Helicobacter pylori infection: past, present and future

The discovery of Helicobacter pylori (H. pylori) by Warren and Marshall in 1982 was preceded by nearly a hundred year of inconspicuous publications in regard to spiral bacteria, achlorhydria, gastritis, gastric urease, and antimicrobial therapy for peptic ulcers. The infection has now been implicated in the etiopathogenesis of chronic gastritis, peptic ulcer disease (PUD), gastric carcinoma, and gastric mucosal associated lymphoid tissue (MALT) lymphoma. The understanding of the etiopathogenesis of dyspepsia and the approach to its management continues to evolve such that PUD and MALT lymphoma are now being considered as infectious diseases in which elimination of the causative agent cures the conditions. Various diagnostic tests with good diagnostic accuracies have been developed and effective multiple antimicrobial therapies are now available for the eradication of the infection. Despite the substantial progress made, there still exits a considerable gap to be filled. A significant number of information generated from studying the bacterial characteristics and host response to the infection has not yet been translated into clinical practice. A major challenge is the absence of a specific antibiotic monotherapy for effective treatment of the infection. Pan African Medical Journal. 2016; 23:216 doi:10.11604/pamj.2016.23.216.8852 This article is available online at: http://www.panafrican-med-journal.com/content/article/23/216/full/ © Abiodun Christopher Jemilohun et al. The Pan African Medical Journal ISSN 1937-8688. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Pan African Medical Journal – ISSN: 19378688 (www.panafrican-med-journal.com) Published in partnership with the African Field Epidemiology Network (AFENET). (www.afenet.net) Review Open Access


Introduction
It has been abundantly demonstrated that H. pylori plays a major role in several upper gastrointestinal diseases which present as dyspepsia since its discovery in the early 1980s by Warren and Marshall [1,2]. Helicobacter pylori is usually found under the mucus layer in the gastric pits in close apposition to gastric epithelial cells where it causes chronic active gastritis [3]. It is a major etiological factor for PUD, gastric carcinoma, and gastric MALT lymphoma [1,4]. This discovery has generated significant interest in research on the organism with attendant continual evolution of the approach to investigation and treatment of patients with dyspepsia. Prior to the discovery of H. pylori the hyperacidity theory held sway in the pathogenesis of PUD. Various diagnostic tests with good diagnostic accuracies have been developed and effective multiple antimicrobial therapies are now available for the eradication of the infection [2,5].
Peptic ulcer disease and MALT lymphoma are now being considered as infectious diseases in which elimination of the causative agent cures the conditions because of the current level of understanding of the organism [3].
Despite the substantial progress made, there still exits a considerable gap to be filled. Some important information generated from studying the bacterial characteristics and host response have not yet been translated into clinical practice. A major challenge is the absence of a specific antibiotic monotherapy for effective treatment of the infection. This review appraises H. pylori in regard to the historical perspective, the current scientific understanding of the organism on its various aspects that are relevant to the clinician and the future direction in treatment.

Methods
Relevant literatures on the subjects in texts books and scientific journals were reviewed. Extensive internet literature search for both original research and review articles in biomedical databases was made through Google scholar, PubMed, HINARI and Ovid. Keywords employed were "Helicobacter pylori", "Dyspepsia", "Peptic ulcer disease" "Gastric cancer", "Epidemiology" and "Management". The bibliographies of the articles on hand were used to find other references. An electronic reference manager (Mendeley) was used to store the articles. Of the 230 articles reviewed,82 were cited in the final draft of the article.

