H. pylori Biology and Pathogenesis
H. pylori is a gram-negative, microaerophilic, spiral-shaped bacterium that uniquely colonizes the human gastric mucosa — an environment considered hostile to bacterial life due to its extreme acidity (pH 1–3). H. pylori survives by producing urease, an enzyme that hydrolyzes urea (diffusing from gastric tissue) into ammonia and CO₂. The ammonia locally neutralizes gastric acid in the immediate microenvironment around the bacterium, creating a pH-buffered niche. Urease activity is so specific and abundant that it forms the basis of the most accurate non-invasive diagnostic tests (urea breath test, stool antigen).
Colonization Strategy
H. pylori uses multiple mechanisms to establish permanent colonization:
- Flagellar motility: Polar flagella allow H. pylori to corkscrew through viscous gastric mucus toward the mucus-epithelium interface, where pH is less acidic (~4–6)
- BabA adhesin: Binds Lewisᵇ blood group antigen on gastric epithelial cells — explains why blood group O individuals have higher H. pylori colonization density and higher peptic ulcer risk
- Immune evasion: H. pylori modifies its lipopolysaccharide (LPS) to avoid TLR4 activation; produces VacA vacuolating cytotoxin that suppresses T cell proliferation; induces regulatory T cells (Tregs) via CagA-dependent mechanisms — ensuring the inflammatory response is insufficient for clearance
The CagA Oncoprotein Mechanism
CagA (cytotoxin-associated gene A protein, 120–145 kDa) is the primary virulence factor linking H. pylori to gastric cancer. The cag pathogenicity island encodes a Type IV secretion system (T4SS) — a molecular syringe that physically injects CagA from the bacterium into the cytoplasm of the attached gastric epithelial cell. Once inside:
- CagA is phosphorylated by Src and Abl kinases at EPIYA motifs (Glu-Pro-Ile-Tyr-Ala sequences)
- Phospho-CagA binds and activates SHP-2 phosphatase (PTPN11), which normally activates RAS/ERK signaling — leading to uncontrolled proliferation
- CagA disrupts the PAR1b kinase/tight junction complex → loss of apical-basolateral cell polarity (the "hummingbird phenotype") — a hallmark of epithelial-to-mesenchymal transition (EMT)
- CagA also activates NF-κB → IL-8 secretion → neutrophil recruitment → chronic inflammatory damage
- Non-phosphorylated CagA disrupts E-cadherin/β-catenin complex → nuclear β-catenin accumulation → Wnt target gene upregulation (c-Myc, cyclin D1)
The end result of CagA-induced SHP-2 activation + polarity disruption + β-catenin signaling is a gastric epithelium that is proliferating abnormally, losing differentiation markers, and accumulating DNA damage — the precancerous cascade that progresses from superficial gastritis → atrophic gastritis → intestinal metaplasia → dysplasia → gastric adenocarcinoma (the Correa cascade, described in 1992).
