Your SIBO Subtype Doesn't Matter — Until You Fix the Cell That Created It

Why hydrogen, methane, and hydrogen sulfide are three symptoms of the same upstream failure — and what that means for every protocol you've ever tried.

There are now three recognized subtypes of small intestinal bacterial overgrowth. Hydrogen-dominant SIBO. Intestinal methanogen overgrowth. And hydrogen sulfide SIBO — or what Pimentel's group at Cedars-Sinai is now calling ISO, intestinal sulfide overproduction.

Each has its own gas signature, its own symptom profile, its own treatment protocol. Rifaximin for hydrogen. Rifaximin plus neomycin for methane. Bismuth and low-sulfur diets for hydrogen sulfide. Three subtypes, three playbooks.

And all three miss the same thing.

They're treating the gas. They're not treating the cell that made the gas possible.

* The Hydrogen Economy You've Never Heard Of

Before you can understand why SIBO subtypes are a downstream phenomenon, you need to understand something about how gases work in the gut.

Every bacterium that ferments carbohydrates in your intestine produces hydrogen. It's a metabolic byproduct. And here's the problem — hydrogen accumulation is thermodynamically unfavorable for the bacteria producing it. If hydrogen builds up, fermentation slows down. The system stalls.

So the gut has evolved hydrogen sinks. Three groups of organisms compete to consume hydrogen, and each produces a different gas as its waste product.

Acetogens convert hydrogen and CO₂ into acetate — a short-chain fatty acid that feeds the colonocyte. This is the healthy pathway. The colonocyte gets fuel. The anaerobic environment is maintained. Everyone wins.

Methanogens — primarily the archaeon Methanobrevibacter smithii — convert hydrogen and CO₂ into methane. Methane directly inhibits intestinal smooth muscle contractility. Transit slows. Constipation follows.

Sulfate-reducing bacteria convert hydrogen and sulfate into hydrogen sulfide. H₂S freely crosses cell membranes and poisons cytochrome c oxidase — Complex IV of the mitochondrial electron transport chain. The colonocyte's power plant shuts down.

Roughly 60% of humans rely on sulfate-reducing bacteria as their primary hydrogen sink. About 30-40% rely on methanogens. This ratio is constitutive — it's determined early in life and largely stable. It means that when dysbiosis occurs, the gas phenotype that manifests is predetermined by which hydrogenotroph was already dominant.

The subtype doesn't tell you what went wrong. It tells you which hydrogen consumer was waiting in the wings when the system broke.

So what actually broke?

* The Cell Nobody's Looking At

The colonocyte — the epithelial cell lining the colon — is the most metabolically unusual cell in the human body. While most cells run primarily on glucose, the colonocyte derives 70-90% of its energy from beta-oxidation of butyrate, a short-chain fatty acid produced by obligate anaerobic bacteria in the colon.

This isn't a minor metabolic quirk. It's the central regulatory mechanism of the entire colonic ecosystem.

When colonocytes oxidize butyrate, they consume enormous amounts of oxygen. This oxygen consumption maintains what's called physiological epithelial hypoxia — an oxygen partial pressure of less than 1% at the luminal surface. This hypoxic environment is essential. Obligate anaerobic bacteria — the ones that produce butyrate, that maintain barrier function, that keep pathogenic organisms in check — can only thrive under these conditions.

Byndloss and colleagues demonstrated this definitively in a 2017 Science paper. When butyrate-producing bacteria are depleted, PPAR-γ signaling in colonocytes goes silent. Without PPAR-γ, colonocytes shift from oxidative phosphorylation to anaerobic glycolysis. Oxygen consumption drops. The epithelium becomes oxygenated. And facultative anaerobic bacteria — Escherichia, Klebsiella, Salmonella — expand by using that oxygen for aerobic respiration.

Litvak and colleagues formalized this in their 2018 Science review: colonocyte metabolism functions as a "control switch" between homeostatic and dysbiotic communities.

The expansion of Enterobacteriaceae is not a cause of disease. It is a microbial signature of epithelial dysfunction.

And here's what nobody is saying clearly enough: those Enterobacteriaceae — the ones that expand when the colonocyte fails — are hydrogen producers. They are the fermentative organisms whose hydrogen output feeds the entire downstream gas cascade.

Fix the colonocyte, and you cut the hydrogen supply at its source.

Three Gases, One Failure

Let me trace each subtype back to this single upstream event.

Hydrogen-Dominant SIBO

When colonocytes fail, the oxygen gradient collapses. Enterobacteriaceae expand in the colon. These organisms are motile and tolerant of varied environments. They migrate upward through the ileocecal valve into the small intestine — especially when motility is compromised.

Once in the small intestine, they ferment carbohydrates and produce massive amounts of hydrogen. In pure hydrogen-dominant SIBO, neither methanogens nor sulfate-reducing bacteria are present in sufficient numbers to consume the hydrogen efficiently. It accumulates. You measure it on the breath test. You diagnose SIBO.

