## 1 Introduction  
Since the 1950s, plastic production has steadily increased owing to its advantages of being inexpensive, light, strong, durable, and corrosion-resistant. Plastic production increased from 1.7 million tons in 1950 to 359 million tons in 2018, more than 200 times the volume produced 68 years ago [1; 2; 3]. Owing to the rapid increase in the usage of plastics globally, a significant amount of nonrecycled, landfilled and reused plastics has been released into the environment and has accumulated.  
As plastics, which are hardly biodegradable, accumulate in the environment, a new environmental issue that has emerged is microplastic pollution. Microplastics are plastics that have been shattered by various weathering processes or fabricated into small sizes for specific purposes with diameters of 5 mm or less. Microplastics found in terrestrial, aquatic, and atmospheric environments have various polymer types, shapes, and sizes [4; 5; 6]. Although microplastics do not have short-term lethal effects on living organisms, long-term exposure can induce chronic toxicity through various mechanisms [7; 8]. Toxic chemicals used in the production of plastics can induce toxicity, and the small and pointy structure of microplastics can damage organisms and cause inflammation. Ingestion of tiny microplastics has been shown to cause malnutrition and alterations in reproductive function in some organisms [9; 10]. Additionally, persistent organic pollutants adsorbed on plastics can reach concentrations up to a million times higher than ambient concentrations, and these compounds can be further desorbed inside the organism, exacerbating the bioaccumulation of the persistent organic pollutants [11; 12].  
Biofilms are microbial colonies that attach to and grow on biotic and abiotic surfaces [13]. Microplastics provide a new artificial adhesive substrate for biofilm formation [14; 15]. The formation and heteroaggregation of biofilms have been reported to affect not only the physical properties of microplastics [16] but also the chemical properties owing to the presence of bacteria in the biofilm [17; 18]. Such modified microplastics have been found to enhance the carrier effect on heavy metals while also increasing the associated toxicity [19; 20; 21]. Furthermore, when harmful microorganisms form a biofilm on microplastics, antibiotic resistance increases, increasing the toxic potential in the aquatic ecosystem [22]. In the wastewater treatment process, biofilm formation on the microplastic surface affects the microplastic removal efficiency of the sedimentation tank through the change in density [23]. In the membrane water treatment process, microplastics act as a substrate for microorganisms to adhere and grow, causing increased membrane biofouling [24].  
Biofilm formation on the surface of microplastics can be affected by environmental conditions, as well as the properties of plastics and biological interactions between microorganisms, but specific research results to explain the mechanism are still lacking [25]. Therefore, this study aims to analyze the difference in the biofilm formation of microorganisms based on various types of microplastics. Using the _Pseudomonas aeruginosa_ PAO1 strain as a model microorganism, biofilm formation on the surfaces of four types of microplastics including polyethylene (PE), polypropylene (PP), polytetrafluoroethylene (PTFE), and polystyrene (PS) were analyzed through batch experiments with microplates. The difference in biofilm formation rate based on the material and surface characteristics of microplastics was investigated, as well as the effect of the interaction between microorganisms and plastics on initial attachment and biofilm development. Finally, the effect of quorum sensing (QS) (a cell-cell communication mechanism) on biofilm formation on PE microplastic surfaces was analyzed.