Environmental occurrence and dissemination of colistin resistance in contaminated aquatic systems: a systematic review of eight observational studies.

INTRODUCTION

Colistin is a last-resort antibiotic used against multidrug-resistant Gram-negative bacterial infections. The emergence of plasmid-mediated colistin resistance, particularly mcr genes, has raised concerns about the environmental persistence and dissemination of resistance determinants. Aquatic environments may serve as reservoirs and dissemination pathways for colistin resistance; however, the available evidence remains fragmented. This systematic review aimed to synthesize evidence on the occurrence, resistance mechanisms, and environmental and potential public health implications of colistin resistance in contaminated aquatic systems.

METHODS

A systematic literature search was conducted in five electronic databases up to December 2024 in accordance with PRISMA guidelines. Studies investigating colistin resistance in wastewater, surface water, aquaculture systems, and related aquatic environmental matrices were eligible for inclusion. Following screening and eligibility assessment, eight observational studies were included. Owing to substantial methodological and outcome heterogeneity, a narrative synthesis was performed. Isolate-level phenotypic resistance, sample- or site-level molecular positivity, and metagenomic relative gene abundance were analyzed as separate, non-comparable outcome strata.

RESULTS

A total of 26,740 records were retrieved, of which 20,691 unique records were screened after removal of duplicates. Eight studies met the predefined eligibility criteria. The studies investigated diverse aquatic environments, including slaughterhouse effluents, municipal wastewater, urban rivers, aquaculture systems, and wastewater-associated bioaerosols. Plasmid-mediated mcr genes were identified across several environmental settings and frequently occurred alongside other antimicrobial resistance determinants. Chromosomal mechanisms of colistin resistance were also reported. The reported resistance measures varied substantially between studies, ranging from isolate-level phenotypic resistance and sample/site-level molecular detection to metagenomic relative abundance.

DISCUSSION: The available evidence indicates that contaminated aquatic environments can harbor colistin resistance determinants and may contribute to their environmental dissemination. However, the limited number of eligible studies and substantial methodological heterogeneity prevent direct quantitative comparison or estimation of a common environmental prevalence. Standardized surveillance approaches integrating culture-based methods, molecular detection, and genomic/metagenomic characterization are needed to improve comparability and clarify the environmental pathways of colistin resistance within a One Health framework.

SYSTEMATIC REVIEW REGISTRATION: https://www.crd.york.ac.uk/PROSPERO/, identifier CRD42024613277.