To evaluate the usefulness of NAC as pre-treatment attempt associated with a culture-guided antibiotic therapy as rescue therapy after multi-attempts antibiotic failure, in the first phase of the study, the effect of NAC on biofilm formation and on its demolition will be tested in vitro by adding NAC in cultured H pylori isolates from a limited number of patients (n10) with an history of at least 4 failed eradicating attempts. Two biopsy samples of gastric mucosa will be obtained during endoscopy for microbiological purpose. Specimens will be used for H pylori culture. In specimens from 5 patients, H pylori will be cultured together with NAC 2 mg, while in the other 5 cases, H pylori culture will be carried out without NAC. In these latter, after the biofilm formation, H pylori isolates will be faced with different doses of NAC (2 mg, 10 mg, and 20 mg) to observe the behavior of biofilm. In the second phase of the study, 40 patients, after at least four unsuccessful H pylori eradication attempts, will be consecutively recruited. During endoscopy, in each patient, at least 4 mucosal biopsies will be obtained from the gastric body. Two of these biopsies will be used for biofilm study at the scanning electron microscopy (SEM). The remaining biopsies will be used for H pylori culture. From culture, the in vitro antibiotic susceptibility testing together with genetic analysis of H pylori isolates will be performed. On the basis of antibiotic susceptibility testing alone, patients will then randomly assigned to receiving two different eradication schedules: Group A, patients receiving NCA 600 mg once a day for a week and subsequently a culture-guided one-week regimen including a PPI plus two antibiotics against H pylori; group B (control), patients receiving solely a culture-guided one-week antibiotic treatment including a PPI plus two antibiotics. Sensitive antibiotics will be always chosen on the basis of the more favorable minimum inhibiting concentration value. all patients will have a diary to record the side effects and symptoms during therapy. Compliance will measured by counting the tablets returned after the 7-day treatment. Patients will take a control C13 urea breath test at least two months after the end of therapy. They will be also invited to repeat endoscopic examination for monitoring biofilm persistence or absence. Finally, results obtained with the two treatment arms will be compared and evaluated also on the light of genetic analyses of H pylori isolates.