Small intestinal bacterial overgrowth

01OverviewDefinition, clinical context and the essential points that orientate the chapter.

The normal proximal small bowel contains fewer organisms than the colon because acid, bile, pancreatic secretion, migrating motor complexes, intact anatomy and the ileocaecal valve limit colonisation. When these defences fail, organisms deconjugate bile salts, consume nutrients and injure mucosa. The result ranges from gas and diarrhoea to steatorrhoea, weight loss, B12 deficiency and hypoalbuminaemia. Folate may be normal or high because bacteria synthesise it, but this is not a diagnostic rule.

Diagnostic tests are imperfect. Quantitative culture of a carefully obtained small-bowel aspirate is conceptually direct but samples only one site, is easily contaminated and has non-uniform thresholds. Breath tests infer substrate fermentation from hydrogen or methane, but rapid transit can mimic an early rise, distal overgrowth can be missed by glucose and methane producers complicate interpretation. The BSG position on chronic diarrhoea reflects these limitations: in a patient with strong anatomical or motility risk, a monitored empirical antibiotic trial may be more pragmatic.

SIBO recurs when the substrate persists. A useful consultation maps surgery, radiation, Crohn's strictures, diverticula, systemic sclerosis, diabetes, opioids, acid suppression and pseudo-obstruction. It also distinguishes intestinal methanogen overgrowth, which involves archaea and constipation, from classic diarrhoeal SIBO. Long-term care belongs with gastroenterology or an intestinal-failure service when repeated courses, malnutrition or complex anatomy are involved.

Key points

SIBO is an excess or abnormal composition of small-bowel microorganisms causing symptoms or malabsorption; bloating alone is too nonspecific to establish it.
Predisposing mechanisms include blind loops, strictures, fistulae, diverticula, scleroderma dysmotility, previous surgery, small-bowel stasis and loss of gastric acid or ileocaecal barrier.
Symptoms include bloating, discomfort, diarrhoea, steatorrhoea, weight loss and B12 or fat-soluble vitamin deficiency; methane-associated overgrowth may present with constipation.
Jejunal aspirate culture is invasive and lacks fully standardised thresholds; glucose and lactulose breath tests have important false-positive and false-negative limitations.
BSG chronic-diarrhoea guidance favours an empirical antibiotic trial when pre-test probability is high rather than relying on poorly performing routine breath tests.
A treatment response does not prove SIBO because antibiotics alter colonic flora, inflammation and motility; document what changed and whether it recurs.
Correct a stricture, blind loop, fistula, medicine driver or motility problem where possible, and avoid repeated antibiotics without anatomical review.
Replace B12 and other deficits, review nutrition and consider refeeding risk in severe disease.

02AetiologyUnderlying causes, associations and risk factors, with why each one matters.

Structural stasis

Blind loops, diverticula, strictures, fistulae and postoperative anatomy create poorly cleared segments where colonic-type organisms accumulate.

Dysmotility

Scleroderma, diabetes, neurological disease and severe intestinal dysmotility weaken the migrating motor complex that normally clears fasting small bowel.

Reduced antimicrobial barriers

Hypochlorhydria and loss of the ileocaecal valve reduce gastric and anatomical restraint on proximal bacterial colonisation.

03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.

1
Abnormal small-bowel colonisation
Excess or altered organisms persist because luminal clearance and normal compartment barriers are impaired between meals.
2
Fermentation and bile deconjugation
Bacteria ferment carbohydrates and deconjugate bile salts, producing gas, osmotic symptoms and impaired fat absorption after food intake.
3
Nutrient competition and mucosal injury
Organisms consume vitamin B12 and may damage epithelium, causing diarrhoea, steatorrhoea, weight loss and deficiency in severe disease.

04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.

High-probability anatomy

Blind loop, jejunal diverticulosis, bypassed segment, fistula, small-bowel stricture or loss of the ileocaecal valve plus new bloating and diarrhoea makes overgrowth plausible.

Motility phenotype

Systemic sclerosis, autonomic neuropathy, chronic intestinal pseudo-obstruction or opioid exposure produces stasis, distension and variable diarrhoea or constipation.

Malabsorptive disease

Steatorrhoea, weight loss, macrocytic or mixed anaemia, low B12, fat-soluble vitamin deficiency and oedema suggest clinically important overgrowth or a competing mucosal or pancreatic cause.

