Effect of environmental factors on attachment of Escherichia coli and Staphylococcus aureus to polypropylene microplastics

Microplastics (MPs) serve as substrates for bacterial colonization, facilitating pathogen transport in aquatic environments. Therefore, this study investigated the attachment of Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) to five MP polymers and evaluated the effects of pH, temperature, hydrodynamic conditions, natural organic matter (NOM), and salinity using polypropylene (PP)-MPs. PP-MPs supported the highest bacterial attachment efficiencies due to their hydrophobicity (water contact angle  = 98.4°) and weak negative surface charge (zeta potential = -16.6 mV). Bacterial attachment decreased as pH increased (4.5-8.6) because of increased electrostatic repulsion. Increasing temperature (20 to 35 °C) decreased E. coli attachment, possibly through reduced curli production and increased motility, whereas it increased S. aureus attachment, which may be attributed to enhanced EPS production and surface hydrophobicity. Under varying hydrodynamic conditions, E. coli attachment peaked at 150 rpm, whereas S. aureus attachment remained high under both static and high-mixing conditions. Increasing NOM concentrations suppressed E. coli attachment through enhanced electrostatic repulsion but increased S. aureus attachment owing to the charge heterogeneity of its cell envelope. Elevated NaCl concentrations promoted bacterial attachment by screening electrostatic repulsion, consistent with the extended Derjaguin-Landau-Verwey-Overbeek theory, although this effect was less pronounced for S. aureus due to its high attachment. E. coli attachment decreased from 79.72% in sterilized water to 17.10% in lake water and 4.87% in WWTP effluent, whereas S. aureus retained high attachment efficiencies of 78.26% and 90.14%, respectively. These findings highlight how environmental conditions and species-specific traits collectively govern MP-associated bacterial attachment.