carbohydrate substrate. With the glucose test, the patient drinks 75 g of glucose mixed with 1 cup of water.4,9 Breath samples are collected before ingestion of the glucose mixture and then every 15 minutes for 90 to 120 minutes.4 Glucose hydrogen breath testing is more specific but less sensitive than other methods, increasing the risk of false positives.2

With the lactulose test, the patient ingests 10 g of lactulose mixed with 1 cup of water.4,9 This test mimics the glucose test except for the time frame; samples are collected for 180 to 240 minutes.4 Lactulose breath testing is less specific but more sensitive than other methods.2

Principles of the hydrogen breath test

With the fructose test, the patient ingests 25 g of fructose mixed with 1 cup of water.9 The fructose test uses the same process as the other tests but is completed over 180 minutes.4

The reliability of breath testing relies on patient preparation before the test (see Breath testing guidelines). It's recommended that the patient refrain from taking antibiotics and probiotics for 2 to 4 weeks before the breath test. Patients should also stop taking medications that increase motility 1 week before the test.2,4 The patient should avoid complex carbohydrates, dairy, fiber, and alcohol for 24 hours before the test.2 Additional recommendations include avoiding fermented foods on the day before the test, fasting for 8 to 12 hours before the test, and avoiding smoking or strenuous activity on the day of testing.4,9

Checking methane levels will increase the accuracy of the test; however, breath testing isn't as specific as other tests, such as small intestine aspirate and culture, and may result in false positives.1,2,4 Breath testing also isn't standardized regarding indications, how to test, and how to interpret results.9 In some cases, breath testing is preferred because it's more cost effective and less invasive than small intestine aspirate and culture.

Additional diagnostic testing may be completed to support the diagnosis of SIBO. For example, patients with SIBO tend to have low vitamin B12 levels due to malabsorption or bacteria consuming the available B12, so checking vitamin B12 levels is often a part of diagnostic testing.2 An increased folate level and deficiencies in vitamins A, D, E, and K may also be observed.2

In some cases, antibiotics may be used as a diagnostic tool.2 The patient is given antibiotics and monitored for symptom improvement, suggestive of a SIBO diagnosis. It's necessary to consider the risks associated with the use of antibiotics as a diagnostic tool, including decreasing the number of normal flora in the GI tract and increasing the risk of antibiotic-resistant infections.


Complications associated with SIBO include poor absorption of nutrients, vitamin deficiency, dehydration, osteoporosis, and kidney stones.1

The patient with SIBO may become malnourished because bacteria in the small intestine use up nutrients normally used by the body.11 Bacteria can also damage the small intestine, making it more difficult to absorb nutrients, in addition to breaking down bile salts, leading to poor digestion of fats and diarrhea.1,11 Compounds produced by bacteria in the small intestine contribute to diarrhea, which can play a role in malnutrition and weight loss. This is more common in children and can lead to nutritional deficiencies, weight loss, and slowed growth.8

Difficulty digesting fats contributes to vitamin A, D, E, and K deficiency.1 Vitamin A deficiency can lead to vision problems, such as night blindness, and decreased production of pigments required for the retina to work properly.12 Vitamin D deficiency is associated with Ricketts (a disease that causes bones to become soft and bendy), bone pain, and muscle weakness.13 Vitamin E deficiency can cause loss of feeling in the arms and legs, problems controlling body movements, muscle weakness, vision problems, and a weakened immune system.14 Vitamin K deficiency is most often associated with an increased risk of bleeding but can also cause bone loss, bruising, oozing from the nose and gums, excessive bleeding, and heavy menstrual periods.15

Patients with SIBO may also develop vitamin B12 deficiency and associated weakness, fatigue, tingling, numbness of the hands and feet, and mental confusion.1 Poor calcium absorption over a prolonged period, which occurs with SIBO and vitamin D deficiency, may contribute to osteoporosis and kidney stones.1


SIBO is treated with antibiotics, nutritional deficiency correction, and dietary changes such as a lactose-free diet.1,8 The treatment approach for methane-producing bacteria may be different because the prominent methanogen in the GI tract is resistant to antibiotics.9 In most cases, the initial treatment is the administration of antibiotics to decrease the number of bacteria in the small intestine. Note that the goal is to reduce, not eliminate, the bacteria. Antibiotics may need to be taken long-term to prevent the return of the bacterial overgrowth.1

