The plasticity of the Staphylococcus aureus genome confers to this specie the ability to gain accessory genes encoding virulence factors as well as antibiotic resistance determinants such as Staphylococcal cassette chromosome mec SCCmec that encodes methicillin-resistance. Methicillin resistance first emerged in hospital-acquired strains originally of successful nosocomial pandemic clones, and is also risen in virulent community-acquired strains containing the Panton-Valentine leucocidin. In 2002, we observed the worrying emergence of a new methicillin-resistant S. aureus (MRSA) clone that contains the tst gene encoding toxic shock syndrome toxin (TSST-1) responsible for both hospital and community-acquired infections. Our study was focused on: (i) new tools for MRSA description, mainly on a SCCmec typing tool, (ii) the characterisation of this new clone by molecular and phenotypic methods, (iii) the epidemiology of this clone, (iv) the pathogenic potential of this clone and the investigation of the genetic features that enhance transmission and virulence, particularly the role of TSST-1. The tst+ MRSA clone is an epidemic clone of genetic background ST5 by multilocus sequence typing that occurs mainly in young people and is responsible for a wide diversity of clinical syndromes including toxin-mediated and suppurative diseases. This clone harbours a peculiar cassette “truncated SCCmec type I” which could play an important role in virulence and transmission but are still under investigation. TSST-1 does not play a determining role (either directly or indirectly) in the pleiotropic pathogenesis of this clone
