Antibiotic resistance in Helicobacter pylori : Recent insights

The discovery that Helicobacter pylori (H. pylori) infection is the main cause of most gastro duodenal diseases has dramatically changed the management of these diseases. Antibiotics are the mainstay of treatment. However, prevalence of H. pylori antibiotic resistance is increasing worldwide, and is the main factor affecting the efficacy of current therapeutic regimens. In Sri Lanka, although eradication therapy for H. pylori by antibiotics is commonly advocated, due to a lack of susceptibility data, it has not been possible to formulate suitable guidelines for clinical use.


Introduction
Helicobacter pylori (formerly known as Campylobacter pylori or pyloridis) was first isolated from humans in1982. 1 The first isolation of H. pylori in pure culture and its association with gastritis and peptic ulcer disease led to the awarding of the Nobel Prize in Medicine in 2005 to Barry Marshall and Robin Warren, two physicians in Australia.It is a spiral shaped Gram negative bacterium that lives within the mucus layer overlying the stomach and duodenum.The outstanding biochemical characteristic of helicobacter is its rapid urease activity.All clinical isolates are known to be urease positive. 2e prevalence of H. pylori infection correlates with socio-economic status and is seen more in developing rather than developed countries. 3In Sri Lanka, the prevalence of H. pylori estimated by using histology, biopsy urease and PCR ranges from 9.7% to 70% 4,5 .This extremely wide prevalence appears to be due to different types of tests used in different studies.The sensitivity and specificity of PCR has been found to be very high, but is considered an experimental tool as it is not standardized. 6The sensitivity of the biopsy urease test reduces with treatment. 6We are currently carrying out a study to determine which test is superior in confirming the presence of H. pylori from patients with dyspeptic symptoms.
Every exposure to H. pylori does not lead to persistent colonization, either due to lack of adaptation to the particular host 7 or to use of antibiotics. 8,9After H. pylori acquisition, virtually all persons develop persistent colonization that lasts for life. 2 Chronic diffuse superficial gastritis, gastric ulceration, gastric adenocarcinoma and gastric lymphoma are associated with chronic H. pylori colonization.
Eradication of H. pylori in individuals may be the best course of action in the prevention of the above diseases.However, patients who receive H. pylori eradication standard triple therapy [proton pump inhibitor (PPI), amoxicillin (AMOX) and clarithromycin (CLA)] often encounter eradication failure over their treatment period. 10Development of antibiotic resistant strains of H. pylori may be the main factor affecting efficacy of current therapeutic regimens.
Worldwide, H. pylori antibiotic resistance towards different antibiotics has increased.Such a phenomenon may affect therapeutic management in different countries. 11The prevalence of bacterial resistance varies in different geographic areas, and has been correlated with the consumption of antibiotics in the general population. 12,13Prevalence of antibiotic resistance in H. pylori in Sri Lanka is not known and published studies in this area are sparse.

Current anti H. pylori regimens
Eradication therapy of H. pylori infection consists of different regimens.The standard first line regimen is triple therapy with AMOX, CLA and PPI given for 7 to 14 days. 14,15In areas with a high prevalence of CLA resistance, first-line therapy containing dual therapy (PPI and AMOX for 5 days) followed by triple therapy with a PPI, CLA and tinidazole [or metronidazole (MET)] for 5 days has been used, which is termed as sequential regimen. 16A new regimen termed as concomitant therapy is a 4-drug regimen (PPI, CLA, AMOX, and MET) for 7 to 10 days, which is more simple than sequential therapy and is more suitable for patients in high endemic areas of dual resistance to CLA and MET. 16Further, a sequential-concomitant hybrid therapy (dualconcomitant) consisting a PPI and AMOX for 7 days followed by a PPI, AMOX, CLA and MET for 7 days has shown promising success rate in attempts to overcome dual resistance. 17smuth containing quadruple therapy with MET, tetracycline (TET) and bismuth with PPI for 10 to 14 days used as second-line and/or salvage therapy has shown good efficacy in multicenter studies. 18In patients who failed with first line CLA-based triple therapy, levofloxacin (LEV)based triple therapy (LEV, AMOX, and a PPI) has proven to be superior to quadruple therapy and had fewer side effects as salvage therapy in a meta-analysis. 19e optimal therapeutic regimen has not been defined for refractory H. pylori infection.In this situation, the Maastricht IV/Florence Consensus Report recommends that whenever possible, anti-H.pylori treatment should be guided by antimicrobial susceptibility testing. 15However, antimicrobial sensitivity data for patients who failed eradication therapy with second line regimens is still not widely available.A recent prospective study has shown that a 10-day quadruple therapy with LEV, AMOX, bismuth, and rabeprazole as third-line treatment is effective and safe. 20][23]