A commonly prescribed antibiotic is rifaximin, given at 600 to 1,600 mg/day for 5 to 28 days.4 Rifaximin works for both Gram-positive and negative bacteria, as well as aerobic and anaerobic bacteria.2,5 Rifaximin isn't used as the sole antibiotic for methane-prominent bacterial growth.10 Common adverse reactions associated with rifaximin include dizziness and peripheral edema.16

Other antibiotics that may be used include ciprofloxacin and metronidazole.4 Adverse reactions associated with ciprofloxacin include diarrhea, nausea, photosensitivity, dizziness, headache, and hyper-/hypoglycemia.16 Be aware that the absorption of ciprofloxacin is decreased with intake of calcium, magnesium, aluminum, iron, and zinc.16 Educate the patient to avoid these substances for 6 hours before and 2 hours after taking the medication.16 Common adverse reactions associated with metronidazole include dizziness, headache, abdominal pain, anorexia, and nausea.16 Teach the patient to avoid alcohol while taking metronidazole and for 3 days after its completion.16 Alcohol intake can cause flushing, nausea, vomiting, headache, and abdominal cramps.16

The treatment of methane-prominent bacterial overgrowth includes neomycin and rifaximin.10 Adverse reactions associated with neomycin include diarrhea, nausea, and vomiting.16

Caution breastfeeding patients about the associated risks when taking any of these antibiotics. It's advised that breastfeeding be avoided with rifaximin, ciprofloxacin, and metronidazole.16 Neither safety with breastfeeding while taking neomycin nor safety in pediatric patients has been established at this time.16 These antibiotics are excreted in breastmilk and can cause problems in the infant if breastfeeding continues.16 Patients who wish to continue breastfeeding should be encouraged to pump while taking the antibiotic and discard the milk. This will ensure that the milk supply remains constant. If the patient has previously pumped breastmilk, it can be given to the infant during the course of antibiotics. If previously pumped breastmilk isn't available, formula can be provided.

All of these antibiotics should be used cautiously during pregnancy.16 Ask the patient about pregnancy or attempts to become pregnant before the initiation of the antibiotic.

Consider that these antibiotics don't only decrease bacteria in the small intestine, but they also decrease the normal flora in the large intestine. Rifaximin and ciprofloxacin are associated with an increased risk of Clostridium difficile-associated diarrhea (CDAD).16 Due to the increased risk of CDAD, advise the patient to notify the healthcare provider if diarrhea, abdominal cramping, fever, or bloody stools develop.16

Probiotics may be used to help reduce the bacterial burden associated with SIBO.2 They can also increase the effectiveness of antibiotics.2 Probiotics should be chosen carefully, and the patient closely monitored. If the patient is experiencing constipation instead of diarrhea, probiotics may not be an appropriate choice because they can lead to increased methanogenic bacteria—the bacteria responsible for constipation. Research on the use of probiotics in patients with SIBO is mixed; some evidence shows that they may worsen symptoms in some patients.2 More research needs to be conducted on the use of probiotics in the treatment of SIBO.

Herbal supplements are sometimes used by patients with SIBO.2 In fact, some studies show them to be as effective as antibiotic therapy.2 A variety of commercial supplements are available; however, they aren't approved treatment options. Overall, research is lacking on the use of these herbal supplements for the treatment of SIBO.2 The patient wishing to try herbal supplements should speak with their healthcare provider before starting them.

If present, micronutrient deficiencies should be corrected. In many cases the patient will need vitamin B12, fat-soluble vitamins, thiamine, niacin, calcium, and iron replacement.1,10 Other vitamins may also need to be replaced. Oral replacement of these vitamins may be ineffective if SIBO isn't treated and in cases when the small intestine is damaged.

Some studies show positive results with an elemental diet (a liquid diet of predigested macronutrients), which may be used in patients who are unable to tolerate antibiotics or for those who are unsuccessful on antibiotics.4,10 In some cases, this diet is used instead of antibiotics.4 The macronutrients are primarily absorbed in the proximal small bowel, decreasing the amount of nutrients reaching the distal portion of the small bowel where the bacterial overgrowth is located.4 Although this diet is promising, adherence can be an issue because it isn't well tolerated by patients and can be expensive.2,4

Another diet is the low FODMAP diet. FODMAP stands for fermentable oligosaccharides, disaccharides, monosaccharides, and polyols.4 SIBO causes the fermentation of carbohydrates, such as lactose and fructose, and dietary FODMAPS.5 A low FODMAP diet decreases the exposure of the small intestine to carbohydrates and their fermentation products.4 This diet may be helpful because the bacteria associated with SIBO ferment carbohydrates, and the low FODMAP diet removes sugars that can be fermented in the GI tract.2,4