Current status of knowledge Historical perspective (Historical timeline)
The discovery of H. pylori, by Warren and Marshall, was preceded by nearly a hundred year of inconspicuous publications relating to spiral bacteria, achlorhydria, gastritis, gastric urease, and antimicrobial therapy for ulcers [6]. Investigation of gastric bacteria properly began in the latter half of the 19th century when microscope resolution had sufficiently advanced [7].
Bottcher and Letulle firstly hypothesized that bacteria caused ulcer disease in 1875, after they discovered bacteria in the floor and margins of gastric ulcers. In 1889 Walery Jaworski described spiral organisms (Vibrio rugula) in gastric washings. He suggested that these organisms might be implicated in causation of gastric disease.
Similar spiral organisms were found in stomach of humans and other species by several scientists between then and the 20th century. For instance, in 1893 Bizzozero noted spirochetes in the gastric mucosa of dogs, which were named H. bizzozeroni. Kasai and Kobayashi in 1920 isolated spirochetes in cats and transmitted them to rabbits to produce ulcers [7].
Warren in 1979 identified Campylobacter pylori as the putative causative agent of human gastritis [7,8]. Culture of the organism (H. pylori) was elusive until 1982 when it was obtained by Barry Marshall [6]. Earlier attempts to culture the organism proved abortive because incubation was usually limited to 48 hours.
Success at culture was incidental, as one of them spanned a holiday period and hence, lasted for 5 days, thereby yielding a growth.

Epidemiology and transmission
Helicobacter pylori infection occurs globally, but the prevalence differs among countries and population clusters within the same country [1]. The prevalence of the infection is associated with age, socioeconomic class, and country of origin [13]. Prevalence rates ranging from 20-50% are reported in the adult populations of the developed world but the prevalence is much more in the developing countries with prevalence as high as 90% in some countries [1,4].
Higher prevalence exists in regions of low socioeconomic and poor sanitary conditions, and in rural as contrasted to urban areas. The socioeconomic status of the family during childhood appears to be the major marker of infection [14]. There is an age-related increase in prevalence in the developed countries which is a reflection of birth cohort effect [14]. The general belief is that infection takes place mostly in childhood and the rate of acquisition has reduced with improved sanitary condition and probably antibiotic use among children in the developed world. Genetic susceptibility to infection has been demonstrated by studies that showed a higher concordance rate of the infection among monozygotic twins reared apart or together than in age-matched dizygotic twins [15].
H. pylori has been found in water; stomach of animals like cat and sheep; and milk of goat, sheep and cow [16]. Despite these, available evidence suggests that humans are the primary reservoir and that H. pylori in animals represents an anthroponosis [17]. Helicobacter pylori has been found in saliva and dental plaques of humans [18,19]. Some studies have suggested that dental plaques serve as reservoir for both person to person transmission of the organism and re-infection after successful eradication of organism [19]. The primary mode of transmission is person to person and clusters are found in families. This can be through oraloral, feco-oral, and gastro-oral (through gastric secretions, vomitus, and improperly disinfected endoscopes). The infection could also be transmitted from water [20].

Bacteriology and pathogenesis
Helicobacter pylori is a microaerophilic slow-growing gram negative spiral organism. It is 0.5 to 8.0µm wide and has multiple ensheathed flagella at one end [21]. It has the capability of dual existence in bacillary and coccoid forms [22]. The bacillary form is motile while the coccoid form is non-motile. The genome of the organism codes for about 1,500 proteins [1]. Bacterial etiology of dyspepsia has been disputed in the past because of the knowledge that the stomach is unfavorable to bacterial growth due to its high acid content but H. pylori is able to overcome this by means of virulence factors which enhance colonization of the gastric epithelium and induction of tissue damage.
The actual outcome of H. pylori infection (gastritis, PUD, gastric MALT lymphoma or gastric cancer) is determined by a complex interplay between bacterial, host and environmental factors.

Bacterial factors
These include factors that enhance mucosal colonization and factors that mediate tissue injury.

