Mini MCAT passage: MRSA detection mechanisms

Beta-lactam antibiotics bind to Staphylococcus aureus penicillin-binding proteins (PBPs), thereby inhibiting cell wall synthesis and therefore bacterial replication. To overcome beta-lactam antibiotics, Methicillin-resistant S. aureus (MRSA) creates a mutated cell-wall protein called PBP2a, which has diminished affinity for penicillins. PBP2a is encoded by the mecA gene. The presence of the mecA gene can be tested by polymerase chain reaction (PCR), whereas the presence of PBP2a is tested by a latex agglutination technique. Both of these methods are highly sensitive for methicillin resistance, but are relatively costly.

To assess the utility of lower-cost methods of rapidly detecting MRSA, researchers compared the sensitivity and specificity of mecA PCR with the standard, more economical disk-diffusion method, shown in Figure 1. In this method, an antibiotic disk is placed in a petri dish populated with MRSA. At 18 hours of incubation, the zone of bacterial clearance around the disk is measured. The researchers found that, of 500 Staphylococcus aureus isolates tested, 300 were methicillin-resistant by mecA probe. Of the 300 resistant isolates, 270 were detected by disk diffusion. Of the remaining mecA-negative isolates, 150 were detected by disk diffusion.

Figure 1 Zones of clearance around antibiotic-impregnated disks: quadrant A demonstrates a large (>19mm) zone of clearance, with quadrants C and D demonstrating smaller (≤19mm) zones