Members of the genus Helicobacter are usually spiral, curved, or fusiform rod-shaped Gram-negative bacteria. Helicobacter species have been isolated from the gastrointestinal and hepatobiliary tract of many different mammalian hosts, including humans, dogs, cats, pigs, cattle, and other domestic and wild animals. The various helicobacters can be divides into two groups: Helicobacter species that primarily colonize the stomach (gastric helicobacters), and those that colonize the intestines (enterohepatic helicobacters). Humans are the primary host-reservoir for H. pylori, which is a spiral-shaped, Gram-negative, catalase- and oxidase-positive, and urease positive rod. H. pylori is associated with antral gastritis, duodenal (peptic) ulcer disease, gastric ulcers, gastric adeno carcinoma, and gastric mucosa-associated lymphoid tissue (MALT) lymphomas.

Morphology and Identification

A. Typical Organisms Helicobacter species, including H. pylori, have many characteristics in common with campylobacters. Helicobacter species are motile and have single and/or multiple monopolar flagella that are typically sheathed and can vary greatly in their flagellum morphology.

 B. Culture While H. pylori can be readily isolated from gastric biopsy specimens, culture sensitivity may be limited by several factors, including delayed specimen transport and processing, prior antimicrobial therapy, or contamination with other mucosal bacteria. Special transport media (eg, Stuart’s trans port medium) should be used to main the organisms’ viability when transport to the laboratory is anticipated to exceed 2 hours. H. pylori usually grows within 3–6 days when incubated at 37°C in a microaerophilic and humid atmosphere; however, incubation of up to 14 days may be necessary before resulting the culture as negative. To achieve a higher yield for recovery of the organism, the biopsy specimen may be homogenized prior to streaking onto the agar plate. The agar media for primary isolation include enriched agar media supplemented with blood and/or blood products (eg, chocolate agar) or antibiotic-containing media such as Skirrow’s medium, in order to suppress overgrowth by other competing bacterial flora. The colonies have varying appearance on blood agar ranging from gray to translucent and are 1–2 mm in diameter.

 C. Growth Characteristics H. pylori is oxidase positive and catalase positive, and has a characteristic Gram-stain morphology; the organism is motile, and is a strong producer of urease.

Pathogenesis and Pathology

 H. pylori is able to survive in the acidic environment of the stomach and ultimately establish lifelong colonization of the gastric mucosa in the absence of antimicrobial treatment. While H. pylori grows optimally at a pH of 6.0–7.0, it would be killed or not grow at the pH within the gastric lumen (pH 1–3). Several factors contribute to the organism’s ability to overcome the acidic environment of the stomach, contributing to colonization, inflammation, changes in gastric acid production, and tissue destruction. Gastric mucus is relatively impermeable to acid and has a strong buffering capacity. On the lumen side of the mucus, the pH is low (1.0–3.0); on the epithelial side, the pH is about 5.0–7.0. After entering the stomach, H. pylori utilizes its urease activity to neutralize the gastric acid; intracellular urease activity as well as urease located on the bacterial cell surface allow for the breakdown of urea into ammonia and CO₂ ; NH₃ is converted to ammonium (NH₄ ⁺) and extruded from the bacterial cell leading to neutralization of the gastric acid. Flagella-mediated motility then allows the organisms, protected from the gastric acid, to move through the gastric mucus toward the epithelium. H. pylori is found deep in the mucous layer near the epithelial surface where a near physiologic pH is present. H. pylori then colonizes the gastric-type but not intestinal type epithelial cells, and releases several effector proteins and toxins, including adhesion factors, neutrophil-activating protein A, a heat shock protein, and various cytotoxins. Several outer membrane proteins of H. pylori have been identified to facilitate cell adhesion; these proteins include BabA (blood antigen-binding protein A), which binds to fucosylated Lewis b receptor on gastric epithelial cells, and SabA, which binds to sialyl Lewis X receptors. H. pylori doesnot appear to invade the gastric mucosa, but rather release various toxins that ultimately exert tissue damage; among these are mucinase, phospholipase, neutrophil-activating protein A, heat shock protein 60, cytotoxin-associated gene A protein (CagA), and vacuolating cytotoxin A (VacA). CagA is secreted by H. pylori and then translocated into the gastric epithelial cells via a type IV secretion system; it interferes with the epithelial cells’ cytoskeletal structure and induces IL-8 production, leading to neutrophil attraction. VacA affects the balance between cell death and proliferation, and also acts as an activator of IL-8 mediated acute inflammation. In addition to these two factors, CagA and VacA, that are responsible for the extensive tissue inflammation and damage, H. pylori organisms may also secrete urease, which acts as a chemoattractant and activator of host phagocytic and inflammatory cells. Several studies indicate that CagA is directly associated with acute gastritis, gastric ulcer development, and gastric carcinoma. While the prevalence of CagA-positive H. pylori organisms is about 60% in western countries, the prevalence of CagA positive H. pylori in Southeast Asian countries is close to 90%, perhaps accounting for the relatively higher incidence of gastric cancer in this part of the world.

