Pyrimidine-2-amines increase susceptibility of methicillin-resistant Staphylococcus aureus to penicillin G

β-lactamase (penicillinase) based resistance renders early β-lactams like penicillin G useless against methicillin-resistant Staphylococcus aureus (MRSA). Finding novel antimicrobials is difficult, and resistance has been observed against most treatment options. Increasing the susceptibility of MRSA to early β-lactams would help bring back the clinical utility of a powerful and safe class of antimicrobials. Moreillon and coworkers have already demonstrated that β-lactamase inhibition can render penicillin G more powerful than oxacillin. Yet, they also highlighted that direct β-lactamase inhibitors are not the best tool for the job. Here, we reveal that certain pyrimidin-2-amines (P2A) reduce the minimum inhibitory concentration (MIC) of penicillin G against resistant S. aureus strains reliably by up to 64-fold, when present at 50 μM. P2As do not inhibit β-lactamases directly, but reduce its expression; This is in stark contrast to direct penicillinase inhibitors like sulbactam and clavulanate that covalently bind the secreted protein. The MIC of penicillinase-insensitive oxacillin were not altered, clearly because these cannot overcome PBP2a; This demonstrates the advantage of penicillin G over penicillinase-insensitive β-lactams. Gene knockout experiments show that factors commonly associated with decreased resistance to cell wall-active antimicrobials, like Stk1 and VraS, are not the target of P2As. Yet, multiple gene knockouts were resistant to P2A activity, suggesting these chemicals act through an as-yet-unknown central controller. A preliminary structure-activity relationship has also provided insights into pharmacophoric features. We have demonstrated that Moreillon’s principle, that penicillin G is useful against MRSA upon cessation of β-lactamase activity, can be practically implemented by suppression of penicillinase expression instead of direct β-lactamase inhibition.