Antibiotic treatment has two conflicting effects: the desired, immediate effect of inhibiting bacterial growth and the undesired, long-term effect of promoting the evolution of resistance. Although these contrasting outcomes seem inextricably linked, recent work has revealed several ways by which antibiotics can be combined to inhibit bacterial growth while, counterintuitively, selecting against resistant mutants. Decoupling treatment efficacy from the risk of resistance can be achieved by exploiting specific interactions between drugs, and the ways in which resistance mutations to a given drug can modulate these interactions or increase the sensitivity of the bacteria to other compounds. Although their practical application requires much further development and validation, and relies on advances in genomic diagnostics, these discoveries suggest novel paradigms that may restrict or even reverse the evolution of resistance.

The cell envelopes of Gram-positive bacteria comprise two major constituents, peptidoglycan and teichoic acids. Wall teichoic acids (WTAs) are anionic glycophosphate polymers that play important roles in bacterial cell growth, division, and pathogenesis. They are synthesized intracellularly and exported by an ABC transporter to the cell surface, where they are covalently attached to peptidoglycan. We address here the substrate specificity of WTA transporters by substituting the Bacillus subtilis homologue, TagGH Bs , with the Staphylococcus aureus homologue, TarGH Sa . These transporters export structurally different substrates in their indigenous organisms, but we show that TarGH Sa can substitute for the B. subtilis transporter. Hence, substrate specificity does not depend on the WTA main chain polymer structure, but may be determined by the conserved diphospholipid-linked disaccharide portion of the WTA precursor. We also show that the complemented B. subtilis strain becomes susceptible to a S. aureus-specific antibiotic, demonstrating that the S. aureus WTA transporter is the sole target of this compound.

We developed a competition-based screening strategy to identify compounds that invert the selective advantage of antibiotic resistance. Using our assay, we screened over 19,000 compounds for the ability to select against the TetA tetracycline resistance efflux pump in E. coli and identified two hits: β-thujaplicin and disulfiram. Treating a tetracycline resistant population with β-thujaplicin selects for loss of the resistance gene, enabling an effective second-phase treatment with doxycycline.