<i>Acinetobacter</i> spp. with lower susceptibility to quaternary ammonium compounds enriched in microbial communities of frequently used sinks

Sanitary environments that undergo frequent cleaning and disinfection may harbor microbial communities with potential health risks. While biofilms in healthcare settings are well studied, comparatively less is known about sink-drain microbiomes in public and educational buildings, where hundreds of people may interact with shared sink fixtures. This study characterized the spatial and temporal heterogeneity of sink-drain biofilm microbiomes in academic buildings. We sampled 16 sinks from two buildings (four floors each, with sinks closest and furthest to the bathroom entrance), which are cleaned daily with quaternary ammonium compound (QAC) disinfectants, during periods of low and high student traffic (during and after academic breaks, respectively) across winter, spring, and summer. We observed significant spatial and temporal variations in microbial assemblages. Individual sinks accounted for 43% (PERMANOVA, <i>P <</i> 0.0001) of the variation in microbial communities. Microbiomes in each building were dominated by two genera, which together accounted for 30% of the community composition: <i>Acinetobacter</i> and <i>Enhydrobacter</i> (also classified as <i>Moraxella</i>) in the newer building, and <i>Sphingomonas</i> and <i>Mycobacterium</i> in the older building. <i>Acinetobacter</i> abundance varied seasonally and showed higher relative abundance during periods of high traffic. Metagenomic analysis of selected sinks revealed a high prevalence of <i>qac</i> genes and metagenome-assembled genomes (MAGs) harboring antimicrobial resistance genes (ARGs), including <i>A. parvus.</i> Notably, 34%-53% of <i>qac</i> genes were co-localized on contigs associated with mobile genetic elements. These findings suggest that disinfected sink drains serve as persistent reservoirs of diverse microorganisms and potentially mobile resistance elements.IMPORTANCESink drains are recognized as environmental reservoirs for multidrug-resistant bacteria and have been linked to healthcare-associated outbreaks. In public and educational buildings, these microbiomes are shaped by frequent human activity, making them potential sources of exposure and contributors to the environmental dissemination of antibiotic resistance genes. Quaternary ammonium compound (QAC) disinfectants are widely used on surfaces; however, they can select for resistant taxa and co-select for antibiotic resistance. In this study, despite routine cleaning of sink surfaces with QACs, public restroom sink drains remain colonized by resilient biofilms, posing a potential risk to multiple users. Additionally, factors such as human traffic and seasonal variation may influence drain usage and microbial community composition. Elucidating how seasonal dynamics and human activity shape sink-drain biofilms is essential for understanding their role in the environmental transmission of antimicrobial resistance and informing mitigation strategies in nonclinical settings.