Abstract
Background:
Antimicrobial resistance (AMR) is recognized as a critical global health threat, traditionally studied in the context of pathogenic bacteria.

Aim:
Commensal bacteria—non-pathogenic microorganisms that inhabit humans, animals, and environmental niches—serve as important reservoirs and vectors for antimicrobial resistance genes (ARGs). Their role in acquiring, maintaining, and disseminating ARGs remains underappreciated despite significant implications for public health.

Methods:
We conducted a narrative synthesis of peer-reviewed literature published between 2020 and 2025, retrieved from PubMed, Scopus, and Web of Science. Search terms included “commensal bacteria,” “antimicrobial resistance,” “horizontal gene transfer,” and “One Health.” Studies were included if they reported on resistance acquisition mechanisms, environmental persistence, or clinical impacts of commensal bacteria. Relevant surveillance program reports (e.g., GLASS, EARS-Net) were also reviewed.

Results:
Recent studies reveal that commensal bacteria acquire ARGs through horizontal gene transfer mechanisms such as conjugation, transformation, and transduction. These bacteria contribute significantly to the environmental resistome via human and animal waste, food chains, and water systems. Certain commensals can transition to opportunistic pathogens in immunocompromised hosts, exacerbating clinical outcomes. Surveillance gaps persist, particularly in low- and middle-income countries, where environmental and veterinary data are underrepresented. Emerging One Health strategies show promise but face barriers in implementation, including resource limitations and fragmented cross-sectoral coordination.

Conclusion:
Mitigating AMR requires an integrated approach that addresses both pathogenic and commensal bacteria within the One Health framework. Strengthened surveillance, targeted antibiotic stewardship, and public health interventions are essential to curb the silent but substantial contribution of commensals to global AMR.

Key words: Antimicrobial resistance; Commensal bacteria; Horizontal gene transfer; Gut microbiome; Nosocomial infections; One Health.