Heterogeneity of Methicillin-SusceptibleStaphylococcus aureusStrains at a German University Hospital Implicates the Circulating-Strain Pool as a Potential Source of Emerging Methicillin-ResistantS. aureusClones

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Abstract
Recently, we demonstrated rapid dissemination of different methicillin-resistantStaphylococcus aureus(MRSA) clones at the Institute for Microbiology at the University of Magdeburg (B. Ghebremedhin, W. König, and B. König, Eur. J. Clin. Microbiol. Infect. Dis.24:388-398, 2005). The majority of them harbored the readily transmissiblemeccassette type IV. Thus, theoretically, methicillin-susceptibleStaphylococcus aureus(MSSA) might capture themecAgene from circulating MRSA, or MRSA strains might catch mobile toxin genes from MSSA. Therefore, we characterized MSSA strains circulating at the University Hospital in Magdeburg. Among a total of 84 MSSA strains under study, about 40% possessed thetst(toxic shock syndrome toxin) gene and up to four additional enterotoxin genes.tst-positive MSSA strains belonged to all knownagrgroups (I to IV) and to 14 differentspatypes (t008, t012, t015, t019, t024, t056, t065, t127, t133, t162, t271, t287, t399, and t400), and they were classified by multilocus sequence typing (MLST) as ST1, ST8, ST30, ST39, ST45, ST101, ST121, ST395, and ST426. In contrast, simultaneously circulating MRSA strains (n= 24) harbored in general two or three genes of the enterotoxin gene cluster, and thetst-positive MRSA isolates belonged to the well-known epidemic types ST22, ST45, and ST228 and were classified asspatypes t001, t028, and t032. From our results, one may conclude that the pool of circulating MSSA strains is an important parameter with regard to the epidemiology of hospital- and community-acquired MRSA clones and their potential virulence.

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