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Cefoxitin disk diffusion versus mecA PCR for MRSA detection


Methicillin-resistant Staphylococcus aureus (MRSA) is an antibiotic-resistant organism of increasing clinical importance in the hospital setting as well as in the general community. This organism overcomes environmental antibiotic pressures by acquiring resistance genes from other bacterial species or from free DNA in the environment.
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, MRSA creates a mutated cell-wall protein called PBP2a, which has diminished affinity for penicillins. PBP2a is encoded by the mecA gene. Presence of the mecA gene can be tested by polymerase chain reaction (PCR), whereas 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 in under-resourced settings, 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 isolate, 270 were detected by disk diffusion. Of the remaining mecA-negative isolates, 150 were detected by disk diffusion. They concluded that disk diffusion is an acceptable method of MRSA detection in under-resourced laboratories.
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. Quadrant B shows no zone of clearance.
According to the passage, which of the following mechanisms does not increase the prevalence of MRSA bacteria?
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Choose 1 answer: