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Staphylococcal Cassette Chromosome

Nosocomial S. epidermidis isolates are known for their pronounced resistance to many of our commonly used antibiotics. As in S. aureus, methicillin resistance in S. epidermidis is conferred by SCCmec. However, in contrast to MRSA, little attention is paid to methicillin-resistant S. epidermidis (MRSE) in hospital settings, and it is not combated with the same intensity by hygiene measures as is MRSA. As a result, a high prevalence of resistant isolates is recorded worldwide. Approximately 80% of S. epidermidis isolates from device-associated infections are methi-cillin- and multiresistant, whereas commensal strains obtained from the community are mostly antibiotic-susceptible [105]. A recent study of SCCmec distribution provides evidence that S. epidermidis can harbor all types of SCCmec [106]. Genome sequencing of the methicillin-resistant S. epidermidis RP62A revealed the presence of a type II SCCmec inserted in orfX in the large inverted chromosomal region at the righthand end of the S. epidermidis chromosome (Fig. 9.2). Ninety-eight percent identity of the element on the nucleotide level with S. aureus type II SCCmec is an indication of the capacity for horizontal gene transfer between the two species [17]. This hypothesis is supported by a range of studies showing the existence of primordial SCCmec-like elements in coagulase-negative staphylococci which can be mobilized and transferred to other strains or species [66, 77, 107, 108]. SCCmecs are now regarded as site-specific mobile elements in which extensive recombination and gene shuffling takes place. Obviously, they not only serve as shuttles for the transfer of methicillin resistance, but can also carry other sta-phylococcal genes. Thus, S. epidermidis ATCC 12228 contains an SCC element (named SCCpbp4) which instead of the mec complex harbors the penicillin-binding protein gene pbp4, the teichoic acid biosynthesis gene tagF, a restriction modification system (hsdS, hsdM), and cadmium and mercury resistance genes along with recombinase genes and IS elements [17, 66]. These findings strongly suggest that S. epidermidis and other coagulase-negative staphylococci represent the gene pool in which an ongoing generation of novel SCC types takes place and from which methicillin resistance in S. aureus might originate. In this respect, the genome data underscore the necessity of taking multiresistant S. epidermidis and coa-gulase-negative staphylococci seriously as reservoirs for the spread of resistance genes within microbial communities. As a consequence they should be controlled by appropriate hygiene measures in a similar manner as MRSA.

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