〔Abstract〕 Objective To explore the effects of polystyrene microplastics(PS-MPs) on the growth, activity, oxidative stress levels, biofilm formation and virulence ofKlebsiella pneumoniae. Methods Klebsiella pneumoniaewas exposed to PS-MPs at different concentrations(10, 50 and 100 μg/ml) and particle sizes(0.1, 1.0 and 5.0 μm), and the growth curves were measured. The bacterial activity was determined by CCK-8(cell counting kit-8). The level of intracellular reactive oxygen species(ROS) was determined by fluorescence probe. The biofilm forming ability was determined by crystal violet staining. Real-time quantitative fluorescent PCR(qRT-PCR) was used to detect the relative expression levels of biofilm-forming genes(luxS, mrkA, wbbM, pgaA, wzm) and virulence genes(ureA, uge, wabG, fimH). Results A high concentration(100 μg/ml) of 0.1 μm PS-MPs had a stronger inhibitory effect on the growth and activity ofKlebsiella pneumoniae, and the intracellular ROS level significantly increased, indicating that smaller particle size and higher concentration of PS-MPs were more toxic to bacteria. PS-MPs of 100 μg/ml particle size groups(0.1, 1.0 and 5.0 μm) significantly promoted the biofilm formation ofKlebsiella pneumoniae. The relative expression levels of biofilm formation related genes(luxS, mrkA, wbbM, pgaA, wzm) and virulence genes(ureA, uge, wabG, fimH) increased. Conclusion By inducingKlebsiella pneumoniaeto produce a high level of ROS, PS-MPs can cause oxidative stress, inhibit the growth and activity of bacteria, and enhance the biofilm formation ability and virulence, thus affecting the biological characteristics ofKlebsiae pneumoniae.