Resistance to which antibiotic categorizes a strain of Staphylococcus aureus as MRSA?

A strain of Staphylococcus aureus is categorized as Methicillin-Resistant Staphylococcus aureus (MRSA) when it exhibits resistance to oxacillin. This classification is significant in clinical microbiology as it indicates a particular strain that is resistant to a broad range of beta-lactam antibiotics, including methicillin.

The mechanism of resistance is primarily due to the acquisition of the mecA gene, which encodes for a penicillin-binding protein (PBP2a) that has a lower affinity for beta-lactam antibiotics, effectively allowing the bacteria to survive despite the presence of these drugs. MRSA is a major concern in healthcare settings because it is associated with a higher rate of treatment failure, leading to more severe infections that can be difficult to manage.

While resistance to the other antibiotics listed may indicate different types of resistance or susceptibility patterns in Staphylococcus aureus, they do not specifically categorize the strain as MRSA. For instance, resistance to colistin is more relevant to Gram-negative organisms, and trimethoprim-sulfamethoxazole and tetracycline resistance does not carry the same clinical implications for categorizing Staphylococcus aureus as MRSA.