Obstruction or ischaemia alarmRed flag

Persistent vomiting, colicky focal pain, peritonism, fever or lactate elevation is not uncomplicated SIBO and requires urgent imaging and surgical assessment.

Competing common disorder

IBS, coeliac disease, bile acid diarrhoea, pancreatic insufficiency, giardiasis and medicine-related diarrhoea can mimic or coexist; low pre-test probability makes indiscriminate testing misleading.

05InvestigationsWhat to request, why it matters and how to interpret it.

Investigation order

Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.

01
Anatomy and medicine reviewFirst step
Why
Obtain operation and radiotherapy history, assess strictures, fistulae, diverticula, dysmotility and review opioids, anticholinergics and acid suppression.
Interpretation and limitations
A clear predisposing mechanism increases test and treatment meaning; absence does not exclude SIBO but lowers the value of repeated empirical antibiotics.
02
Nutrition and malabsorption profile
Why
Check FBC, ferritin, B12, folate, albumin, INR, calcium, magnesium, vitamin D and additional fat-soluble vitamins according to severity.
Interpretation and limitations
Patterns support biological impact but are nonspecific. Combined iron and B12 deficiency can yield a normal MCV.
03
Glucose or lactulose breath test
Why
Measure hydrogen and methane after a standard substrate only where the local service uses a validated protocol and the result will change management.
Interpretation and limitations
Rapid transit, recent antibiotics, diet preparation and methane biology create false results. A curve is not interpretable without symptoms and pre-test probability.
04
Small-bowel aspirate culture
Why
Obtain a protected sample during specialist endoscopy in selected complex or treatment-refractory cases.
Interpretation and limitations
Contamination, patchy distribution and varying quantitative thresholds limit certainty; culture may guide therapy when a credible dominant organism is found.
05
Cross-sectional small-bowel imaging
Why
Use CT or MR enterography to identify stricture, diverticula, fistula, blind loop, tumour or inflammatory disease when anatomy is unknown or symptoms recur.
Interpretation and limitations
Imaging does not diagnose bacterial burden but can reveal the correctable reason overgrowth returns.
06
Alternative-cause testing
Why
Use coeliac serology, faecal calprotectin, stool pathogen testing, faecal elastase or bile acid testing according to the clinical phenotype.
Interpretation and limitations
A response to antibiotics should not prevent completion of an indicated cancer, inflammatory or malabsorption pathway.

06Differential diagnosisRealistic alternatives and the features that help distinguish them.

Irritable bowel syndrome

Bloating alone is nonspecific; a typical pain-stool pattern without structural risk, malnutrition or deficiency favours IBS over SIBO.

Coeliac or inflammatory bowel disease

Positive serology, villous atrophy, raised inflammatory markers or characteristic endoscopy and imaging identifies primary mucosal inflammation.

Pancreatic exocrine insufficiency

Pancreatic disease, low faecal elastase and fat-soluble vitamin loss support enzyme deficiency, though SIBO and pancreatic disease can coexist.

07ManagementImmediate care, first-line treatment, alternatives and escalation.

01AssessHigh pre-test probabilityFirst stepCompatible symptoms occur with clear structural, surgical or motility risk.

1Exclude acute obstruction, ischaemia and sepsis, then document anatomy, medicine drivers, stool, weight and deficiency phenotype.
2Discuss a local validated test or a time-limited empirical antibiotic trial with gastroenterology, defining the expected symptom and nutritional response.
3Correct measured deficiencies and optimise diet without imposing multiple unsupported exclusions.
4Arrange imaging or endoscopy when anatomy has not been assessed or a correctable lesion is plausible.

02TreatTargeted antimicrobial courseSIBO is supported by high clinical probability, testing or a specialist-agreed empirical trial.

1Select a short oral regimen from local microbiology guidance based on allergy, previous exposure, renal function and risk of C. difficile.
2Record baseline stool, bloating and weight so response is assessable; stop rather than extend automatically when no meaningful response occurs.
3Review recurrence for a structural or motility driver and avoid rotating antibiotics indefinitely without diagnostic reset.
4In complex intestinal failure, coordinate antimicrobials, nutrition and prokinetic or surgical options through the specialist MDT.

