Uncovering the mechanism of action of a historical antimicrobial against S. aureus and A. baumannii

Abstract

Natural products have provided most of our modern pharmacopoeia, serving as active molecules or scaffolds for active molecules. Their use in drug development is often inspired by their traditional or historical medical use. For many decades, this discovery pipeline has focused on identifying a single molecule responsible for much of the biological activity of a raw natural product preparation (e.g. a whole-plant extract) and scoping this molecule for clinical potential. However, it is increasingly realised that historical/traditional remedies with significant biological activity may owe this activity to the combined action of multiple molecules. Concomitantly, microbiologists increasingly argue that effectively fighting antimicrobial-resistant infections will rely on combination therapies that combine multiple antimicrobials and/or adjuvant molecules. We previously reconstructed a complex historical remedy, Bald's eyesalve. Our reconstruction of this remedy had strong antibiofilm activity, which relied on the presence of multiple ingredients. Here, we report that Bald's eyesalve has multiple antibacterial effects on exemplar Gram-positive (Staphylococcus aureus) and Gram-negative (Acinetobacter baumannii) pathogens. Bald's eyesalve disrupts bacterial membrane integrity; inhibits expression of genes associated with bacterial adhesins, virulence factors and efflux pumps in both S. aureus and A. baumannii; inhibits quorum sensing in S. aureus; and causes downregulation of genes involved in de novo nucleotide biosynthesis in S. aureus. Lastly, we show that this multifaceted mechanism of action makes it difficult for S. aureus, A. baumannii, and Pseudomonas aeruginosa to evolve resistance against Bald's eyesalve. Bald's eyesalve could be used to identify a defined cocktail of natural products suitable for preclinical testing as a multi-target antibacterial preparation to which resistance may arise more slowly than current single-molecule antibiotics.