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How did MRSA become resistant to methicillin and related penicillins?
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MRSA (methicillin-resistant Staphylococcus aureus) became resistant to methicillin and related penicillins primarily through the acquisition of a specific gene known as mecA from a distantly related bacterial species.1
This gene encodes a unique penicillin-binding protein (PBP) that has a low affinity for methicillin. By producing this alternative protein, the bacterium can continue to synthesize its cell wall even in the presence of the antibiotic, which would otherwise inhibit this essential process. This mechanism allows the bacteria to survive and proliferate despite the presence of drugs that were specifically designed to overcome earlier forms of bacterial resistance, such as the production of the enzyme penicillinase.12