Colonization factors
Flagella: The possession of spiral shaped, unipolar, sheathed flagella [23] allows the organism to move rapidly from the lumen of the stomach, where the pH is low, through the mucus layer to an area where pH is near neutral to permit optimal growth. Urease: H. pylori has a great capacity for urease production, probably more than almost all other bacterial species. Urease hydrolyses urea to produce ammonia (NH3) and carbon dioxide (CO2). The presence of NH3reduces the acidity of the stomach; which may be necessary for providing a congenial environment for H. pylori. Ure1, a pH-gated channel helps to regulate the production of urea [1]. Possession of urease however, may not be Page number not for citation purposes 4 absolutely essential for colonization because urease negative H. pylori have been cultured from patients with duodenal ulcer.
Adherence factors: H. pylori has tissue tropism for the gastric epithelium. It possesses fibrillar adhesins, located on its surface which binds closely to the carbohydrate receptors on the mucosal cell leading to the formation of an adherence pedestal [1]. The best-characterized of these adhesins is BabA, which is a 78-kD outer-membrane protein that binds to the fucosylated Lewis B blood group antigen. BabA is relevant in H. pylori associated disease and may influence disease severity, although the results of several studies are contradictory [1]. This property prevents the organism from being shed during cell and mucus turnover. can be used to detect VacA-producing H. pylori strains. VacA is an exotoxin, which inserts itself into the epithelial cell membrane to form a hexameric anion-selective, voltage-dependent channel through which bicarbonate and organic anions can be released [26].
It also acts on the host mitochondrial membrane to induce apoptosis leading to release of cytochrome c from the intermembrane space [27].

Cytotoxin-Associated Antigen (CagA):
It is a highly antigenic protein encoded by the cagA gene that is part of the cag pathogenicity island (Cag PaI). The presence of Cag PaI is associated with a more prominent tissue inflammatory response than is seen with strains lacking it. This increase in inflammation is associated with an increased risk of developing a symptomatic outcome of the infection, especially PUD and gastric adenocarcinoma [28]. Antibodies to CagA can be used to detect CagA-producing H. pylori strains. The cag PaI encodes a type IV secretory apparatus that injects CagA into mammalian cells [29] where it triggers cytokine production. Cag PaI-positive H. pylori also induces apoptosis via the mitochondrial pathway [27]. Apoptosis of epithelial cells compromises epithelial barrier which protect the epithelium against luminal acid and pepsin.
Outer Membrane Inflammatory Protein(OipA): It is possessed by most strains with CagA. It acts synergistically with CagA to produce a more intense inflammatory response than would have otherwise occurred in either [30].
Heat shock proteins: These are highly antigenic heat shock proteins known as HspA and HspB. Their role in the pathogenesis of the infection is still not fully known. It has been observed that, even in the absence of VacA and CagA, H. pylori can sensitize human gastric epithelial cells and enhance susceptibility to tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) mediated apoptosis [31]. The infection chronicity is sustained through suppression of H. pylori specific memory CD4+ T-cell responses by antigen specific CD4+ CD25 high regulatory T cells [32].

