Dysregulation of cell envelope homeostasis in Staphylococcus aureus exposed to solvated lignin

Lignin is an aromatic plant cell wall polymer that facilitates water transport through the vasculature of plants. Although lignin's ability to reduce bacterial growth been previously reported, it's hydrophobicity complicates the ability to examine its biological effects on living cells in aqueous growth media. We recently described the ability to solvate lignin in Good's buffers with neutral pH, a breakthrough that has allowed examination of lignin's antimicrobial effects against the human pathogen Staphylococcus aureus. We previously showed that lignin damages the S. aureus cell membrane, causes increased cell clustering, and inhibits growth synergistically with tunicamycin, a teichoic acid synthesis inhibitor. In this current study, additional experiments were performed to better understand the physiological and transcriptomic responses of S. aureus to lignin. Intriguingly, lignin restored the susceptibility of genetically resistant S. aureus isolates to ß-lactam antibiotics, dysregulated intracellular pH, and impaired normal cell division. Additionally, RNAseq analysis of lignin-treated cultures revealed a number of gene expression changes related to cell envelope, cell wall physiology, fatty acid metabolism and stress resistance. Altogether, these results represent the first comprehensive analysis of lignin's antibacterial activity against S. aureus that provide clarity in deciphering the mechanisms of lignin's antibacterial activity, while supporting the notion that lignin has potential to be repurposed for biomedical applications. Overall design: Examination of solvated lignin on S. aureus global gene expression at 20 minutes and 2 hours post-treatment, in aerobic Tryptic Soy Broth (TSB) cultures