Cationic Lignin as an Efficient and Biorenewable Antimicrobial Material

In this work, we utilized an inexpensive and naturally abundant polymer lignin and functionalized it with quaternary ammonium groups to yield a cationic antimicrobial, QAL. As opposed to non-cationic alkali lignin (AL), a relatively low concentration of cationic QAL (∼25–150 μg/mL) exerted strong bacteriostatic and bacteriolytic effects against both wild-type and kanamycin (kan)-resistant E. coli (∼90% dead cells, ∼90–100% growth inhibition with a 1 h treatment). Treatment with 25 μg/mL QAL exposed lipid (Nile red staining) and roughened the bacterial cell envelope (from ∼4.9 to 12.9 nm). Inner membrane damage was also evident as an increased amount of leakage of the cytoplasmic enzyme was evidenced by the increase in treatment time and QAL concentration. Additionally, a Langmuir-like monolayer coverage of QAL onto bacteria was identified, which agreed with zeta potential measurements and suggested electrostatic binding as the major mechanism of antimicrobial action of QAL. Lastly, QAL showed no/minimal cytotoxicity against human embryonic kidney cells (90–100% cell viability) within the concentration range (0–300 μg/mL) in which QAL killed and completely inhibited the growth of bacteria. The development of such efficient, biorenewable antimicrobial materials from lignin can pave the way for effectively addressing antibiotic resistance and enabling biomass valorization simultaneously.