Host response
H. pylori does not invade the gastric mucosa but its attachment to the surface mucosal epithelial cells sets off a remarkable cellular and humorous host immune responses. It provokes a chronic inflammatory response involving recruitment of neutrophils, followed by T and B lymphocytes, plasma cells and macrophages, which altogether cause gastric epithelial damage by their activities [33]. Human antibody response to H. pylori lipopolysaccharide (LPS) Pro-inflammatory polymorphisms of the interleukin-1β gene favor the development of stomach body predominant gastritis that is associated with hypochlorhydria, gastric atrophy, and gastric adenocarcinoma. Absence of these pro-inflammatory polymorphisms leads to development of H. pylori associated antral predominant gastritis which is associated with a normal to high level of acid secretion [1].
Disease spectrum It has been demonstrated that H. pylori is involved in the pathogenesis of several diseases [1,39]. These include: Gastritis: This could occur in form of either acute or chronic gastritis. The inflammatory mechanisms described above are some of the ways by which H. pylori causes gastritis. Acute infection is typically manifested as a transient mild illness characterized by epigastric pain, nausea, histological finding of neutrophilic gastritis and a transient hypochlorhydria [4].It is not known how often acute infection clears spontaneously. However, studies in children suggest that spontaneous loss of infection may be common [15]. Acute H. pylori infection is diagnosed by the presence of a positive UBT and negative IgG anti-H. pylori antibodies [4]. Chronic gastritis often presents in form of chronic active, non-atrophic superficial antral gastritis, with a picture of focal epithelial cell damage [4]. This is usually asymptomatic, although it may be associated with PUD. Occlusion of mucosal end-arteries due to impaired fibrinolytic activity may contribute to the focal nature of PUD [41]. NSAID ingestion also increases the chances of developing GU.
Patients with DU have been known, before the discovery of H pylori, tosecrete about twice as much acid as controls because they have twice as many parietal cells [42]. Acid hypersecretion in DU is virtually always due to H. pylori infection because secretion returns to normal after the infection is eradicated. The predominantly antral gastritis in DU diminishes the number of somatostatin-producing cells in the antrum. This leads to a reduction in production of somatostatin, reduced somatostatin-mediated inhibition of gastrin release from the G cells, and eventual development of hypergastrinemia [40].The increased acid secretion may, on its own, increase the risk of duodenal ulceration or may induce gastric metaplasia in the duodenum, which becomes colonized by H. pylori, then inflamed (duodenitis), and finally ulcerated [42]. Successful eradication of H. pylori leads to PUD healing and less frequent recurrence of the ulcer [43].
Gastric adenocarcinoma: H. pylori has been implicated as the strongest risk factor in the pathogenesis of gastric adenocarcinoma, Page number not for citation purposes 6 especially the distal type [1,44] and has thus, been classified as a class I (or definite) carcinogen by the WHO [9]. The pathogenesis of gastric cancer includes a sequence of events that begins with H. pylori-induced chronic superficial gastritis, progressing towards atrophic gastritis, intestinal metaplasia, dysplasia and eventually gastric cancer. This sequence takes decades to complete [9,44].
Several aspects of the inflammatory milieu that have been implicated as carcinogens include: increased oxidative stress and the formation of oxygen-free radicals leading to DNA damage, increased pro-inflammatory cytokine production such as IL-1β and TNF which stimulate greater cell turnover and reduced apoptosis, and potential for faulty or incomplete DNA repair [44].
Bacterial virulence has been shown to be an important factor in carcinogenesis. For instance, patients infected with CagA-positive strains have been shown to have a higher risk of developing gastric carcinoma than those infected with CagA-negative H. pylori strains [9,44].
H. pylori-infected cells also express some factors similar to those commonly implicated in carcinogenesis e.g. matrix metalloproteinase-7 (MMP-7) which has been found to be upregulated in colorectal cancer [45]. MMP-7 is important in the normal and pathological remodelling of epithelial-matrix interactions and is up-regulated in gastric cancer too. It plays an important role in promoting tissue invasion and metastasis of cancer cells. This upregulation is also dependent on the cag-PaI [45].
Although H. pylori is now thought to account for 80% or more of gastric cancers, it is noteworthy that only 3% of infected patients progress to gastric cancer [9]. This suggests that H. pylori infection only on its own is generally not sufficient to cause gastric carcinoma [46]. A combination of bacterial factors, environmental insults, the host immune response and other genetic factors is responsible.

MALT lymphoma:
The molecular pathogenesis of MALT lymphoma is incompletely understood but seems to also involve strain- via impairment of p53-dependent apoptosis [48]. In keeping with this, eradication of H. pylori "cures" the lymphoma by removing antigenic stimulus for T cells [3]. , others did not show any association [50].

Extra-gastroduodenal diseases:
H. pylori has also been suggested to be causally related to several extra-gastroduodenal diseases [51]. These associations are generally weak because they were not obtained from randomized controlled studies [52]. A list of such diseases is shown in Table 1.

Diagnosis
In view of the importance of H. pylori in the etiopathogenesis of major gastrointestinal and extra-gastrointestinal diseases, it is pertinent to look into cost-effective and reliable diagnostic tests for early detection and therapy. Helicobater pylori diagnostic tests can be broadly classified into invasive and non-invasive tests [2,5].
Invasive tests require endoscopic gastro-duodenal biopsy samples while the non-invasive tests do not [2,5].
Despite the good sensitivity and specificity of most of the commonly used tests, determination of gold standard has been difficult because none of them is perfect. A combination of at least two tests is commonly used as gold standard, though some researchers now use UBT as their gold standard [53]. It is not uncommon to use at least one invasive test whenever two tests are used as gold standard [54].

Invasive tests
Histology: Biopsies are obtained from the gastric antrum and corpus [55]. Multiple levels of each biopsy are routinely stained with haematoxylin and eosin (H&E) and with a special stain such as  [57].

Fluorescent in situ hybridization (FISH):
This is another molecular test for diagnosing H. pylori that is particularly useful in detection of H. pylori clarithromycin resistance/sensitivity [58].

