Background: Helicobacter pylori is a well-established risk factor for gastric carcinogenesis. Increasing evidence suggests that the oral cavity may serve as an extragastric reservoir for the bacterium, potentially contributing to persistent infection and reinfection. Orthodontic appliances can modify oral biofilm ecology and may facilitate bacterial colonization. This study aimed to investigate the association between oral H. pylori colonization and gastric cancer, while exploring the potential modifying role of fixed orthodontic appliances. Materials and Methods: In this cross-sectional observational study, 212 participants were recruited from gastroenterology and dental clinics between January 2023 and March 2025. Oral samples were collected and analyzed for H. pylori DNA using polymerase chain reaction (PCR). Gastric diagnoses were established through endoscopic examination and histopathological evaluation, classifying participants into gastric cancer, precancerous gastric lesions, non-atrophic gastritis, and control groups. Demographic, clinical, and oral health variables were recorded. Multivariable logistic regression models were used to evaluate the association between oral H. pylori detection and gastric cancer while adjusting for potential confounders, including age, sex, smoking status, oral hygiene indicators, and socioeconomic factors. Results: Oral Helicobacter pylori DNA was detected in 35/54 (64.8%) patients with gastric cancer, 30/56 (53.6%) with precancerous lesions, 21/52 (40.4%) with non-atrophic gastritis, and 15/50 (30.0%) controls. Gastric H. pylori infection was identified in 41/54 (75.9%) gastric cancer cases compared with 18/50 (36.0%) controls. Oral H. pylori positivity was more frequent among patients undergoing active orthodontic treatment (22/36, 61.1%) than among those without orthodontic appliances (79/188, 42.0%). In multivariable analysis, oral H. pylori positivity remained independently associated with gastric cancer (adjusted OR 3.02, 95% CI 1.51–6.03, p = 0.002). Conclusions: Our findings support an association between oral–gastric microbial interactions and H. pylori–associated disease, and suggest that the oral cavity may serve as a potential reservoir for gastric infection dynamics. The presence of orthodontic appliances may be associated with altered oral microbial ecology and could be linked to sustained H. pylori colonization.

Background: Helicobacter pylori is a well-established risk factor for gastric carcinogenesis. Increasing evidence suggests that the oral cavity may serve as an extragastric reservoir for the bacterium, potentially contributing to persistent infection and reinfection. Orthodontic appliances can modify oral biofilm ecology and may facilitate bacterial colonization. This study aimed to investigate the association between oral H. pylori colonization and gastric cancer, while exploring the potential modifying role of fixed orthodontic appliances. Materials and Methods: In this cross-sectional observational study, 212 participants were recruited from gastroenterology and dental clinics between January 2023 and March 2025. Oral samples were collected and analyzed for H. pylori DNA using polymerase chain reaction (PCR). Gastric diagnoses were established through endoscopic examination and histopathological evaluation, classifying participants into gastric cancer, precancerous gastric lesions, non-atrophic gastritis, and control groups. Demographic, clinical, and oral health variables were recorded. Multivariable logistic regression models were used to evaluate the association between oral H. pylori detection and gastric cancer while adjusting for potential confounders, including age, sex, smoking status, oral hygiene indicators, and socioeconomic factors. Results: Oral H. pylori DNA was detected more frequently in participants with gastric cancer compared with controls. After adjustment for potential confounders, the presence of oral H. pylori was significantly associated with increased odds of gastric cancer. Interaction analysis suggested that individuals with fixed orthodontic appliances demonstrated higher rates of oral H. pylori detection, supporting the hypothesis that orthodontic biofilm retention may facilitate bacterial persistence within the oral cavity. Conclusions: Our findings support the concept of an oral–gastric microbial axis in H. pylori–associated disease and suggest that the oral cavity may represent a potential reservoir contributing to gastric infection dynamics. The presence of orthodontic appliances may influence oral microbial ecology and could play a role in sustaining H. pylori colonization. These results highlight the importance of interdisciplinary approaches integrating dentistry and gastroenterology in the understanding and management of H. pylori infection and gastric cancer risk.