03ResetLow probability or non-responseTesting is equivocal, no risk factor exists or symptoms persist after an adequate trial.

1Reconsider IBS, lactose or FODMAP intolerance, bile acid diarrhoea, coeliac disease, pancreatic insufficiency and microscopic colitis.
2Check whether a breath result could reflect rapid transit, preparation failure or methane rather than hydrogen biology.
3Avoid repeated broad-spectrum courses and address adverse effects, resistance and C. difficile risk.
4EscalationEscalate weight loss, bleeding, anaemia, nocturnal symptoms or obstruction through the appropriate organic-disease pathway.

Key medicines and prescribing safety3 treatments · regimens, roles and cautions

A minimally absorbed antibiotic used by some UK services for selected SIBO or intestinal methanogen overgrowth.

Rifaximin

Specialist-selected short oral course using the current local off-label or formulary regimen; dose and duration vary by phenotype and service.

Not a universal first-line choice and not proof of diagnosis. Cost, resistance, C. difficile, recurrence and interactions require review; avoid endless repeat prescriptions.

Reduces excess small-bowel organisms when SIBO is clinically credible and rifaximin is unsuitable or not approved.

Alternative systemic antibiotic

A short course of an agent such as co-amoxiclav, metronidazole or another locally approved choice, adjusted for renal function, allergy and previous cultures.

Agent-specific neuropathy, tendon, hepatic, interaction and C. difficile risks. Choose with microbiology and reassess rather than rotate empirically.

Treats B12 deficiency caused or worsened by bacterial consumption and underlying anatomy.

Hydroxocobalamin

Current BNF intramuscular loading and maintenance based on neurological involvement and persistence of malabsorption.

Treat neurological features without delay. Also assess folate, iron and copper and address the overgrowth driver.

08ComplicationsImportant consequences, why they occur and why they matter clinically.

Vitamin and nutrient deficiency

Vitamin B12 and fat-soluble vitamin depletion can cause anaemia, neuropathy, bone disease and coagulopathy when overgrowth is prolonged.

Weight loss and malnutrition

Maldigestion, diarrhoea and food avoidance cause sarcopenia and reduced treatment resilience, especially with short bowel or systemic disease.

Recurrence and antimicrobial harm

Uncorrected anatomy or dysmotility promotes relapse, while indefinite empirical antibiotic cycling increases adverse effects, resistance and Clostridioides difficile risk.

09Monitoring and follow-upTreatment response, safety checks and longer-term review.

Measure the specific target symptoms, stool form, weight and function before and after a defined antibiotic course.
Repeat B12, FBC, ferritin, albumin and fat-soluble nutrients according to baseline abnormalities and replacement.
Review for antibiotic rash, diarrhoea, neuropathy, interactions and C. difficile during and after treatment.
Track recurrence interval and cumulative antibiotic exposure; shortening intervals should trigger anatomical or motility reassessment.
Assess whether opioids, anticholinergics or unnecessary acid suppression can be reduced safely.
In severe or postoperative disease, coordinate nutrition, line or stoma care and imaging through an intestinal-failure service.

10Special situationsVariants, exceptions and circumstances that change the usual approach.

Breath is an indirect sample

The test measures host exhalation after microbial metabolism, so transit and gas phenotype can imitate or conceal small-bowel overgrowth.

Methane is made by archaea

Constipation-associated methane biology is better termed intestinal methanogen overgrowth and may respond differently from hydrogen-predominant diarrhoeal disease.

High folate is not reassurance

Bacterial folate production can coexist with B12 depletion and major malabsorption.

Recurrence contains diagnostic information

Rapid relapse after repeated short responses suggests unresolved anatomy or motility rather than an antibiotic-duration problem.

Antibiotic response is not specificity

Many gut disorders fluctuate or respond nonspecifically to microbial change. A good response supports but does not prove the label.

11Common pitfallsFrequent interpretation and management errors.

01
Diagnosing SIBO from bloating alone.
02
Treating a breath-test curve without checking transit, preparation or symptoms.
03
Cycling broad antibiotics indefinitely without anatomy review.
04
Missing obstruction because vomiting is attributed to overgrowth.
05
Ignoring B12 and fat-soluble vitamin deficiency.
06
Calling constipation-associated methane identical to classic diarrhoeal SIBO.