Non-invasive tests
Serology: chronic H. pylori infection elicits a circulating antibody response that can be quantitatively measured by serological assay technique like enzyme-linked immunosorbent assay (ELISA) [59].
Though tests for IgG, IgA and IgM antibodies can be done, only IgG antibody test is reliable. It involves the use of serum or plasma, and lately tests on whole blood (in office test) [57]. Microwells coated with H. pylori antigen are exposed to H. pylori antibody (in serum, plasma or whole blood) in the presence of an indicator. The color change resulting from the antigen-antibody reaction is read visually or with the use of a spectrophotometer. Other serological methods that can be used include immunoblot, flow microsphere immunofluorescence and chromatographic tests.
Because of its easy availability, affordability, and simplicity, it is commonly used in prevalence studies of H. pylori. Its major drawback is its poor discriminatory power between current infection and previous exposure, since it may still be positive several months after H. pylori eradication. It is therefore generally not useful in confirming cure after antimicrobial therapy but it is useful for the initial diagnosis of H. pylori infection and epidemiological surveys [59].
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Urea breath test (UBT): This is an indirect method of detecting
the presence of H. pylori in the stomach premised on the ability of H. pylori to produce the urease enzyme. Urea labeled with either 13 C or 14 C is ingested by the patient [60]. If urease is present in the stomach as a result of H. pylori infection, labeled CO 2 will be split off and absorbed into the circulation, where its presence can be determined by analysis of expired breath by means of a spectrometer. The result is expressed in delta/mil. 13 C UBT is preferred to 14 C UBT especially in children and pregnant women because it is stable and non-radioactive [61].
UBT is now being considered as the gold standard by some researchers [61]. It is the non-invasive test with the highest sensitivity and specificity (>95%) and is the preferred means of

Stool antigen test (SAT):
The test is based on the detection of H.
pylori antigen in the stool. Helicobacter pylori adhering to gastric epithelium in infected persons appear in their stool as a consequence of the normal shedding of the epithelium. This means that the test is a direct test of active infection which gives it an advantage over serology. It is an enzyme immunoassay test which is available in both polyclonal and monoclonal forms. The monoclonal immunoassay is newer and more sensitive and specific than the polyclonal assay [58] and may be considered as an alternative to UBT in the initial diagnosis of patients with dyspepsia who do not require immediate endoscopy [62]. SAT is simple and relatively cheap. It can be carried out in most routine laboratories.
It is slightly less reliable when used soon after the end of H.
pylori eradication therapy. It is now generally recommended to wait for about 12 weeks to reliably confirm eradication [58].  [9,64]. The accuracy of some of the commonly used Helicobacter pylori diagnostic methods is depicted in Table 2.

Treatment
It is appropriate for all patients with dyspepsia who are positive for H. pylori to undergo H. pylori eradication therapy because of the strong association between H. pylori and the diseases manifesting clinically as dyspepsia [1][2][3]. This is particularly important because H. pylori eradication has been associated with significant reduction in the rate of recurrence of PUD and cure of MALT lymphoma. The specific recommendations for treatment include patients with PUD and MALT lymphoma; patients with atrophic gastritis; first degree relatives of patients with gastric cancer; patients with unexplained iron deficiency anemia; and patients with chronic idiopathic thrombocytopenic purpura [2,5].
The fact that no single agent is sufficient in eradicating the H.
pylori is a major challenge [1,2,4,5]. Because failure rate is above 20% in most commonly used first-line therapies [65], there is often the need for a second line therapy.
The choice of a second-line treatment is contingent upon the initial treatment. Most authorities agree that culture is not required after a first line eradication failure [2,66]. If a metronidazole-based treatment was used a clarithromycin-based regimen (or at least a metronidazole-free regimen) should be used thereafter, and vice versa [67]. This recommendation is grounded on the observation that acquired bacterial resistance to metronidazole or clarithromycin is often a result of previous treatment failure [68].
Another challenge in the treatment of H. pylori infection is the determination of the optimal duration of therapy. While there is controversy as to whether 7, 10, or 14 days is optimal [69,70]; it has been generally observed that longer [71] durations provide better results than a 7 day duration [72]. The current European and United States" recommendations support a 14 day duration therapy [2,5]. Notwithstanding the foregoing, available evidence suggests that a 7-day treatment duration is sufficient when quadruple therapy is used as a second line treatment [73].

