Final answer:
MRSA is resistant to β-lactam antibiotics due to the acquisition of the mecA gene, which encodes for a low-affinity penicillin-binding protein, PBP2a.
Step-by-step explanation:
The nucleotide alteration that leads to methicillin-resistant Staphylococcus aureus (MRSA) is primarily associated with the acquisition of a new low-affinity penicillin-binding protein (PBP). This methicillin resistance is genetically encoded, often through horizontal gene transfer, and renders MRSA resistant to all β-lactam antibiotics. Specifically, the resistance is due to the acquisition of the mecA gene, which encodes for the altered PBP2a with reduced affinity for methicillin and other β-lactams, enabling bacteria to grow and divide even in the presence of these antibiotics. The introduction of methicillin to clinical practice was followed by the emergence of MRSA, making this pathogen a serious concern for infections, including skin infections, pneumonia, and septicemia.