In this work, the effects of extracellular polymeric substances (EPS) on the aggregation and biological responses of different micro(nano)plastics (MNPs, <1000 µm) were investigated. EPS increased the colloidal stability of PS MPs in NaCl or CaCl₂. For the three PS NPs (PS-NH₂, PS-COOH, and PS-naked), EPS also enhanced their colloidal stabilities in the presence of NaCl. However, the effect of CaCl₂ on the colloidal stabilities of PS NPs in the presence of EPS depended on their surface functional groups. In CaCl₂, both Derjaguin-Landau-Verwey-Overbeek theory and molecular bridging explained the interaction between MNPs (both NPs and MPs) and EPS. Laser Direct Infrared and scanning electron microscope imaging showed that opalescent EPS corona formed on PS MPs via intermolecular-bridging by Ca²⁺, and the critical coagulation concentrations (70 mM in NaCl, 1.5 mM in CaCl₂) in EPS were much lower than that for PS NPs (1000 mM for NaCl; 65 mM for CaCl₂). PS-NH₂ NPs showed the highest increase in the growth of bacteria (Bacillus subtilis), followed by PS MPs and PS-naked NPs, while PS-COOH NPs had no significant effect. Biological response of PS NPs was unaffected by EPS, while EPS further enhanced the positive effects of PS MPs on bacterial growth.