High Colonization Pressure Might Compromise the Efficiency of Routine Methicillin-Resistant Staphylococcus aureus Screening

To the Editor—Routine screening for methicillin-resistant Staphylococcus aureus (MRSA) in the intensive care unit is a widely recommended [1] and quite wellstudied [2–4] intervention. Yet, the recent study by Huang et al. [5] finally imparts a clinical imperative (the reduction of bacteremias) to the old epidemiological rationale of MRSA transmission control. The study’s sequential design allows for the assessment of multiple interventions, and its unique and very astute monitoring of methicillin-susceptible S. aureus bacteremias as a control excludes the possibility of natural fluctuations or other confounding factors, which were not accounted for in previous studies [2–4]. Thus, it is all the more deplorable that Huang and colleagues did not provide an estimation of the MRSA colonization pressure during the study interval. Colonization pressure is an important risk factor for MRSA acquisition in the intensive care unit [6]. A study of vancomycinresistant Enterococcus transmission [7] concludes that a high colonization pressure may supersede the effect of other transmission variables, including infection-control measures. It does not seem unreasonable to extrapolate this phenomenon to MRSA, especially in light of high rates of gut carriage of the organism in colonized patients [8]. The MRSA carriage prevalence and incidence reported in the article by Huang et al. [1] evoke low colonization pressure. Indeed, no successful control of the spread of MRSA has been achieved by screening strategies in an environment of high colonization pressure (i.e., 150%). Future studies of MRSA control in the intensive care unit need to report the colonization pressure that is prevalent in the study population, because the findings reported might not be applicable to intensive care units with high colonization pressure. In these cases, other, more aggressive measures might be required [8].