Epidemiology of antibiotic resistance
Published data has shown that the prevalence of antibiotic resistance appears to be partly determined by geographical factors. 23Singh et al in 2009 tested 68 H. pylori isolates from North India and determined that all isolates were resistant to MET, while 65% were resistant to AMOX and 4.7% were resistant to CLA.However, none of the isolates were found to be resistant to TET. 24 In yet another study in Calcutta, about 90% of strains were MET resistant. 25In China, increased resistance to CLA and MET was seen from 2000 to 2009 (from 12.8 to 23.8% and 12.8 to 56.6% respectively), while AMOX resistance decreased from 2.1% to 0.3%. 26A high resistance to MET has been also reported from Saudi Arabia.The rate of resistance to MET in 2008 was 69.5%, while CLA resistance was 21% and no resistance to AMOX was seen. 27ese findings support the idea that H. pylori gene pools differ regionally and emphasize the potential importance of studies of Indian and other non-Western H. pylori populations in developing a global understanding of this gastric pathogen and associated disease. 25 Europe, there were huge differences in antibiotic sensitivities between southern and northern countries.Higher resistance rates of CLA in adults were observed in southern European countries such as Spain where the rate of CLA resistance was 35.6% in patient isolates of H. pylori. 17Mendonca et al. (2000) analyzed 90 Brazilian dyspeptic patients and showed that resistance of H. pylori to CLA, MET, TET, AMOX and FUR was 7%, 42%, 7%, 29%, and 4%, respectively. 28A metaanalysis reported that overall H. pylori antibiotic resistance rates (31 studies from 1993 to 2009) for AMO, CLA, MET, TET, LEV and multidrug-based therapies had increased in different continents. 11e prevalence of antibiotic resistance in various regions appearsto be associated with the pattern of general use of antibiotics in the region.An example is the high resistance rates to CLA seen in countries where this antibiotic is widely administered as mono-therapy for respiratory infections while there is low resistance seen in countries with a low consumption of macrolides. 29Another factor which might account for these existing geographic differences in antibiotic resistances is the presence of different H. pylori strains in particular geographic areas. 30

Antibiotic resistance mechanisms
The main antibiotic resistance mechanism which leads to H. pylori eradication failures arepoint mutations on the bacterial chromosome that cause changes in each drug's site of action.Other mechanisms such as acquisition of foreign genes carried on mobile genetic elements can also play a role. 31e bacteriostatic activity of CLA depends on its capacity to inhibit protein synthesis by binding to the 50S bacterial ribosomal subunit.Resistance to CLA is due to 23S rRNA mutations which inhibit the binding between CLA and the ribosomal subunit dedicated to the specific antibiotic related protein synthesis.Furthermore, expressions of active drug efflux mechanisms also play an important role in resistance to CLA. [32][33][34][35] Amoxicillin is a β-lactam antibiotic which acts by interfering with peptidoglycan synthesis.It blocks transporters involved in peptidoglycan synthesis named penicillin binding proteins (PBP).Production of β-lactamase, i.e. the main mechanism of penicillin resistance in other bacteria, does not play a role in AMOX resistance in H. pylori. 36ctericidal activity of MET depends on the reduction of its nitro-groups in anionic radicals, nitroso-derivates and hydroxylamines which are able to damage the DNA-helicoidal structure.MET resistance in H. pylori is complex and is primarily associated with mutational inactivation of the redox-related genes (frxA, rdxA).Different mutations involving the rdxA gene which encodes for an oxygen insensitive NADPH nitro-reductase are recognized as the main mechanism conferring MET resistance in H. pylori. 37Meanwhile, there are reports that the MET resistance phenotype may arise in H. pylori without mutations in rdxA or frxA, suggesting the presence of additional MET resistance mechanisms. 38uoroquinolones (FLU) exert a dose-dependent bactericidal effect by binding the sub-unit A of DNA gyrase (topoisomerase Ⅱ), an essential enzyme for the maintenance of DNA helicoidal structure.The mechanism of FLU resistance in H. pylori has been found to be linked to mutations in the quinolone resistance-determining regions of the gyrase A (gyrA) gene. 39,40utations in the gyrase B (gyrB) gene have also been identified in LEV resistant strains.However, this rarely occurs and is often associated with gyrA mutations. 23tracycline acts as a bacteriostatic agent by inhibiting codon anticodon link at the level of 30S ribosomal subunit and blocking the attachment of aminoacyl-tRNA to the acceptor site.Mechanisms of resistance to TET are associated with de novo mutations in the 16S rRNA gene. 41actors other than point mutations in the 30S ribosomal subunit may contribute towards the development of TET resistance.Possible mechanisms are ribosomal protection, 42 decreased membrane permeability and reduced intracellular accumulation due to a specific tetracycline efflux pump system. 43

Conclusion
Worldwide H. pylori resistance towards different antibiotics used in current eradication regimens is increasing and may affect the therapeutic management of malignant and non-malignant gastro duodenal diseases associated with H. pylori colonization.Antibiotic sensitivity data from different regions of the world highlights the need for constant surveillance of H. pylori antibiotic resistance, so that tailoring therapy is possible in clinical practice.The prevalence of antibiotic resistance in H. pylori in Sri Lanka is not known and studies in this area are needed for a better approach to treatment with current and alternative antibiotic regimens.