The hydrophobicity of lignin enhances E. coli survival during NaClO exposure

• Lignin significantly increases E. coli resistance to chlorination. • Lignin adsorbs bacteria through hydrophobic interactions. • The hydrophobicity of lignin helps E. coli survival. High turbidity, attributed to suspended solids and colloidal particles, can lead to microbiological control failure during water disinfection. Lignin, a naturally occurring particle suspended in water, may contribute to turbidity. However, its ability to adsorb bacteria and protect them from chlorination remains poorly understood. In this study, we investigated Escherichia coli inactivation by NaClO in the presence and absence of natural particles. Among the five tested microparticles, lignin significantly enhanced E. coli survival under NaClO exposure, diminishing its bactericidal effect. Furthermore, surviving bacteria showed upregulation of gene expression related to energy metabolism and oxidative stress, as indicated by transcriptional analysis and qPCR measurements. These findings were further supported by increased ATP content, decreased ROS levels, and a prolonged lag phase in growth curves, enabling bacteria to rapidly restore their growth activity. Further analysis revealed that E. coli binds to the surface of lignin through hydrophobic interactions involving hydroxyl, methyl, aromatic rings, and ether groups. Lignin exhibited a strong bacterial adsorption capacity, as described by pseudo-second-order (PSO) kinetics and the Freundlich model, leading to reduced membrane permeability to NaClO. Importantly, the protective effect of lignin on bacteria was positively correlated with its hydrophobicity, with greater hydrophobicity resulting in enhanced bacterial protection. These findings suggest that lignin significantly increases E. coli resistance to chlorination, potentially contributing to disinfection failure in water treatment. This study highlights the role of lignin as a potential hazard that compromises microbiological control, offering new insights into chlorination failure caused by turbidity.