Microplastics (diameters <5mm) are a global concern in aquatic ecology and are readily colonized by bacteria in the environment. Plastics have different physicochemical surface properties based on their intended applications. For example, polyethylene (PE) has a net negative charge while polypropylene (PP) has a net neutral charge in seawater. Bacteria prefer inert and hydrophobic surfaces for colonization which subsequently alters surface topography of microplastics. In this study, we investigated colonization of different microplastics by bacteria. Six types of microplastics were incubated for 8 weeks in microcosms with water from Lake Erie. Microcosms were inoculated with one of three species: Acinetobacter calcoaceticus, Burkholderia cepacia, and Escherichia coli. These bacterial species are ubiquitous in water bodies associated with human populations. Bacterial abundance was determined using scanning electron microscopy (SEM) and fluorescent microscopy with DAPI (4', 6-diamidino-2-phenylindole) staining. Confocal laser scanning microscopy (CLSM) was used for quantification of extracellular polymeric substance (EPS), and alterations in surface topography were determined by measuring contact angles of water droplets on microplastics. Lower contact angle values are characteristic of rougher, hydrophilic surfaces which are conducive for bacterial colonization.
Results/Conclusions
Fluorescent and SEM microscopy revealed that bacterial abundance differed among bacterial species and plastic types after 8 weeks. As the study progressed, E.coli remained the most abundant while A.calcoaceticus abundance decreased on most surfaces. Polypropylene had the lowest bacterial abundance among the plastic types in most cases. Production of EPS was elevated on A.calcoaceticus colonized surfaces. Burkholderia cepacia, when grown on PP and PS (polystyrene), reduced contact angles by 50% and 40%, respectively, compared to week 0. Our results indicate that bacterial colonization of microplastics is affected by the physicochemical properties of the substrates and physiological properties of colonizing bacteria. They also suggest that early colonization and biofilm formation on microplastics prime the substrates for further bacterial colonization in the environment.