Interactions between bacteria and nano (micro)-sized polystyrene particles by bacterial responses and microscopy

Microorganisms play an important role in biogeochemical cycles, and are inevitably found associated with plastic debris. The interplay between microbes and plastics may change the characteristics of certain plastics over time and drive the environmental fate of plastics. In this study, we evaluated interactions of bacteria with nano- and microplastics. Here, polystyrene (PS) polymer particles of various diameters, specifically 60, 220, 430, 700, 1040, 1700, and 2260 nm, were used as the plastics. Escherichia coli (E. coli, gram-negative) and Bacillus sp. (gram-positive) were chosen as model bacteria. The effects of nano- and microPS particles on E. coli and Bacillus sp. cells were investigated by measuring the growth and viability of the cells in laboratory-scale flasks and their generation of reactive oxygen species (ROS) upon their exposure to these particles of 100 mg/L. The particles inhibited the growth and viability of both types of bacterial cells, but their inhibitory effects varied depending on the diameter of PS particle. The 60-nm-diameter PS particles were visually observed to enter the cells as well as accumulate on their surfaces and enhanced ROS generation of the cells. Unexpectedly, the 1040-nm-diameter PS particles, similar in size to the bacterial cells, inhibited the growth of both E. coli and Bacillus sp. cells the most. The E. coli and Bacillus sp. cells formed microPS-biofilm complex by secreting an extracellular polymeric substance (EPS) in response to their exposure to the ∼ 1-μm-diameter PS particles. A positive correlation between relative ROS levels and specific growth rates of the E. coli cells were observed with a Pearson correlation coefficient r value of 0.676 (p < 0.05).