This arm will include patients who have to receive eradication therapy for the first time as first line or experienced patients who have to receive eradication as rescue therapy for third or further line, respectively.

After the confirmation of Helicobacter pylori infection, eligible patients whether first line or rescue therapy will be assigned to receive:

- A modified and optimized bismuth quadruple therapy that comprises: 40 mg of esomeprazole bid, amoxicillin 1g tds, metronidazole 500mg tds and bismuth subcitrate 240mg bid, all medications taken for 14 days.

Esomeprazole will be given at least 30 minutes before meals every 12 hours. Antibiotics will be given after meals every 8 hours and bismuth subcitrate at least 2 hours far from meals every 12 hours.

Participants achieved helicobacter pylori eradication if they perform negative at 13C-urea breath test (13C-UBT) 6-8 weeks after completion of modified bismuth-containing treatment

Side effects of treatment will be assessed on a structured clinical interview with a specific questionnaire completed immediately after the end of eradication therapy and at the final re-evaluation at 8 weeks. During the interview, patients will be asked to grade the severity of each adverse event experienced as "mild" (transient and well tolerated), "moderate" (causing discomfort and partially interfering with common everyday activities), or "severe" (causing considerable interference with patients' daily activities). Incapacitating or life-threatening complications will be classified as serious and will be reported to regulatory agency (National Organization of Medicines). Adherence to treatment will be assessed by providing all patients with a pre-structured printed table with all dosages illustrated, asking to tick each time a pill was consumed and bring it back along with any tablet not consumed, for pill counting.

In patients that are eligible to receive a rescue therapy, H. pylori clinical strains will be isolated from gastric biopsies. Antibiotic susceptibility testing of Helicobacter pylori will be performed utilizing E-test strips (BioMerieux, Marcy l'Étoile, France), according to the manufacturer's instructions, on Mueller Hinton agar medium (Beckton Dickinson) supplemented with 10% horse blood. MIC clinical breakpoints used to define resistance according to EUCAST will be: clarithromycin (>0.5 mg/L), levofloxacin (>1 mg/L), tetracycline (>1 mg/L), metronidazole (>8 mg/L) and amoxicillin (>0.12 mg/L).