MRSA: We Can Overcome, But Who Will Lead the Battle?

Abstract

During the past two decades, methicillin-resistant Staphylococcus aureus (MRSA) has become the most prevalent and important antimicrobial-resistant pathogen, causing serious nosocomial and community-acquired infections. This trend continues unabated today, now involving additional classes of antimicrobial agents such as glycopeptides.1 The emergence of MRSA is primarily due to the successful spread of a limited number of clones of S. aureus representing the most prevalent lineages within its population structure2 that have acquired one of the resistance-encoding SSCmec elements. Hospitals and other healthcare institutions have been the primary sites where such strains are transmitted and are “breeding” sites where MRSA can maintain its highest reproductive rate. High rates of transmission are the consequence of patients’ increased susceptibility to acquisition combined with crowding and high rates of contact with healthcare workers (HCWs), who are considered important vectors in the chain of MRSA transmission. The continuing spread of MRSA indicates that recommended preventive strategies have been either inadequate or improperly implemented. For years, MRSA-positive patients and HCWs have been cycled back into the community, where transmission continues, albeit at a lower rate. In the late 1950s and early 1960s, much attention was focused on elucidating the determinants of S. aureus carriage, dispersion, and transmission.3 We can learn from that experience that transmission will occur within households or other close communities and that colonization of patients will not end spontaneously in all patients. In 1964, Noble et al. found that acquisition of new, resistant strains was enhanced by the use of antibiotics and that nosocomially acquired strains were detected again at readmission after more than 150 days.4 Hare and Thomas, in 1956, published an article on the routes of S. aureus transmission. Three routes were implicated: (1) transport of S. aureus by hands, handkerchiefs, clothing, bedding, or any other object coming into contact with the nose or skin of infected individuals; (2) the release of S. aureus into the atmosphere as a result of friction and dislodgment of dried particles from the skin and hair; and (3) transport of “infected particles” by air currents to other individuals. Of particular note, droplet transmission was not found to be a significant route of transmission.5 From a follow-up study, they learned that S. aureus carriers can contaminate their clothes and surroundings and that most have enough S. aureus cells on various sites of their skin and clothing to render them capable of acting as donors of infection.6 Antibiotic pressure may act as an enhancing factor for transmission of resistant S. aureus, as was shown in 1964 by Ehrenkranz.7 From these 40to 50-year-old studies, one can conclude that multiple reservoirs and routes of transmission exist when considering the epidemiology of S. aureus, regardless of whether they are resistant to antibiotics. Currently, we are confronted with many unidentified and untreated sources of MRSA, including asymptomatic carriers who are spreading MRSA into the community, possibly to (future) patients and to HCWs. When those individuals are (re)admitted or return to work in a healthcare setting, their MRSA carriage is usually unrecognized and they may thus contribute inadvertently to the nosocomial spread of MRSA. In regions where MRSA is highly endemic, it is difficult to determine exactly when, where, and to whom transmission occurs. By contrast, in areas where the prevalence of MRSA is kept low, it is possible to determine all patients and HCWs who have been in contact with an identified MRSA carrier and to test all contacts for MRSA carriage. This search for contacts together with isolation and treatment of carriers are cornerstones of the Dutch “search and destroy” policy that has been so successful.8,9 An important component of this approach is having a highly sensitive screen for detecting secondary carriers (ie, one should apply a broad definition of what constitutes a MRSA contact). In a hospital, in addition to roommates, all HCWs and patients on a ward who have had direct or indirect contact with a MRSA carrier are at risk for transmission; they all are at risk of acquiring MRSA. Using a sensitive culture method is likewise important for maintaining a highly sensitive screen for MRSA; this usually requires use of a selective enrichment broth.10 The method of isolation chosen should exploit the knowledge gained during the past 50 years regarding the epidemiologic behavior of S. aureus. This means that patients should be placed in a separate