Mutations in a Membrane Permease or hpt Lead to 6-Thioguanine Resistance in Staphylococcus aureus

We recently discovered that 6-thioguanine (6-TG) is an antivirulence compound that is produced by a number of coagulase-negative staphylococci. In Staphylococcus aureus, it inhibits de novo purine biosynthesis and ribosomal protein expression, thus inhibiting growth and abrogating toxin production. Mechanisms by which S. aureus may develop resistance to this compound are currently unknown. Here, we show that 6-TG-resistant S. aureus mutants emerge spontaneously when the bacteria are subjected to high concentrations of 6-TG in vitro. Whole-genome sequencing of these mutants revealed frameshift and missense mutations in a xanthine-uracil permease family protein (stgP [six thioguanine permease]) and single nucleotide polymorphisms in hypoxanthine phosphoribosyltransferase (hpt). These mutations engender S. aureus the ability to resist both the growth inhibitory and toxin downregulation effects of 6-TG. While prophylactic administration of 6-TG ameliorates necrotic lesions in subcutaneous infection of mice with methicillin-resistant S. aureus (MRSA) strain USA300 LAC, the drug did not reduce lesion size formed by the 6-TG-resistant strains. These findings identify mechanisms of 6-TG resistance, and this information can be leveraged to inform strategies to slow the evolution of resistance.