Microplastics have become a global concern due to their increasing use and discharge into the environment. These ubiquitous particles are known to have extremely low degradation rates and accumulate mostly in the marine environment. The evidence for bioaccumulation and indicators of stress linked to microplastics is also stated in the literature. However, the real environmental impact of microplastics has not yet been revealed. Therefore, it is crucial to understand the interaction mechanisms between microplastics and (micro)organisms under controlled (standard) laboratory conditions and environmentally relevant conditions to reflect the true environmental-situation. In this study, we aimed to understand how microplastics extracted from commercially available toothpaste samples interacted with four types of bacteria under both standard and seawater conditions. For this purpose, bacterial inhibitions were examined, and mechanisms of inhibition were evaluated by biochemical parameters (total protein, lipid peroxidase, total antioxidant capacity, and extracellular carbohydrate levels) of bacteria and physicochemical properties (zeta potential, particle size, surface chemistry) of microplastics. Results showed that gram-positive Bacillus subtilis and gram-negative Pseudomonas aeruginosa were affected in controlled and sea water media, respectively. The inhibition of the bacteria relied on the high zeta potentials of the microplastics, and, biochemically, protein and lipid peroxidase activity of bacteria were important in both media. On the other hand, while biochemical responses were similar in both media, the difference between the cell wall and microplastics surface charge was important only in seawater. (C) 2019 Elsevier B.V. All rights reserved.