Cell-size-dependent of bacterial communities antibiotic resistome profiles and risk assessment in a river: a metagenomic analysis

Cell size represents a critical phenotypic trait that governs microbial activity and nutrient assimilation. However, whether bacteria of different sizes exhibit similar antibiotic resistome profiles and associated risks remains unclear. In this study, bacterial communities from a river ecosystem were categorized into two size fractions: small-sized bacteria (UB; 0.1–0.45 μm) and large-sized bacteria (LB; >0.45 μm); their antibiotic resistance profiles and risk levels were comparatively analyzed using metagenomics. A total of 69 antibiotic resistance genes (ARGs) subtypes belonging to 13 ARGs types were identified. The UB and LB groups showed distinct composition of ARGs, with comparable overall abundance. ARGs encoding multidrug and tetracycline resistance were dominant in the UB group. Multidrug, bacitracin and aminoglycoside resistance genes are dominant in the UB group, and 52 ARGs subtypes were uniquely detected. Additionally, the ARG-hosting microbial community structure differed significantly between the two size groups. Nonetheless, both groups demonstrated similar antimicrobial resistance risk levels. Together, these findings offer in-depth understanding of ARG distribution in river ecosystems and highlight the previously overlooked role of small-sized bacteria in the environmental antimicrobial resistance.