Municipal landfills are significant sources of environmental and microbial pollution, impacting groundwater and surface water quality. This study investigated the microbial community composition and antibiotic resistance genes (ARGs) in water downstream of landfills of Gilan, Mazandaran, and Golestan provinces, Iran. Water samples were collected from seven sites, and shotgun metagenomic sequencing was used to analyze microbial diversity and ARGs. Heavy metals and BTEX (benzene, toluene, ethylbenzene, and xylene) compounds were measured using inductively coupled plasma (ICP), and gas chromatography-mass spectrometry (GC-MS) methods, respectively. Pseudomonadaceae and Enterobacteriaceae were the most abundant bacterial families, with efflux pump ARGs being the most prevalent. Concentrations of arsenic and cadmium exceeded WHO and US-EPA standards at all sites. Significant positive correlations were observed between Pseudomonadaceae abundance and lead concentration (r =0.998, p = 0.031, CI [0.966, 0.999]), and between Enterobacteriaceae abundance and chromium concentration (r =0.999, p = 0.0078, CI [0.993, 1.000]). A significant negative correlation was found between the abundance of the two-component system (TCS) gene class and BTEX concentration (r = -0.457, p = 0.014, CI [-0.72, -0.09]). Additionally, aluminum concentration negatively correlated with antibiotic inactivation (r =0.999, p = 0.018, CI [-1.000, -0.997]) and antibiotic target protection classes (r = -0.997, p = 0.048, CI [-0.999, -0.990]). These findings indicate that landfill sites significantly influence bacterial communities, promoting resistance to heavy metals and pollutants. The abundance of ARGs near landfills suggests microbial adaptation to pollution, highlighting the need for improved waste management to mitigate the spread of antibiotic resistance.