Diagnosis: Why the Urea Breath Test Is Gold Standard
H. pylori diagnosis should distinguish active infection (treatment decision) from past exposure (serology). Available tests:
| Test | Sensitivity | Specificity | Active Infection | Notes |
|---|---|---|---|---|
| Urea breath test (UBT, ¹³C or ¹⁴C) | 95–97% | 96–98% | Yes | Gold standard for non-invasive diagnosis; requires 4-week washout from PPIs (falsely negative), 2-week washout from antibiotics; ¹³C is non-radioactive (preferred in pregnancy/children); patient swallows labeled urea tablet, exhales into bag, ¹³C-CO₂ measured by mass spectrometry |
| Stool antigen test (SAT) | 94% | 97% | Yes | Equivalent accuracy to UBT; monoclonal antibody tests (not polyclonal) preferred; also requires 2-week antibiotic and 4-week PPI washout; most accessible and lowest cost; FDA-cleared for diagnosis and test-of-cure |
| Serology (IgG antibody) | 85% | 79% | No (past exposure) | Remains positive for years after successful eradication — cannot distinguish active infection from cleared past infection; NOT recommended for diagnosis or test-of-cure per ACG guidelines; only useful in areas where UBT/SAT unavailable |
| Endoscopic biopsy (RUT + histology ± culture) | RUT: 93%; Histology: 93–96% | >95% | Yes | Rapid urease test (CLO test) gives result in 1–24h; histology adds cancer screening and gastritis staging; culture allows susceptibility testing (guides antibiotic choice); requires endoscopy — invasive and expensive; appropriate when endoscopy indicated for other reasons |
H. pylori Eradication: Current First-Line Regimens and Probiotic Adjuncts
- Bismuth quadruple therapy (PBMT) — preferred first-line in most high-resistance regions: Bismuth subsalicylate or subcitrate (525mg QID) + PPI (omeprazole 20–40mg BID or equivalent) + metronidazole (250–500mg QID) + tetracycline (500mg QID) for 10–14 days. Achieves >90% eradication even in clarithromycin-resistant strains. Available as Pylera (bismuth + metronidazole + tetracycline, 3 capsules QID + PPI — simplifies adherence). Side effects: dark stools (bismuth — harmless), nausea, metallic taste (metronidazole). Avoid alcohol during and 48h after metronidazole (disulfiram-like reaction).
- Clarithromycin-based triple therapy — only where local clarithromycin resistance <15%: PPI (standard or double dose) + clarithromycin 500mg BID + amoxicillin 1g BID x 14 days. Efficacy has declined to 70–85% in most regions. The ACG 2017 guidelines recommend obtaining local clarithromycin resistance data before prescribing — if >15% resistance is documented regionally, do not use. Culture-guided therapy (choosing antibiotics based on susceptibility testing from biopsy) is the emerging gold standard.
- Concomitant (non-bismuth quadruple) therapy — alternative in penicillin-allergic patients: PPI + clarithromycin 500mg BID + metronidazole 500mg BID + amoxicillin 1g BID x 14 days (or substitute levofloxacin 500mg QD for penicillin-allergic patients). Comparable to bismuth quadruple where metronidazole resistance is also low; effectiveness varies significantly by regional resistance patterns.
- Probiotic adjunct — start day 1 with antibiotics, continue 4 weeks: Lactobacillus reuteri DSM 17938 (BioGaia tablets — the most studied single strain), Saccharomyces boulardii CNCM I-745 (Florastor), or a multi-strain Lactobacillus combination all have RCT evidence for: (1) reducing antibiotic-associated diarrhea during H. pylori treatment by 40–60%; (2) improving eradication rates by 5–15%; (3) potentially suppressing H. pylori directly (L. reuteri produces reuterin, antimicrobial against H. pylori). Take probiotics at least 2 hours away from the antibiotic doses. Continue for 4 weeks post-eradication to support microbiome recovery.
- Test-of-cure — mandatory 4 weeks after completing therapy: UBT or stool antigen (not serology) at least 4 weeks after completing antibiotics AND at least 2 weeks after stopping PPIs. Re-testing is critically important — approximately 15–20% of first-line treatments fail, and early re-treatment with an alternative regimen before antibiotic resistance develops further improves long-term outcomes. Eradication failure should prompt culture-guided susceptibility testing before second-line therapy.
For H. pylori eradication support: Florastor (Saccharomyces boulardii CNCM I-745, 500mg x 2/day) is the best-evidenced option for preventing antibiotic-associated diarrhea AND improving eradication rates — Saccharomyces is a yeast, not a bacteria, and is unaffected by antibiotics. BioGaia (L. reuteri DSM 17938, 1–2 tablets/day) is the best-evidenced Lactobacillus strain. Take either option at least 2 hours away from antibiotic doses. Continue 4 weeks after completing antibiotics.