Functional Analyses of Cassette Chromosome Recombinase C2 (CcrC2) and Its Use in Eliminating Methicillin Resistance by Combining CRISPR-Cas9.

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Worldwide occurrence of methicillin-resistant Staphylococcus aureus (MRSA) poses enormous challenges for both communities and health care settings. Cassette chromosome recombinases (Ccr) specifically perform excision and acquisition of a staphylococcal cassette chromosome mec (SCC mec) in staphylococci and are responsible for the spread of methicillin resistance. This study explored the roles of CcrC2, a recently discovered Ccr, in the horizontal transfer of SCC mec and developed a potential means to control the spread of methicillin resistance. Knockout of CcrC2 completely aborted the excision of SCC mec, while overexpression of CcrC2 partially removed the SCC mec from the genome and transformed methicillin-resistant Staphylococcus aureus (MRSA) into methicillin-susceptible Staphylococcus aureus (MSSA). Moreover, two nucleotide residues (G5C6) in the direct repeat sequence within an att site were found to be critical for excision and acquisition efficiencies. To block the horizontal transfer of methicillin resistance, a SCC mec killer system was developed by combining the CcrC2-mediated SCC mec excision and the mecA-targeting CRISPR-Cas9 machinery. The SCC mec killer transformed MRSA to MSSA and disrupted the mecA-carrying SCC mec intermediate, thereby eliminating methicillin resistance determinant mecA gene inside a MRSA cell and blocking the horizontal transfer of SCC mec. The SCC mec killer was versatile for efficiently removing multiple types of SCC mec elements. It is envisioned that this approach could offer a new means to control the spread of methicillin resistance.