Small Intestinal Bacterial Overgrowth
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Direct answer
Bloating that worsens through the day, early satiety, loose stools and deficiency states in a patient with a motility disorder, an anatomic blind loop or chronic acid suppression should raise small intestinal bacterial overgrowth — excessive colonic-type flora colonising the small bowel, deconjugating bile salts, consuming vitamin B12 and damaging the brush border. The American Gastroenterological Association's current position is deliberately deflationary: breath testing is not recommended for routine evaluation, its sensitivity and specificity are too poor, and the pragmatic pathway is to diagnose clinically and treat empirically with antibiotics, reserving breath tests for selected cases. Rifaximin 550 mg three times daily for 14 days is the standard course for hydrogen-predominant disease; methane-dominant breath patterns (associated with constipation) are treated with rifaximin plus neomycin. Because recurrence is the rule rather than the exception, the durable part of management is treating the underlying cause — prokinetics for dysmotility, surgery for strictures and blind loops, and a critical look at the PPI prescription.
What you must remember
- Definition: over 10 to the power 5 colony-forming units per millilitre of colonic-type bacteria in proximal small-bowel aspirate — the old gold standard, now rarely used because aspiration is invasive and counts correlate imperfectly with symptoms.
- The protective triad lost: gastric acid, small-bowel motility (migrating motor complex), and the ileocaecal valve — each has a disease list attached: acid suppression (PPIs), dysmotility (diabetic autonomic neuropathy, scleroderma, opioid-induced stasis, gastroparesis), and valve loss (ileocaecal resection, fistula), plus anatomic stasis (blind loops, strictures, adhesions, diverticula).
- Clinical fingerprint: bloating and postprandial discomfort, chronic watery diarrhoea, steatorrhoea from bile-salt deconjugation, weight loss, B12 deficiency (bacteria consume it) with a paradoxically normal or high folate (bacteria synthesise it) — the B12-low/folate-high pair is the classic biochemical signature.
- Breath test, if used: glucose 75 g or lactulose 10 g, with a hydrogen rise of 20 ppm or more within 90 minutes interpreted as positive; methane above roughly 10 ppm at baseline indicates methanogen colonisation (Methanobrevibacter smithii, an archaeon, not a bacterium), which associates with constipation and slower transit.
- AGA stance: treat empirically where pre-test probability is reasonable; the evidence base for breath testing and for dietary supplements marketed for SIBO is weak — a favourite "current guidance" viva point.
- Antibiotic choices: rifaximin first (minimal absorption, favourable safety, repeatable); alternatives include metronidazole, ciprofloxacin, tetracycline and the double therapy of neomycin plus rifaximin for methanogens; repeat courses are common — symptomatic recurrence within months affects a large minority.
- Underlying-cause management: prokinetics (prucalopride, erythromycin low-dose) in dysmotility, diabetic glycaemic control, surgical correction of strictures and blind loops, and deprescribing unnecessary long-term PPIs — the intervention most often forgotten on the ward round.
- Nutritional repair: B12 replacement, fat-soluble vitamins, calcium and vitamin D for bone health, and dietary review — the deficiency state that brought the patient in must be explicitly corrected, not assumed to resolve.
A worked outpatient case
A 61-year-old with long-standing type 2 diabetes and autonomic neuropathy (postural drop, resting tachycardia) reports two years of bloating, alternating diarrhoea and 5 kg weight loss; B12 is low with normal folate, and coeliac serology, faecal calprotectin and a stool panel are negative. Per current guidance, no breath test is needed: the pre-test probability in diabetic dysmotility is high enough to justify empirical rifaximin 550 mg three times daily for two weeks. He improves markedly, and relapses by month four — the expected pattern, which triggers the two durable measures: a prokinetic for the underlying dysmotility, and a planned intermittent-antibiotic strategy (cycling rifaximin or alternating agents) alongside glycaemic optimisation, with B12 replacement continuing. Had his dominant complaint been constipation with a documented methane-predominant breath pattern, neomycin would join the rifaximin. The examinable content is precisely this: when to treat empirically, what to add on relapse, and what to fix underneath.
Where students slip
Two opposite errors share the stage. The first is breath-test evangelism — ordering glucose breath tests on every bloated patient and treating numbers rather than symptoms, contrary to the current AGA position. The second is missing the anatomical cause: a blind loop, stricture or ileocaecal-valve loss converts recurrent SIBO into a surgical question, and no number of antibiotic courses fixes stasis. Candidates also forget the biochemical signature (low B12 with normal-to-high folate), swap the gases (methane goes with constipation, hydrogen with diarrhoea), and overlook the PPI on the drug chart as the iatrogenic enabler.
Frequently asked questions
What is the gold-standard — and the recommended — diagnostic approach?
Jejunal aspirate culture exceeding 10 to the power 5 CFU/mL is the classical gold standard; current AGA guidance recommends against routine breath testing and supports clinical diagnosis with empirical antibiotic treatment.
Which patients are at highest risk of SIBO?
Those with dysmotility (diabetes autonomic neuropathy, scleroderma, opioid use), anatomic abnormalities (blind loops, strictures, diverticula, ileocaecal valve loss), achlorhydria and long-term PPI use, and the elderly.
How is hydrogen-predominant SIBO treated?
Rifaximin 550 mg three times daily for 14 days; repeat courses are acceptable for recurrence, with attention to the underlying motility or anatomic cause.
How does methane change management?
Methane on breath testing reflects archaeal methanogens, associates with constipation, and is treated with combination neomycin plus rifaximin rather than rifaximin alone.
Why does SIBO cause low B12 with normal or high folate?
Intraluminal bacteria consume ingested vitamin B12 but synthesise folate, producing the paradoxical pair — a frequently examined biochemical clue.