Understanding and controlling methicillin-resistant Staphylococcus aureus infections.

Abstract

During the past four decades, methicillin-resistant Staphylococcus aureus (MRSA) has spread throughout the world and has become highly endemic in many geographic areas. Recent studies of methicillin-sensitive S. aureus (MSSA) and MRSA isolates collected over many years and analyzed by multilocus sequence typing (MLST), staphylococcal cassette chromosome mec (SCCmec) typing, and use of a computerized algorithm based on related sequence types (BURST) have revealed that the evolution and spread of MRSA occurred because of the introduction of the mobile SCCmec element into several different clones of MSSA.1 Early MRSA strains appear to have developed from a MSSA strain (ST250-MSSA) that was prevalent in European countries, including Denmark, in the 1950s.1,2 Of interest, this MSSA ancestor to early MRSA strains is no longer prevalent among disease-associated isolates. More recent epidemic strains of MRSA (EMRSA-2, -6, -7, -12, -13, and -14) that spread effectively in hospitals during the 1980s and 1990s are indistinguishable by MLST, suggesting that a relatively small number of MRSA clones have unique qualities that facilitate their transmission over wide geographic areas. For example, EMRSA clones ST8-MRSAIII and ST239-MRSA-III have been recovered from patients in Finland, France, Germany, the Netherlands, Sweden, the United Kingdom, and the United States.1 One can conclude from the above findings that the occurrence of epidemics or high levels of endemicity observed in a given geographic area can be explained, at least in part, by whether strains with epidemic potential are circulating in healthcare facilities. In this issue of Infection Control and Hospital Epidemiology, Vriens et al.3 note that although the prevalence of MRSA is still less than 1% in the Netherlands, occasionally strains of MRSA imported from other countries have spread rapidly between patients and healthcare workers in their surgical intensive care unit (ICU). The outbreaks occurred despite the fact that virtually all hospitals in the Netherlands routinely use aggressive measures to control the spread of MRSA. These measures include placing in isolation any patient transferred from a hospital outside the Netherlands, discontinuing isolation of such patients only after screening cultures have revealed that they are not colonized with MRSA, routinely screening all patients and personnel exposed to a patient with MRSA with treatment (decolonization) of any carriers, and wearing a mask, a gown, and gloves whenever entering the room of a patient with MRSA.3,4 The authors questioned whether the ability of MRSA to spread rapidly in the surgical ICU was unique, or whether similar spread of MSSA strains was going undetected. To examine this issue, a 2month, prospective, observational study was conducted to determine the frequency of transmission of MSSA in the surgical ICU. The study involved weekly screening cultures of patients and personnel, prospective recording of the number and type of contacts that each healthcare worker had with patients, and pulsed-field gel electrophoresis (PFGE) of MSSA isolates recovered from patients and personnel. The prevalence of MSSA among patients (24%) and healthcare workers (22%) was similar to what has been described in many other publications. Among the more than 4,100 contacts between patients and healthcare workers, in only 21 instances did both the patient and the healthcare worker carry MSSA. PFGE typing revealed that none of the healthcare workers had the same strain of MSSA as the patients with whom they had contact. Furthermore, none of the patients were colonized with the same strain. Concurrent study of nosocomial transmission of MRSA was not performed. Instead, the authors compared their find-