Ceftaroline is active against heteroresistant methicillin-resistant Staphylococcus aureus clinical strains despite associated mutational mechanisms and intermediate levels of resistance.

Methicillin-resistant Staphylococcus aureus (MRSA) is an important infectious human pathogen responsible for diseases ranging from skin and soft tissue infections to life-threatening endocarditis. ß-Lactam resistance in MRSA involves acquisition of penicillin-binding protein 2a (PBP2a), a protein with low affinity for ß-lactams that mediates cell wall assembly when the normal staphylococcal PBPs (PBP1 to -4) are blocked by these agents. Many MRSA strains display heterogeneous expression of resistance (HeR) against ß-lactam antibiotics. The ß-lactam-mediated homoresistant (HoR) phenotype is associated with both expression of the mecA gene and activation of the LexA-RecA-mediated SOS response, a regulatory network induced in response to DNA damage. Ceftaroline (CPT) is the only FDA-approved cephalosporin targeting PBP2a. We investigated the mechanistic basis of CPT activity against HeR-MRSA strains, including a set of strains displaying an intermediate level of resistance to CPT. Mechanistically, we found that 1 exposure of HeR-MRSA to subinhibitory concentrations of CPT selected for the HoR derivative activated the SOS response and increased mutagenesis. Importantly, CPT-selected HoR cells remained susceptible to CPT while still being resistant to most ß-lactams, and 2-CPT activity in HeR-MRSA resided in an attenuated induction of mecA expression in comparison to other ß-lactams. In addition, 3-CPT intermediate-resistant strains displayed a significant increase in CPT-induced mecA expression accompanied by mutations in PBP2, which together may interfere with the complete repression by CPT of both PBP2a and PBP2a-PBP2 interactions and thus be a determining factor in the low level of CPT resistance in the absence of mecA gene mutations. The present study provides mechanistic evidence that CPT represents an alternative therapeutic option for the treatment of heteroresistant MRSA strains.

TCA cycle-mediated generation of ROS is a key mediator for HeR-MRSA survival under ß-lactam antibiotic exposure.

Methicillin-resistant Staphylococcus aureus (MRSA) is a major multidrug resistant pathogen responsible for several difficult-to-treat infections in humans. Clinical Hetero-resistant (HeR) MRSA strains, mostly associated with persistent infections, are composed of mixed cell populations that contain organisms with low levels of resistance (hetero-resistant HeR) and those that display high levels of drug resistance (homo-resistant HoR). However, the full understanding of ß-lactam-mediated HeR/HoR selection remains to be completed. In previous studies we demonstrated that acquisition of the HoR phenotype during exposure to ß-lactam antibiotics depended on two key elements: (1) activation of the SOS response, a conserved regulatory network in bacteria that is induced in response to DNA damage, resulting in increased mutation rates, and (2) adaptive metabolic changes redirecting HeR-MRSA metabolism to the tricarboxylic acid (TCA) cycle in order to increase the energy supply for cell-wall synthesis. In the present work, we identified that both main mechanistic components are associated through TCA cycle-mediated reactive oxygen species (ROS) production, which temporally affects DNA integrity and triggers activation of the SOS response resulting in enhanced mutagenesis. The present work brings new insights into a role of ROS generation on the development of resistance to ß-lactam antibiotics in a model of natural occurrence, emphasizing the cytoprotective role in HeR-MRSA survival mechanism.