8.3 Helicobacter pylori Detection and GI Pathogenicity

8.3 Helicobacter pylori Detection and GI Pathogenicity

Quick Answer: Prove active H. pylori with urea breath test, stool antigen, biopsy urease, histology, or culture—not with IgG serology. Culture, when done, is microaerophilic at 35 °C and is not routine. Then map enteric disease to invasion versus toxin (Shiga, cholera toxin, C. difficile TcdA/TcdB, Salmonella T3SS) and correlate fecal leukocytes with multiplex GI PCR.

Outline II.F ends with detection methods for Helicobacter pylori and organism pathogenicity (etiology, transmission, virulence). H. pylori is not a Cary-Blair stool-culture organism. It lives in gastric mucus, not in the colon, and the laboratory question is active infection versus past exposure. The same section then asks you to explain why Shigella makes dysentery and V. cholerae makes rice-water stool, and why a multiplex PCR panel does not replace that thinking.

Helicobacter pylori: tests that prove now versus tests that prove ever

H. pylori is a curved Gram-negative rod, oxidase-positive, catalase-positive, and powerfully urease-positive. Urease hydrolyzes urea to ammonia and CO2, raising local pH so the organism survives gastric acid. That biochemistry is the principle of the urea breath test and the rapid urease (CLO) test.

Urea breath test. The patient drinks 13C- or 14C-labeled urea. If gastric H. pylori urease is present, labeled CO2 appears in breath. False negatives follow proton-pump inhibitors, antibiotics, and bismuth; stop PPI for about two weeks and antibiotics/bismuth for four weeks when the question is eradication. This is an active-infection test.

Stool antigen. Immunoassay for H. pylori antigen in feces also indicates current gastric infection and is the usual noninvasive test of eradication (often ≥4 weeks after therapy). It is not a stool culture and it does not mean the organism is an enteric diarrheal pathogen.

Serology does not prove active infection. IgG can remain positive for years after the organism is gone. Do not use serology to confirm eradication, to test children as a first-line active-disease test, or to explain current ulcer symptoms after treatment. A positive IgG means the patient met H. pylori at some time, not that organisms are in the stomach today.

Biopsy rapid urease (CLO or equivalent). A gastric antral biopsy is placed in a urea-containing well with a pH indicator. Urease-positive organisms turn the well alkaline (yellow to pink/red). Sensitivity falls if the patient is on PPI or if the biopsy misses the colonized patch; histology backs it up.

Histology shows curved organisms in mucus on Giemsa, silver (Warthin–Starry), or immunohistochemistry, plus gastritis. It is still the gold-standard tissue test in many endoscopy suites.

Culture is not routine. When ordered (usually for refractory infection and susceptibility), inoculate enriched chocolate or selective Helicobacter media from a gastric biopsy, incubate microaerophilic at 35 °C (not 42 °C), and hold up to 5–7 days. H. pylori is more fastidious and more temperature-sensitive than C. jejuni. Colonies are small, translucent, and oxidase/catalase/urease-positive. Stool is the wrong specimen for this culture.

Virulence that matters for gastric disease: urease for acid survival, flagella for mucus motility, VacA cytotoxin, and CagA (type IV secretion) associated with peptic ulcer, gastric adenocarcinoma, and MALT lymphoma. Transmission is person-to-person, often fecal–oral or oral–oral in childhood—not undercooked poultry.

Exam scenario. Dyspepsia, positive IgG, negative stool antigen: serology is leftover antibody, not active infection—do not treat on IgG alone. Second scenario: a bench incubates a gastric biopsy at 42 °C on Campy-BAP “because it is a campylobacter.” That plate is the wrong temperature and usually the wrong medium for H. pylori.

Pathogenicity: etiology, transmission, invasion versus toxin

Think in three layers the outline names: who gets sick and from where, how the organism gets in, and whether it invades mucosa or intoxicates from the lumen.

Fecal–oral, food, water, zoonoses. Shigella has a tiny inoculum and spreads person-to-person (day care, institutions) more than food. Salmonella is foodborne and zoonotic: poultry, eggs, produce, and reptiles. C. jejuni is poultry, unpasteurized milk, and puppies. Y. enterocolitica is pork and chitterlings. V. cholerae is fecally contaminated water; V. parahaemolyticus and V. vulnificus are seafood and seawater. Aeromonas and Plesiomonas are freshwater and seafood. STEC is ground beef, produce, and petting zoos (ruminants). C. difficile is spores in the healthcare and community environment after microbiome disruption, not a picnic-potato-salad story. H. pylori is gastric colonization, not acute food poisoning.

Invasion. Shigella enters colonic M cells, multiplies in cytoplasm, and spreads cell-to-cell—dysentery with blood, mucus, and fecal leukocytes. Salmonella uses type III secretion systems: SPI-1 injects effectors for epithelial invasion, SPI-2 for survival inside macrophages; gastroenteritis is ileocolitis, and typhoidal strains disseminate. C. jejuni invades ileum and colon and is linked to Guillain–Barré via molecular mimicry of GM1 ganglioside, a pathogenicity extra the exam may attach to Campylobacter, not to STEC. Y. enterocolitica invades Peyer’s patches (invasin, Yops via T3SS) and causes mesenteric adenitis. Enteroinvasive E. coli behaves like Shigella. Invasive disease is the setting in which a fecal leukocyte, lactoferrin, or calprotectin result is often positive.

Toxin. STEC Shiga toxins (Stx1/Stx2) inhibit protein synthesis on endothelial cells and drive hemorrhagic colitis and HUS—invasion is limited; leukocytes may be fewer than in shigellosis, but blood is common. V. cholerae cholera toxin ADP-ribosylates Gs, locks adenylate cyclase on, and dumps chloride into the lumen: voluminous watery stool without fecal leukocytes. ETEC heat-labile toxin is cholera-toxin-like; heat-stable toxin raises cGMP. C. difficile TcdA and TcdB glucosylate Rho GTPases, kill colonocytes, and produce colitis that can be leukocyte-rich and even pseudomembranous; binary toxin appears in epidemic NAP1/027 strains. Salmonella is not a cholera-toxin disease on this exam; if the stem says T3SS, think invasion.

Correlate fecal leukocytes and multiplex GI PCR

A methylene-blue wet mount or a lactoferrin immunoassay is a rapid invasion clue, not an organism name. Leukocytes plus blood point to Shigella, Salmonella, Campylobacter, Yersinia, EIEC, or C. difficile colitis. Watery, leukocyte-poor stool points to cholera, ETEC, or viral disease—and still needs STEC toxin testing when bloody or when HUS is in the room, because STEC is toxin-mediated.

Multiplex GI PCR panels detect many of the outline organisms in hours, including Salmonella, Shigella/EIEC, Campylobacter, Yersinia, Vibrio, STEC stx, and C. difficile toxin genes. Use them. Then remember their limits: they do not serotype, they do not yield an isolate for public health, they can stay positive after recovery, and C. difficile NAAT on formed stool is colonization. A panel positive for Salmonella still goes to HE/XLD so O and H typing can happen. A panel positive for STEC still needs SMAC plus toxin confirmation and referral. Correlate the PCR with stool consistency, fecal leukocytes, and epidemiology (oysters, travel, day care, antibiotics) or you will treat colonization and miss the virulence story the outline is actually testing.