In human volunteers, ingestion of H. pylori resulted in development of gastritis and hypochlorhydria. There is a strong association between the presence of H. pylori infection and peptic ulcer disease as well as duodenal ulceration. Antimicrobial therapy results in clearing of H. pylori and improvement of gastritis and duodenal ulcer disease.

Histologically, gastritis is characterized by acute and chronic inflammation. Polymorphonuclear and mono nuclear cell infiltrates are seen within the epithelium and lamina propria. Vacuoles within cells are often pronounced. Destruction of the epithelium is common, and glandular atrophy may occur. Thus, H. pylori is a major risk factor for gastric cancer. H. pylori infection is a known independent risk factor for the development of atrophic gastritis, gastric ulcer disease, gastric adenocarcinomas, and gastric mucosa associated lymphoid tissue (MALT) lymphomas .

Clinical Findings

 Following acquisition of the H. pylori, acute infection typically yields an upper gastrointestinal illness (“food poisoning”) with nausea and pain; vomiting and fever may also be present. The acute symptoms may last for less than 1 week or as long as 2 weeks. After the acquisition of the organisms and the initial acute stage of the infection, colonization with H. pylori occurs in many patients. Such colonization persists for years, perhaps decades, or even a lifetime. However, not every exposure to H. pylori leads to persistent colonization; lack of the organism’s adaptation to a particular host or incidentally concomitant antibiotic therapy may perhaps prevent persistent colonization. However, in the absence of medication-induced ulceration, approximately 90% of patients with duodenal ulcers have indeed a H. pylori infection. Colonization with H. pylori has been shown to be present in 50–80% of patients with benign gastric ulceration. Finally, long-lasting H. pylori colonization, which is associated with chronic gastritis and subsequent development of intestinal metaplasia and atrophic gastritis, is a well-known risk factor for the development of gastric adenocarcinoma.

Diagnostic Laboratory Tests

A. Specimens

Gastric biopsy specimens can be used for histologic examination or minced in saline and used for culture. Blood is collected for determination of serum antibodies. Stool samples may be collected for H. pylori antigen detection. Diagnostic testing methods are summarized in Table 1.

Table1. Diagnostic Testing Methods for H. pylori

B. Smears

The diagnosis of gastritis and H. pylori infection can be made histologically; this approach is generally more sensitive than culture. A gastroscopy procedure with biopsy is required. Routine stains (eg, hematoxylin & eosin stain) demonstrate acute/chronic gastritis, and Giemsa or special stains (eg, silver stains or immunohistochemical stains) can show the curved or spiral-shaped organisms.

 C. Culture

 Since H. pylori organisms adhere to the gastric mucosa, the bacteria cannot be recovered from stool specimens like other gastrointestinal pathogens. As described above, culture is usually performed when patients are not responding to treatment, and there is a need to perform antimicrobial susceptibility testing. Tissue for culture is obtained by endoscopy and biopsy of the gastric mucosa.

 D. Antibodies

Several assays have been developed to detect serum antibodies specific for H. pylori. While testing for IgG serum anti bodies against H. pylori is useful to confirm the exposure to the organism, either for epidemiologic purposes or for the evaluation of a symptomatic patient, the antibody titers do not typically correlate with the severity of the disease. Furthermore, IgM antibodies disappear rapidly during the initial course of an acute infection, and are of little diagnostic value. The relevance of IgA testing remains controversial, and both IgA and IgG serum antibodies persist even if the H. pylori infection is eradicated. The role of antibody testing in differentiating active H. pylori infection from past infection and/or completion of therapy is therefore limited.

E. Other, Special Test Methods

Other tests for diagnosis include histologic examination of gastric biopsy specimens, detection of urease production, and detection of H. pylori antigen. The latter is performed on stool specimens. Histopathological examination of gastric biopsy specimens is widely used for the diagnosis of H. pylori in patients undergoing endoscopy with biopsy for evaluation. While the standard hematoxylin and eosin staining method is insufficient for visualizing the organisms, silver staining methods (eg, Warthin-Starry or GMS) or immunohistochemical stains specifically directed against H. pylori antigens are used to detect the organisms. The histological examination of gastric biopsy specimens has a sensitivity and specificity of 95–100% for detection of H. pylori. At the present time, nucleic acid amplification tests (PCR) for H. pylori and other Helicobacter species on various clinical specimens are restricted to research use only. Rapid tests to detect urease activity are widely used for presumptive identification of H. pylori in specimens. Gastric biopsy material can be placed onto a urea-containing medium with a color indicator. If H. pylori is present, the urease rapidly splits the urea (1–2 hours), and the resulting shift in pH yields a color change in the medium. In addition, the production and activity of urease can also be detected by in vivo tests, such as the urea breath test (UBT). In urea breath tests, 13C- or 14C-labeled urea is ingested by the patient. If H. pylori is present, the urease activity generates 13C- or 14C-labeled CO2 that then can be detected in the patient’s exhaled breath. The sensitivity and specificity of the UBT range between 94% and 98%.

