Cationic Lignin as an Efficient and Biorenewable Antimicrobial Material

In this work, we utilized an inexpensive and naturallyabundantpolymer lignin and functionalized it with quaternary ammonium groupsto yield a cationic antimicrobial, QAL. As opposed to non-cationicalkali lignin (AL), a relatively low concentration of cationic QAL(& SIM;25-150 & mu;g/mL) exerted strong bacteriostaticand bacteriolytic effects against both wild-type and kanamycin (kan)-resistant E. coli (& SIM;90% dead cells, & SIM;90-100%growth inhibition with a 1 h treatment). Treatment with 25 & mu;g/mLQAL exposed lipid (Nile red staining) and roughened the bacterialcell envelope (from & SIM;4.9 to 12.9 nm). Inner membrane damagewas also evident as an increased amount of leakage of the cytoplasmicenzyme was evidenced by the increase in treatment time and QAL concentration.Additionally, a Langmuir-like monolayer coverage of QAL onto bacteriawas identified, which agreed with zeta potential measurements andsuggested electrostatic binding as the major mechanism of antimicrobialaction of QAL. Lastly, QAL showed no/minimal cytotoxicity againsthuman embryonic kidney cells (90-100% cell viability) withinthe concentration range (0-300 & mu;g/mL) in which QAL killedand completely inhibited the growth of bacteria. The development ofsuch efficient, biorenewable antimicrobial materials from lignin canpave the way for effectively addressing antibiotic resistance andenabling biomass valorization simultaneously. We demonstrate the action mechanism ofcationic lignin,a green, effective, affordable, and scalable antimicrobial capableof killing antibiotic-resistant bacteria, while presenting a uniquelignocellulosic biomass valorization approach.