6-thioguanine resistance in Staphylococcus aureus LAC

We recently discovered that 6-thioguanine (6-TG) is an anti virulence 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 down regulation effects of 6-TG. Whereas prophylactic administration of 6-TG ameliorates necrotic lesions in subcutaneous infection of mice with 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.