Helicobacter pylori infects 50% of the world population and 80% of the Indian population. The infection is known to be associated with peptic ulcer and gastric cancer. Infection with H. pylori strains carrying the vacAs1i1m1 alleles and the Eastern type cagA is associated with aggressive clinical outcomes in West and in East-Asia. However, why only 10-20% of the H. pylori infected individuals suffer from gastric diseases remained unknown. Recent literature suggests that several other bacteria can colonize human gastric mucosa, but their significance in relation to gastric diseases is not understood. We hypothesize that the interplay among these bacteria and H. pylori is possibly the key factor determining the clinical outcome. Currently, we are investigating the association of H. pylori infection and its virulence markers with gastric and intestinal microbiome for patients with different clinical status. We are also testing the antimicrobial susceptibilities of H. pylori and other gastric bacteria. Our methods include isolation ofH. pylori and other gastric bacteria on blood-agar plates, chromosomal DNA extraction and genotypesdetermination and buildingphylogenetic trees. For microbiome analysis, we extract DNA directly from biopsy samples, amplify the bacterial 16S rDNA by PCR and perform next-generation sequencing (NGS) using Illumina platform. For determining antimicrobial susceptibilities we use agar dilution method. Our ongoing study indicates that the prevalence of H. pylori infection in Trivandrum, Kerala is low as compared to rest of India. However, the genotypes and the antibiotic susceptibility profiles are comparable to the strains isolated from other part of India. Our analysis using metagenomics and culture dependent approaches revealed that, apart from H. pylori, several microbial genera colonize the human gastric epithelium. Presently we are investigating the significance of these infections.