Structure–Activity Relationship of Thiomarinol Antibiotics in Gram-Negative Bacteria

Antibiotic resistance is a serious global public health issue worsened by misuse of antibiotics, which promotes generation of resistance, and a diminished pipeline of new antibiotics, which limits therapeutic options against multidrug-resistant bacteria. In particular, the intrinsic antibiotic resistance displayed by Gram-negative bacteria renders Gram-negative bacteria particularly difficult to treat: the combination of an impermeable outer membrane and active efflux reduces the intracellular concentrations of antibiotics, thereby rendering the antibiotics ineffective. Understanding the functional groups and physicochemical properties of antibiotics that promote activity against Gram-negative bacteria is an ongoing area of research. The hybrid natural product thiomarinol A combines two antibiotics: holothin (a dithiolopyrrolone) and marinolic acid A (an analogue of the antibiotic mupirocin). Thiomarinol A is a broad-spectrum antibiotic, with activity against both Gram-positive and -negative bacteria, retains the modes of actions of its constituent antibiotics, and displays activity against bacteria against which mupirocin is ineffective. In Chapter 2, we show that thiomarinol A accumulates significantly more than mupirocin in Escherichia coli, likely by overcoming the intrinsic resistance mechanisms that render mupirocin ineffective. We also conduct structure–activity relationship studies and find that the dithiolopyrrolone is a privileged functional group for improving activity and accumulation without disrupting binding to the target protein isoleucyl-tRNA synthetase. In Chapter 3, we show that thiomarinol A is less affected by the intrinsic resistance mechanisms of the Gram-negative bacteria Pseudomonas aeruginosa and Acinetobacter baumannii and that thiomarinol A accumulates significantly more than mupirocin in P. aeruginosa. These studies indicate the high accumulation of thiomarinol A in Gram-negative bacteria contributes to the potent antibiotic activity of the hybrid antibiotic and highlight the potential use for dithiolopyrrolones in designing broad-spectrum antibiotics.