Sequential therapy
An ideal H. pylori therapy should have a short duration and ought to lead to an eradication rate of the organism greater than 90%, as was the case initially when the triple-therapy regimens were approved [74]. A decreasing efficacy of common regimens has been observed with failure rates now surpassing 20% [5,75,76]. This development most likely result from increased bacterial resistance to some of the antibiotics in the regimens, especially clarithromycin that is often used [76]. The regimen comprises of a PPI and amoxicillin for 5 days followed by a PPI, clarithromycin, and a 5nitroimidazole (tinidazole or metronidazole) for another 5 days [74].
Sequential therapy originated from Italy and the original therapy utilized tinidazole [74,76]. Several studies from Italy and other parts of the world have reported eradication rates exceeding 90% which is superior to the clarithromycin-based triple therapy [5,74,76]. The tolerability is good in children, adults and the elderly [5]. The basis for this more complex therapy is the belief that amoxicillin could weaken bacterial cell walls in the initial phase of treatment, thereby preventing the development of drug efflux channels that inhibit drugs like clarithromycin from binding to ribosomes [74]. This therapy has a potential to become the standard first-line treatment for H. pylori infection because of this observed advantage.

Third line and 'Rescue' therapies
Treatment of patients who have already undergone first-and second-line therapies is a challenge because of the risk of development of double antibiotic resistance [77]. Though, various therapeutic protocols have been tested by different groups, a standard third-line therapy is currently lacking [2,78]. Endoscopic biopsy culture and antibiotic sensitivity testing is the most suitable option for patients with two eradication therapy failures [2,78]. The third-line therapy should avoid metronidazole and clarithromycin and other antibiotics that are likely to have contributed to development of the resistance.
They can be used to replace clarithromycin and metronidazole in rescue therapies and in first or second line regimens [2,[78][79][80].
They are particularly important because the use of quadruple therapy as the optimal second-line therapy is plagued with problems of a relatively high incidence of adverse effects and regimen complexity [79]. These lead to poor compliance, failure of eradication in about 20 to 30% of patients and subsequent drug resistance [79].

Conclusion
Substantial progress has been made in the understanding of the pathogenicity and treatment of H. pylori but there still exits a considerable gap to be filled. Information generated from studying the virulence factors of H. pylori are yet to be translated to clinical practice. Associations between bacterial characteristics and disease risks have not been defined sufficiently to guide the clinician in treatment decisions [1]. A major challenge exists in developing specific antibiotic monotherapy for effective treatment of the infection. Since the H. pylorigenome has now been sequenced, this provides an opportunity both to identify specific targets for drug therapy, and to facilitate the identification and production of antigens that may be helpful in manufacturing vaccines [81].
Encouraging results from animal models have been obtained in the areas of both therapeutic and prophylactic vaccination; however, translation to human vaccine remains difficult probably because the Page number not for citation purposes 10 immunology of the human stomach is still poorly understood [82].
All these developments and more will be necessary to adequately treat and prevent H. pylori infection in the future, especially in regions of the world with high prevalence.
What is known about this topic  H. pylori plays a major role in the pathogenesis of several upper gastrointestinal diseases that present as dyspepsia;  Reasonably sensitive and specific tests are available for diagnosing H. pylori;  Combination therapy for H. pylori eradication are available.

What this study adds
 The review is a summary of a myriad of materials already written on the topic presented in a concise and easy to read manner;  The sections on historical perspective and future direction on the topic were written to help stimulate the interest of the reader in further research.

Competing interests
The Authors declare no competing interest.

Authors' contributions
Jemilohun AC: conception and design of the study, literature search and review, and manuscript write up. Otegbayo JA: design of study, took oversight of all the stages of the work, critical review of manuscript content. All authors read and agreed to final manuscript. Table 1: Non-gastrointestinal tract diseases possibly associated with Helicobacter pylori infection