Antibiotics adsorb to polyester microplastic fibers and alter bacterial communities in stream mesocosms

Antibiotics and microplastics (MPs) are anthropogenic contaminants that co-occur in wastewater and wastewater-impacted surface waters. Within these environments antibiotics can select for resistant taxa and genes, altering the composition of bacterial communities and potentially contributing to the spread of antibiotic resistance genes (ARGs). Antibiotics can also adsorb to MPs, resulting in high antibiotic concentrations on MP surfaces. We hypothesized that MPs exposed to antibiotics in a freshwater environment would adsorb antibiotics and that this interaction would alter the composition of bacterial biofilms colonizing the MP surfaces, including selecting for antibiotic resistant taxa and increased abundance of ARGs. We tested these hypotheses with a stream mesocosm experiment that included polyester fibers, a mixture of eight antibiotics at two environmentally relevant concentrations, and an inoculum of bacteria from an urban stream. The addition of antibiotics to the streams resulted in significant adsorption of ciprofloxacin, sulfamethoxazole, and trimethoprim to the MPs (p < 0.001), with ciprofloxacin showing the highest adsorption. The addition of antibiotics to the streams also significantly changed the composition of the bacterial communities on the MPs and in the water (p < 0.05), including significant increases in the abundance of bacterial orders linked to antibiotic metabolism and resistance (e.g., Sphingobacteriales, Caulobacterales, and Xanthomonadales). However, the abundance of two ARGs, mexB and tetA, did not vary with antibiotic dosage, and the abundance of an integrase gene, intI1, decreased with antibiotic dosage. These results confirmed our prediction that antibiotics would adsorb to MPs in freshwater and alter the composition of particle-associated and planktonic bacterial communities, but the presence of antibiotics did not increase the abundance of ARGs.