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Staphylococcus epidermidis

S. epidermidis is primarily a normal inhabitant of the healthy human skin and mucosal microflora. In recent decades, however, the bacterium has emerged as a common cause of nosocomial infections. These infections usually occur in association with the use of medical devices, and they preferentially affect immuno-compromised and critically ill patients, causing acute bacteremia and septicemia. From the molecular point of view it is still unclear why S. epidermidis was so successful in becoming established as a nosocomial pathogen. The most interesting question in this respect is whether pathogenic strains obtained from device-associated infections or the hospital environment differ genetically and/or physiologically from commensal isolates outside of hospitals. Currently, the complete annotated genome sequences of two S. epidermidis strains are publicly available: one from S. epidermidis ATCC 12228, a laboratory reference strain used for antibiotic resistance testings [102], and the other from S. epidermidis RP62A (ATCC 35984), a clinical strain isolated from an intravascular catheter-associated sepsis [17].

Fig. 9.2 Pairwise comparison of the Staphylococcus epidermidis ATCC 12228 and Staphylococcus epidermidis RP62A genomes displayed using the Artemis Comparison Tool (ACT; http:// www.sanger.ac.uk/Software/ACT). The red bars represent homologous matches between the genomes and the blue bars indicate a homologous, but inverted chromosomal region.

Genomic islands, IS elements, phages, SCC cassettes, and genes involved in adherence and biofilm formation are marked as colored boxes. Gene symbols: aae, autolysin/adhesin; aap, accumulation-associated protein; atlE, autolysin; bhp, Bap homologous protein; ica, intercellular adhesin. (This figure also appears with the color plates.)

Fig. 9.2 Pairwise comparison of the Staphylococcus epidermidis ATCC 12228 and Staphylococcus epidermidis RP62A genomes displayed using the Artemis Comparison Tool (ACT; http:// www.sanger.ac.uk/Software/ACT). The red bars represent homologous matches between the genomes and the blue bars indicate a homologous, but inverted chromosomal region.

Genomic islands, IS elements, phages, SCC cassettes, and genes involved in adherence and biofilm formation are marked as colored boxes. Gene symbols: aae, autolysin/adhesin; aap, accumulation-associated protein; atlE, autolysin; bhp, Bap homologous protein; ica, intercellular adhesin. (This figure also appears with the color plates.)

Whole-genome analysis of both strains revealed a genome size of approximately 2.499 Mbp for ATCC1228 and 2.616 Mbp for RP62A, coding for 2381 and 2553 ORFs, respectively. Direct comparison of the S. epidermidis RP62A and ATCC12228 sequences indicates over a broad extent a very uniform overall genome organization (Fig. 9.2). Variation in genome size and gene content is mainly due to the insertion of a prophage in S. epidermidis RP62A and differences in terms of other mobile elements such as genomic islands, transposons, and insertion sequences. Figure 9.2 shows on the right a large chromosomal region which is inverted (colored blue). Interestingly, this part of the chromosome contains the attachment site (orfX) for the SCCmec cassettes and a range of genes involved in adherence and biofilm formation (see below). It is tempting to speculate that this part of the S. epidermidis genome is subject to frequent recombination events and acquisition of variable resistance and virulence traits.

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