But the damage isn't from the hydrogen itself. Hydrogen gas is relatively inert to host tissue. The damage comes from the organisms producing it: their LPS activates toll-like receptor 4 and drives inflammatory cascades. Their bile acid deconjugation disrupts FXR signaling. Their nutrient consumption causes malabsorption.

And the inflammation they generate feeds back to the colonocyte. CD38 — a NAD⁺-consuming enzyme — is upregulated on inflammatory immune cells throughout the intestinal mucosa. NAD⁺ levels drop. Sirtuin activity fails. Mitochondrial function deteriorates further.

The colonocyte gets worse. The dysbiosis deepens. The hydrogen keeps flowing.

Intestinal Methanogen Overgrowth

In patients where M. smithii is the dominant hydrogenotroph, colonocyte failure feeds methanogenesis through a mechanism nobody is discussing: acetate competition.

Healthy colonocytes metabolize acetate — along with butyrate and propionate — as an energy substrate. The colonocyte is supposed to be an acetate sink, limiting the amount available in the lumen. When colonocyte oxidation fails, more acetate remains unmetabolized.

M. smithii requires acetate as its sole carbon source. It cannot fix CO₂. The colonocyte's metabolic failure literally provides the carbon substrate for methanogen expansion.

Simultaneously, the hydrogen produced by the expanding Enterobacteriaceae gives M. smithii its energy substrate. The archaeon converts hydrogen and CO₂ into methane, which acts as a gasotransmitter on intestinal smooth muscle, directly inhibiting contractility. Transit slows. Bacterial stasis worsens. More fermentation occurs. More hydrogen is produced. More methane follows.

Data from quantitative PCR studies show M. smithii copy numbers are dramatically higher in constipation-predominant IBS (log₁₀ 6.1) versus diarrhea-predominant IBS (3.4) versus healthy controls (1.9), and the copy number negatively correlates with stool frequency.

Methanogens are also extraordinarily difficult to eradicate because M. smithii is not a bacterium. It's an archaeon with a pseudopeptidoglycan cell wall and ether-linked isoprenoid membrane lipids — making it resistant to most antibiotics. This is why IMO requires combination therapy (rifaximin plus neomycin) and why relapse rates remain high.

But even combination antibiotics don't address the colonocyte failure that provided the acetate surplus and the ecological disruption that expanded the hydrogen producers feeding the methanogens. The metabolic environment that selected for the overgrowth remains intact.

*Hydrogen Sulfide SIBO

This is the subtype where the HCM's predictions are most directly validated, because H₂S doesn't just correlate with colonocyte dysfunction — it causes it.

At high concentrations, H₂S inhibits cytochrome c oxidase at the heme a₃ center. This is Complex IV — the terminal enzyme in the electron transport chain. When Complex IV is poisoned, oxidative phosphorylation stops. The colonocyte cannot generate ATP.

But H₂S does something else that I haven't seen anyone connect explicitly. It also inhibits short-chain acyl-CoA dehydrogenase — SCAD — the first enzyme in the butyrate beta-oxidation pathway.

This means H₂S attacks butyrate metabolism at both ends simultaneously. The colonocyte cannot initiate butyrate oxidation (SCAD blocked) and cannot complete the terminal electron transfer even from whatever oxidation does occur (Complex IV blocked). It's a double hit on the exact pathway the HCM identifies as the central node of gut homeostasis.

The small intestine has approximately one-twentieth the H₂S detoxification capacity of the colon. The enzyme that performs this detoxification — sulfide quinone oxidoreductase, or SQOR — is anchored to the inner mitochondrial membrane and oxidizes H₂S by transferring electrons to coenzyme Q10. When H₂S production exceeds SQOR capacity, the free sulfide poisons the mitochondria that were supposed to detoxify it.

Recent work has revealed that when Complex IV is inhibited by H₂S, colonocytes activate an emergency backup pathway: SQOR transfers electrons to Complex II running in reverse, using fumarate as an electron acceptor and accumulating succinate. This keeps H₂S clearance going but at tremendous metabolic cost — and the succinate that accumulates is itself an oncometabolite and epigenetic modifier.

The clinical implications are significant. The small intestine simply cannot handle the H₂S load that sulfate-reducing bacteria produce when they overgrow. And because SRB consume hydrogen, they can produce false-negative results on standard breath tests. Before tri-gas testing became available, these patients were being told they didn't have SIBO at all.

## Five Bridges From Colon to Small Intestine

There's a question implicit in everything I've described: if colonocyte failure is primarily a colonic event, how does it cause overgrowth in the small intestine?

The answer involves at least five propagation mechanisms.

First, loss of colonization resistance. The healthy colonic ecosystem prevents pathogenic organisms from expanding. When that ecosystem collapses, Enterobacteriaceae and other opportunists expand in the colon and migrate upward.

Second, inflammatory disruption of motility. Colonic barrier failure leads to LPS translocation into the portal and systemic circulation. Systemic inflammation impairs enteric nervous system function, reducing migrating motor complex activity. Without the MMC's cleansing waves, bacteria accumulate in the small intestine.

