Is methicillin-resistant Staphylococcus aureus replacing methicillin-susceptible S. aureus?

Abstract

Despite extensive research on the emergence of and treatments for methicillin-resistant Staphylococcus aureus (MRSA), prior studies have not rigorously evaluated the impact of methicillin resistance on the overall incidence of S. aureus infections. Yet, there are direct clinical and research implications of determining whether methicillin-susceptible S. aureus (MSSA) infection rates remain stable in the face of increasing MRSA prevalence or whether MSSA will be replaced over time. A synthesis of prior studies indicates that the emergence of healthcare-associated MRSA (HA-MRSA) and community-associated MRSA (CA-MRSA) has led to an increase in the overall incidence of S. aureus infections, with MRSA principally adding to, rather than replacing, MSSA. However, colonization with CA-MRSA may at least partially replace colonization with MSSA. So far, evidence indicates that MSSA still accounts for many infections. Therefore, eradication of MRSA alone is not sufficient to address the public health burden of S. aureus.

Figure 1.

Schematic representing the impact of MRSA on the infection incidence rate for S. aureus, depending on both the colonization and infectiveness of MRSA and MSSA. The schematic shows that even if the infection incidence rate of total S. aureus is increasing, it may be due to replacement of MSSA, MRSA incidence in addition to MSSA or a combination of the two processes. Furthermore, the magnitude of the increased S. aureus infection rates depends on the colonization capacity and invasiveness of different strains. The greater the invasiveness of MRSA relative to MSSA, the greater the impact of MRSA on the total S. aureus burden. The horizontal line indicates the S. aureus infection incidence rate before the emergence of MRSA.

Figure 2.

Number of studies identified from North America, Europe and Asia reporting evidence that MRSA is replacing MSSA, adding to MSSA or the study results are equivocal. The figure represents the conclusions from the 45 studies identified for this review. The figure does not account for differences in study quality or size, and it does not distinguish between colonization and infection.