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 β-lactam-mediated HeR/HoR selection remains to be completed. In previous studies we demonstrated that acquisition of the HoR phenotype during exposure to β-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 β-lactam antibiotics in a model of natural occurrence, emphasizing the cytoprotective role in HeR-MRSA survival mechanism.

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c Klebsiella pneumoniae strains producing K. pneumoniae carbapenemase (KPC) cause serious infections in debilitated and immunocompromised patients and are associated with prolonged hospital stays and increased mortality rates. Daptomycin is a lipopeptide used against Staphylococcus aureus infection and considered inactive against Gram-negative bacteria. We investigated the effectiveness of a daptomycin-meropenem combination by synergy kill curve and a pharmacokinetic/pharmacodynamic model. The combination may represent a novel therapeutic strategy against infections caused by KPC-producing K. pneumoniae strains.