Third, bile acid disruption. Colonic dysbiosis alters secondary bile acid production, disrupting the FXR feedback loop to the liver. Changed bile composition reduces the antimicrobial properties of bile in the small intestine, creating a permissive environment for overgrowth.

Fourth, systemic NAD⁺ depletion. CD38, the primary NAD⁺-consuming enzyme, is upregulated by inflammatory signals throughout the GI tract — not just in the colon. Small intestinal epithelial cells experience the same NAD⁺ depletion, weakening their barrier function, antimicrobial peptide production, and motility regulation from the inside.

Fifth, disrupted butyrate supply. Butyrate produced in the colon enters systemic circulation and signals to tissues throughout the body. When colonic butyrate-producing bacteria decline, this systemic signal is lost. Small intestinal HIF stabilization decreases. Mast cell suppression is reduced. The small intestine becomes vulnerable to the overgrowth that follows.

* Why Your Protocol Keeps Failing

If you've treated SIBO and it came back — or if it partially resolved and then plateaued — the reason is almost certainly that you treated the gas without treating the cell.

Rifaximin kills the overgrown bacteria. But it doesn't restore colonocyte oxidative metabolism. It doesn't rebuild the oxygen gradient. It doesn't replenish NAD⁺. It doesn't reverse SLC5A8 methylation. It doesn't fix the migrating motor complex. It doesn't reduce CD38 expression.

The metabolic environment that selected for the dysbiotic community the first time is still there after the antibiotic course ends. The same ecological pressures that expanded the hydrogen producers, the methanogens, or the sulfate-reducing bacteria will do so again. The gas phenotype was never the disease. It was always the readout.

This is why intervention sequencing matters more than intervention selection. Before you reach for the antibiotic, ask: is this colonocyte ready to hold what the antibiotic clears?

If the answer is no, you're just clearing space for the same community to regrow.

The Research That Changed My Thinking

Three papers fundamentally shaped this framework.

Park and colleagues published in iScience in 2024 using SCAD-deficient mice — animals that cannot perform the first step of butyrate beta-oxidation. These mice lost their butyrate-producing bacteria at every taxonomic level. And they did not respond to dietary fiber. The host's inability to oxidize butyrate directly determined the microbial community, regardless of substrate availability.

Libiad and colleagues published in the Journal of Biological Chemistry in 2022, demonstrating that when H₂S poisons Complex IV, colonocytes activate a previously unknown emergency pathway — Complex II running in reverse — to maintain H₂S clearance. Complex II knockdown in colonocytes significantly impaired H₂S clearance. This means the colonocyte's entire mitochondrial machinery is engaged in sulfide defense, and when any component fails, the system cascades toward toxicity.

And the 2025 Frontiers review on SQOR revealed that SQOR expression in the intestinal mucosa declines with age and is reduced by approximately 50% in IBD patients — independent of inflammation severity. The colonocyte's ability to detoxify H₂S weakens over time, lowering the threshold at which bacterial H₂S becomes toxic.

These aren't isolated findings. They're three views of the same mechanism: the colonocyte's mitochondrial capacity determines whether the gut remains in homeostasis or falls into a self-reinforcing pathological state.

Your SIBO subtype is a symptom. The colonocyte is the disease.

Mohammed Attallah develops the Host Capacity Model — a systems biology framework for chronic gut dysfunction centered on colonocyte bioenergetics. Follow on Substack for mechanistic deep dives that challenge conventional thinking about gut health.

References

Byndloss MX et al. Microbiota-activated PPAR-γ signaling inhibits dysbiotic Enterobacteriaceae expansion. Science. 2017;357(6351):570-575.

Litvak Y et al. Colonocyte metabolism shapes the gut microbiota. Science. 2018;362(6418):eaat9076.

Park B et al. Crosstalk between butyrate oxidation in colonocyte and butyrate-producing bacteria. iScience. 2024;27(9):110853.

Libiad M et al. A redox cycle with complex II prioritizes SQOR-dependent H₂S oxidation. J Biol Chem. 2022;298(3):101573.

SQOR review. Front Cell Dev Biol. 2025;13:1685252.

Tiranti V et al. Loss of ETHE1 causes fatal sulfide toxicity in ethylmalonic encephalopathy. Nat Med. 2009;15:200-205.

Singer-Englar T et al. Methanogens and H₂S producing bacteria guide distinct IBS subtypes. Am J Gastroenterol. 2022.

Landry AP et al. Hydrogen sulfide oxidation by sulfide quinone oxidoreductase. Chembiochem. 2021;22(6):949-960.

Verma A et al. Mitochondrial dysfunction in IBD alters intestinal epithelial metabolism of hepatic acylcarnitines. J Clin Invest. 2021;131(1):e137734.

Hartmann J et al. CD38 is expressed on inflammatory cells of the intestine. PLOS One. 2015;10(5):e0126007.

Pimentel M et al. Autoimmunity links vinculin to post-infectious functional bowel changes. Dig Dis Sci. 2015;60(5):1195-1205.

Pimentel M. DDW 2025 presentations on SIBO/IMO/ISO taxonomy and rifaximin + NAC combination therapy.