Clinical Relevance

Staphylococcus aureus is a major pathogen responsible for both nosocomial and community-acquired infections. Although the first S. aureus isolates displaying resistance to methicillin (MRSA) were reported in the early 1960s (Barber, 1961), endemic strains of MRSA carrying multiple resistance determinants did not become a worldwide nosocomial problem until the early 1980s (Hryniewicz, 1999). The presence of MRSA in an institution is paralleled by an increased rate of bacteremia or other severe MRSA infections (Harbarth et al., 2000). MRSA-related bacteremia carries a threefold attributable cost and a threefold excess length of hospital stay when compared with methicillin-susceptible S. aureus (MSSA) bac-teremia (Abramson and Sexton, 1999).

Recently, the apparition of endemic community-acquired MRSA (CA-MRSA) has been reported (Naimi et al., 2003; Saiman et al., 2003). In contrast to hospital-acquired MRSA (HA-MRSA), CA-MRSA are frequently isolated from healthy people and strains are generally susceptible to several older, but clinically important antibiotics (Saimen et al., 2003; Vandenesch et al., 2003; Harbarth et al., 2005). Population dynamic analysis of CA-MRSA revealed that this part of the MRSA population is in constant expansion (Carleton et al., 2004) and was only recently detected as an emerging but worldwide infectious concern (Said-Salim et al., 2003; Vandenesch et al., 2003).

The spread of MRSA in health care centers is difficult to control and requires elaborate infection control guidelines (Pittet et al., 1996, 2000; Herwaldt, 2000). The difficulty to eradicate nosocomial MRSA infections may be explained by (i) the presence of an unknown hidden reservoir of MRSA carriers (ii) the emergence of novel highly epidemic S. aureus clonotypes such as EMRSA-15, EMRSA-16, or EMRSA-17 (Cox et al., 1995; Richardson and Reith, 1993; Aucken et al., 2002), with unspecified selective advantages, and/or (iii) failure in enforcement of infection control prescriptions. Some authors reported that the screening of high-risk patients for MRSA colonization was a cost-effective measure for limiting the spread of the organism in hospitals (Papia et al., 1999). As said before, early detection of MRSA carriers is also crucial for therapeutic decisions with last-line antibiotics against MRSA (e.g., glycopeptides and oxazolidinones) (Sakoulas et al., 2002).

An extensive screening of MRSA carriers at hospital admission, despite its important cost, appears to have a major impact in reducing MRSA nosocomial infection rates (Wernitz et al., 2005), as recently shown by Wernitz and colleagues. Indeed, MRSA carriage or colonization is a major risk factor for becoming infected. The preferred colonization sites are the nose, the throat, and the skin surface (Kluytmans et al., 1997). The spread of MRSA occurs generally after contact with carriers (Grundmann et al., 2005) or "MRSA reservoirs" (parts of which are probably unknown). The spread of MRSA in health care centers is difficult to control and requires elaborate infection control guidelines (Cohen et al., 1991; Nettleman et al., 1991; Pittet et al., 1996; Cosseron-Zerbib et al., 1998; Chaix et al., 1999; Papia et al., 1999; Harbarth et al., 2000) including (i) large-scale screening of suspected carriers, (ii) automated computerized alerts, (iii) specific recommendations for at-risk patients, such as contact isolation (Pittet et al., 1996, 1997; Harbarth and Pittet, 1998), and (iv) significant improvement of hand hygiene compliance (Pittet et al., 2000). These data, together with successful containment effort programs (Cohen et al., 1991; Nettleman et al., 1991; Pittet et al., 1996; Cosseron-Zerbib et al., 1998; Chaix et al., 1999; Papia et al., 1999; Harbarth et al., 2000) prompt for screening high-risk patients even in a highly endemic setting (Rubinovitch and Pittet, 2001). Several international guidelines now recommend the screening of potential MRSA-positive patients at hospital admission (BSAC, 1998; KKI, 1999; Muto et al., 2003). However, despite intensive efforts in the application of such guidelines, MRSA spread remains difficult to control.

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