Staphylococcus aureus bacteremia at 5 US academic medical centers, 2008-2011: significant geographic variation in community-onset infections

Methicillin-resistant Staphylococcus aureus (MRSA) first appeared clinically in the early 1960s [1]. MRSA isolates became endemic in most US hospitals by the late 1980s [2–4]. In the 1990s, a new wave of MRSA infections occurred in community settings [3–8]. In many centers, MRSA isolates predominate as a cause of S. aureus community-onset (CO) infections, including bacteremia [3–5, 9]. Rather than being “escaped” hospital-based MRSA clones, the community MRSA strains were derived from methicillin-susceptible S. aureus (MSSA) strains that acquired a novel resistance element, SCCmec type IV [2, 4, 10–12]. These strain types, especially the USA300 genetic background, were highly virulent, often susceptible to multiple non–β-lactam antibiotics, and carried signature toxin genes (most commonly Panton-Valentine leukocidin [PVL]) rarely found in the older, hospital-acquired strain types [13]. USA300 also had a constitutive upregulation of several key virulence genes [14]. Driven by emergence of USA300, the incidence of MRSA infections rose dramatically in the early 2000s. In 2000, the Centers for Disease Control and Prevention (CDC) estimated that there were >30 000 hospitalizations for MRSA bacteremia [15]; however, more recent published reports have described declines in invasive MRSA infection rates, particularly in healthcare settings [16–20]. In the United States and the United Kingdom, the decline in MRSA bacteremia rates appears to have preceded enhanced infection prevention efforts in hospitals [21]. In a study of US military personnel and their dependents, rates of MRSA bacteremia declined between 2008 and 2011 [22]. However, to date, few reports have documented the incidence of MSSA bacteremia in this time period, or assessed potential geographic variations in CO infection rates, as opposed to hospital-onset (HO) rates. The current study was conducted to define trends in the annual incidence of MRSA and MSSA bacteremia at 5 large, geographically dispersed US academic medical centers to assess the incidence of HO and CO MSSA and MRSA bacteremia during a 4-year period.