Growth and membrane stress responses in E. coli and Acinetobacter sp. upon exposure to functionalized polystyrene microplastics

Microplastic pollution is increasingly recognized as a potential environmental stressor for microorganisms. This study aimed to explore how surface-functionalized polystyrene (PS) microplastics influence selected cellular-level responses in two Gram-negative bacteria, Escherichia coli and Acinetobacter sp., focusing on growth, viability, biofilm formation, and membrane-associated stress. Bacterial cultures were exposed to PS microplastics with three surface chemistries: non-functionalized PS, aminated PS (PS-NH2), and carboxylated PS (PS-COOH). Exposure to PS microplastic induced species- and surface chemistry-dependent alterations in bacterial responses. Compared to the control, non-functionalized PS reduced E. coli growth and viability to 74.8% and 61.3%, respectively, while Acinetobacter sp. showed reductions to 72.1% and 69.3% following PS exposure. Biofilm formation increased significantly to 143.2% in E. coli with PS, and to 207.2% and 190.7% in Acinetobacter sp. with PS and PS-COOH, respectively. Cytotoxicity assays revealed distinct stress patterns: in E. coli, PS exposure elevated MDA and LDH levels to 155.3% and 120.5% of control levels, respectively, while ROS levels remained near baseline (100.2%), indicating predominant membrane rupture and lipid peroxidation. In contrast, Acinetobacter sp. exhibited markedly elevated ROS (118.5% and 123.5%) and MDA (190.7% and 212.8%) levels upon exposure to PS and PS-COOH, while LDH remained comparable to the control, suggesting sublethal oxidative stress and membrane perturbation. These findings demonstrate that even chemically inert PS microplastics can trigger biologically significant responses in bacteria through surface-mediated mechanisms. The observed interspecies and inter-surface variability underscores the complexity of microplastic-microbe interactions and highlights the need for microbial-level assessments in evaluating the ecological risks of microplastic pollution.