Advanced Techniques in Diagnostic Microbiology

412 P. Francois and J. Schrenzel
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.
Molecular Epidemiology
Molecular techniques dedicated to bacterial detection and identification have been
recently reviewed (Nolte et al., 2003; Diekema et al., 2004). In the case of MRSA,
the mecA gene encoding for the low-affinity penicillin-binding protein PBP2 ′ is
the genetic basis of methicillin resistance in MRSA isolates. This gene, originating
from a mobile genetic element designated SCCmec [staphylococcal cassette
chromosome mec (Katayama et al., 2003)], flanked by terminal inverted and direct
repeats (Ito et al., 2001), is invariably inserted into the orfX gene of S. aureus chromosome
(Fig. 24.1). This element contains two site-specific cassette chromosome
recombinases, ccrA and ccrB, responsible for the precise excision and integration
of SCCmec within the bacterial chromosome (Katayama et al., 2003).
To date, five differently organized SCCmec elements have been characterized
(Ito et al., 2003). Three types of SCCmec elements are typically found in HA-
MRSA strains (i) type I, a 34-kb element that was prevalent in MRSA isolates in
the 1960s, (ii) type II, a 53-kb element that was identified in 1982 and is ubiquitous
in Japan, Korea, and the United states, and (iii) type III, the largest (67-Kb) element,
identified in 1985, currently prevalent in Germany, Austria, India, and other South
Asian and Pacific areas (Hiramatsu et al., 2001; Ito et al., 2003). In contrast to