Detection of H. pylori antigen in stool specimens, using enzyme-linked immunosorbent assay tests, has been shown to have great value in diagnosing active H. pylori infection. Furthermore, the stool antigen test is also appropriate as a test of cure for patients with known H. pylori infection who have completed their course of antimicrobial therapy.

 Immunity

 Patients infected with H. pylori quickly develop an IgM anti body response to the infection. Subsequently, IgG and IgA are produced, and these persist, both systemically and at the mucosa, in high titer in chronically infected persons, while the IgM antibodies decline and eventually disappear. Early antimicrobial treatment of H. pylori infection has shown to blunt the antibody response; such patients are thought to be subject to repeat infection.

Treatment

Triple therapy with a proton-pump inhibitor (PPI; standard dose, twice daily), plus amoxicillin (1 g twice daily), plus clarithromycin (500 mg twice daily) is the recommended regimen for initial therapy for 7–14 days. Alternatively, a quadruple therapy with a PPI, plus metronidazole (250 mg four times daily), plus tetracycline (500 mg, four times daily), plus bismuth (dose-dependent on preparation) for 10–14 days may be prescribed. Such initial therapy regimens given for 14 days typically eradicate H. pylori infection in 70–95% of patients. Proton-pump inhibitors (PPIs) directly inhibit H. pylori and appear to be potent urease inhibitors. Tailored antibiotic therapies may be needed based on recurrent or persistent infections due to antimicrobial-resistant strains of H. pylori. Finally, various medications given for 4–6 weeks for suppression of gastric acid production and following initial therapy have shown to enhance ulcer healing.

Epidemiology and Control

H. pylori has been isolated from humans, worldwide, and it is likely that humans are the primary, if not sole reservoir for the organism. While the exact mode of transmission from person-to-person remains somewhat unclear, epidemiologic studies provide strong evidence to support oral–oral and/or fecal–oral transmission of the organism. In most populations, H. pylori seems to be acquired during early childhood. Once acquired, it is likely that colonization persists through out life, unless the individual is treated with appropriate antibiotics. The prevalence of H. pylori infection differs noticeably between developing and developed countries, and a lower socioeconomic status has been shown to be a risk factor for acquisition of H. pylori. The highest prevalence has been reported from developing countries, especially in Southeast Asia, where most children are infected by the age of 10, and prevalence remains high (up to 90%) among adults of all age groups. In developed countries, such as the United States, a significantly lower overall prevalence (approximately 40%) among (older) adults has been observed, likely due to improved hygiene and available treatment for people with active infection. In the United States, when compared to other countries, significantly fewer children and young adults are infected with H. pylori; the bacteria are present on the gastric mucosa of fewer than 20% of persons younger than 30 years old, but then increases in prevalence to 40–60% of people age 60 years and older, including people who are asymptomatic. The rationale for oral-to-oral transmission of H. pylori has been based on studies that demonstrated the ability of detecting viable organisms in regurgitated gastric content; it seems therefore likely that H. pylori can temporarily colonize the oral cavity. Person-to-person transmission of H. pylori is furthermore supported by studies describing intrafamilial clustering of infection. Fecal–oral transmission of H. pylori appears to be more likely in developing countries, where contaminated lack of sanitation and contaminated water sup plies may be a more significant risk factor for acquiring the infection. On rare occasion, transmission of H. pylori from person-to-person via improperly cleaned endoscopes has also been described.

مواضيع ذات صلة

Fermentation Media

Coagulase Test

البرتونيلة Bartonella

الكوكسيلة البورنيتية Coxiella burnetii

الريكتسية التيفية Rickettsia typhi

الريكتسية البروفاسيكية Rickettsia prowazekii

الركيستيا الريكيتسية R.rickettii (حمى جبال روكي المنقطة)

الركيتسيا Rickettsia والشرقية Orientia

الكلاميدوفيلا الرئوية Chlamydophila pneumonia

الكلاميدوفيلا الببغائية Chlamydophila psittaci

الكلاميديا التراخومية Chlamydia trachomatis

Clostridium perfringens