Pathway 1 — Direct epithelial irritation
When an antibiotic dissolves in the stomach and passes through the gut, the compound itself makes contact with the epithelial surface. Many antibiotic compounds are chemically irritating — disrupting the lipid membranes of epithelial cells, activating inflammatory signalling, and in some cases generating reactive oxygen species.
Macrolides activate motilin receptors, causing uncoordinated smooth muscle contractions that mechanically stress the gut wall alongside chemical irritation.
Metronidazole is directly cytotoxic to some epithelial cell populations.
Amoxicillin-clavulanate — clavulanic acid contributes disproportionately to mucosal irritation relative to amoxicillin alone.
The EGFR/ERK mucosal repair cycle replaces damaged surface epithelial cells within hours to days in a healthy stomach. When the course ends, the direct irritation stops and repair takes over — typically resolving within a week.
Pathway 2 — The SCFA deficit mechanism
This is the pathway most patients are not told about — and the one most responsible for gut problems that persist after antibiotics are finished.
Broad-spectrum antibiotics kill Bifidobacterium, Lactobacillus, Faecalibacterium prausnitzii, and other SCFA producers without distinguishing them from the pathogenic target. These populations can be reduced by 80–90% within days of starting a broad-spectrum course.
Day 1–3Without SCFA-producing bacteria, butyrate production in the colon collapses. Colonocytes that depend on butyrate for 70–90% of their energy supply are deprived of their primary fuel within 24–48 hours of bacterial population collapse.
Days 2–4Energy-deprived colonocytes cannot maintain the tight junction proteins — claudin, occludin, ZO-1 — that seal the spaces between cells. The intestinal barrier becomes "leaky" — permeable to bacterial products, antigens, and inflammatory triggers.
Days 3–7Luminal contents breaching the degraded epithelial barrier trigger mucosal immune activation — NF-kB signalling, cytokine release, and inflammatory cell recruitment. The gut becomes inflamed from the inside, independently of what is happening in the stomach. This inflammation persists until the barrier is restored.
Day 5 onwardsButyrate, derived from microbial fermentation of dietary fibre, is the primary energy source for colonocytes and a critical regulator of tight junction integrity. Its loss following antibiotic-induced dysbiosis compromises the intestinal barrier, increasing mucosal permeability and susceptibility to inflammatory activation.
The H. pylori compounding — pathway 3
In H. pylori-positive patients on antibiotics — most commonly during eradication therapy — a third pathway is active simultaneously. H. pylori has already activated NF-kB in the gastric mucosa and suppressed EGFR/ERK repair. The antibiotic's direct epithelial irritation compounds with an already-inflamed gastric lining, and the microbiome collapse from eradication therapy adds the SCFA deficit mechanism on top. All three pathways operate simultaneously — which explains the severity of eradication therapy side effects.
What long-term mucosal repair requires
Microbiome recovery (probiotics + prebiotic fibre): reseeds SCFA-producing commensals (Lactobacillus rhamnosus GG, Saccharomyces boulardii) and provides the substrate they need to rebuild butyrate production. Probiotics must be taken 2 hours apart from antibiotics to survive the course.
Mucosal repair support (NF-kB inhibition + EGFR/ERK activation): addresses the inflammation from gram-negative LPS expansion (quercetin inhibits NF-kB) and directly activates mucosal cell regeneration in the gastric and intestinal lining (glabridin activates EGFR/ERK). Probiotics cannot do this — they rebuild the community, not the environment.
References
- Thursby E, Juge N. Introduction to the human gut microbiota. Biochemical Journal. 2017;474(11):1823–1836. PMID 28512250. Butyrate's role as primary colonocyte energy source and regulator of tight junction integrity — the foundational mechanism of the SCFA deficit pathway.
- Sokol H et al. Faecalibacterium prausnitzii is an anti-inflammatory commensal bacterium. PNAS. 2008;105(43):16731–16736. PMID 19066305. Establishes Faecalibacterium prausnitzii as the dominant butyrate-producing species whose antibiotic-induced depletion drives the SCFA deficit mechanism.
- Crowe SE. Helicobacter pylori infection. New England Journal of Medicine. 2019;380:1158–1165. PMID 30699316. H. pylori NF-kB and EGFR/ERK mechanism — the basis for Pathway 3 in H. pylori patients on antibiotics.