Dominance of EMRSA-15 and -16 among MRSA causing nosocomial bacteraemia in the UK: analysis of isolates from the European Antimicrobial Resistance Surveillance System (EARSS).

Abstract

Sir, The scale of the public health problem posed by methicillin-resistant Staphylococcus aureus (MRSA) in England and Wales increased significantly during the 1990s. Most dramatically, the proportion of cases of S. aureus bacteraemia due to MRSA increased from 1–2% in 1990–1992 to c. 40% in 2000. This increase coincided with the emergence of two particular epidemic MRSA (EMRSA) strains, EMRSA-15 and EMRSA-16, which now dominate MRSA submissions to the S. aureus Reference Service (SaRS) of the Laboratory of Hospital Infection. Nevertheless, EMRSA-15 and -16 have not formally been shown to be the principal agents of MRSA bacteraemia in the UK. Increasing MRSA prevalence rates have also been documented in many other countries. In response to widespread concern about resistance, the European Commission (Directorate General V) has funded pan-European surveillance of resistance. This programme, designated the European Antimicrobial Resistance Surveillance System (EARSS), currently has participants from all countries of the EU, plus Iceland, Norway and Switzerland. Since its commencement in late 1998, EARSS has focused on MRSA from nosocomial bacteraemia and penicillin-resistant pneumococci from community-acquired bacteraemia or meningitis. In the participating countries, data are collected from sentinel laboratories by national co-ordinators, then submitted to a central database maintained at the National Institute of Public Health and the Environment (RIVM), Bilthoven, The Netherlands. As methods of susceptibility testing vary between participating laboratories, the laboratories are encouraged, where possible, to obtain MIC data. In the UK, 26 geographically diverse hospitals are currently submitting data to EARSS. These hospitals record all their S. aureus bacteraemias and send all isolates identified as MRSA to the PHLS Antibiotic Resistance Monitoring and Reference Laboratory (ARMRL) for confirmation of resistance. This is undertaken by the agar incorporation method, and MICs are determined not only for methicillin, but for a range of other antibiotics including vancomycin, teicoplanin, erythromycin, ciprofloxacin, gentamicin, fusidic acid and rifampicin. Between late 1998 and the second quarter of 2000, 591 MRSA isolates were received by ARMRL under the aegis of EARSS, representing 37% of all S. aureus from bacteraemias reported by the participating laboratories. This prevalence rate is comparable to that among all S. aureus bacteraemias reported to the PHLS over approximately the same period. Comparative MRSA prevalence data for other countries participating in EARSS can be obtained from the EARSS website. All MRSA isolates submitted to ARMRL were additionally characterized by phage typing in the SaRS, which showed that 60.2% were EMRSA-15 and 35.4% were EMRSA-16. Isolates of EMRSA-15 were encountered in 25 of the 26 participating hospitals and EMRSA-16 occurred in 19. Although EMRSA-15 and -16 were already known to occur in many hospitals throughout the UK, our finding that they comprised 95.6% of MRSA from cases of bacteraemia is the first direct confirmation of their predominant role in bloodstream infections. This raises the intriguing question of whether the increase in the prevalence of MRSA bacteraemia reflects greater virulence of EMRSA-15 and -16, or simply that, by virtue of being among the most prevalent S. aureus strains in or on colonized patients, EMRSA-15 and -16 are among the strains most likely to cause endogenous bacteraemia. Further work on the epidemiology and pathogenicity of EMRSA-15 and -16 may give further insight into the success of EMRSA-15 and -16 in invasive disease.