Treatment of SIBO should also include correction of underlying problems such as adhesions. Failure to address underlying problems could result in a return of SIBO. It's also important to determine if the patient has additional conditions such as lactose intolerance.2 As many as 40% to 45% of patients will develop recurrent SIBO.2,4 These patients will have a relapse of symptoms within 9 months of treatment.4 Diagnosis should be made with small intestine aspirate and culture so targeted antibiotic treatment can be used.4

Nursing implications

When caring for the patient with SIBO, monitor intake and output, bowel movements, and weight closely because dehydration is a risk. Identifying a mismatch between intake and output can help prevent dehydration and associated complications. Ask the patient to describe bowel movements. Determine if the patient is having loose stools, constipation, or a change in bowel movements. This may be an uncomfortable topic for the patient; ensure that the patient understands the reason behind describing bowel movements.

The patient with SIBO is at risk for weight loss, so monitor weight closely. Always remember that weight should be taken on the same scale and at the same time each day. Ask the patient about their appetite and any discomfort experienced after eating. Determine if the patient has nausea, vomiting, belching, bloating, or flatulence. Lab data should be closely monitored, and any abnormal findings reported to the healthcare provider.

Educate the patient about medications and monitor for associated adverse reactions. Teach the patient about the importance of adherence with the medication regimen and encourage them to notify the healthcare provider about troublesome adverse reactions before discontinuing medications. Lastly, educate the patient about reporting a return of symptoms after the completion of antibiotics. For a condensed version of nursing care expectations, see Nursing assessment and care.

Improved outcomes

SIBO is a common , yet underdiagnosed, condition.17 Use your new knowledge to identify patients at risk for SIBO. Help fellow nurses learn about this condition by sharing what you know. Together we can improve outcomes for patients with SIBO.

Breath testing guidelines

Before the test:

  • Fast for 8 to 12 hours.
  • Avoid:
    • —antibiotics or probiotics for 2 to 4 weeks
    • —medications that increase motility for 1 week
    • —complex carbohydrates, dairy, fiber, or alcohol for 24 hours
    • —fermented foods for 24 hours
    • —smoking or strenuous activity.

consider this


You're caring for a middle-aged patient who arrives at the primary care office with complaints of abdominal pain, especially after eating; bloating; belching; and diarrhea. Upon examination, you determine that the patient has lost 30 lb since the last visit 6 months ago. The patient reports no dietary changes or intentional weight loss. The patient currently takes omeprazole for gastroesophageal reflux disease, metoprolol for hypertension, and metformin for diabetes. What are your next steps? What additional data need to be collected?

cheat sheet

Vitamin deficiencies


Vitamin A

  • Vision problems (such as night blindness)
  • Decreased production of pigments required for proper retinal function

Vitamin D

  • Ricketts
  • Bone pain
  • Muscle weakness

Vitamin E

  • Loss of feeling in the arms and legs
  • Difficulty controlling body movements
  • Muscle weakness
  • Vision problems
  • Weakened immune system

Vitamin K

  • Increased risk of bleeding
  • Bone loss
  • Bruising
  • Oozing from the nose and gums
  • Heavy menstrual periods

Vitamin B12

  • Weakness
  • Fatigue
  • Tingling
  • Numbness of the hands and feet
  • Mental confusion

key points

Nursing assessment and care


Ask about:

  • bowel movements
  • appetite and discomfort after eating
  • belching, bloating, and flatulence
  • medication adherence.


  • weight
  • lab data
  • adverse reactions to medications.

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10. Pimentel M. Small intestinal bacterial overgrowth: management. UpToDate. 2019.
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13. MedlinePlus. Vitamin D deficiency. 2017.
14. Lab Tests Online. Vitamin K deficiency.
15. National Institutes of Health Office of Dietary Supplements. Vitamin E. 2021.
16. Vallerand AH, Sanoski CA. Davis's Drug Guide for Nurses. 17th ed. Philadelphia, PA: F.A. Davis; 2021.
17. Achufusi TGO, Sharma A, Zamora EA, Manocha D. Small intestinal bacterial overgrowth: comprehensive review of diagnosis, prevention, and treatment methods. Cureus. 2020;12(6):e8860.
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