Staphylococcus aureus: Not always right under your nose.

From Rush University Medical Center and Stroger Hospital of Cook County, Chicago, Illinois. Received February 11, 2009; accepted February 17, 2009; electronically published July 1, 2009. Infect Control Hosp Epidemiol 2009; 30:727-729 2009 by The Society for Healthcare Epidemiology of America. All rights reserved. 0899-823X/2009/3008-0002$15.00. DOI: 10.1086/598339 Methicillin-resistant Staphylococcus aureus (MRSA) infections are frequent causes of nosocomial infection in the hospital, especially in intensive care units. Patients in burn trauma units are at increased risk for both colonization and infection with MRSA, likely because of alteration of the normal skin barrier and decreased immune function. Burn units may serve as reservoirs for the spread of MRSA to other patients via contamination of the environment and of the hands of healthcare workers. Because of the high morbidity of MRSA infection in this population, an improved understanding of the colonization dynamics and pathway to subsequent infection is needed to prevent infection and to better inform infection control efforts. S. aureus nasal colonization among the general population can be persistent, intermittent, or never present, with colonization often preceding infection. A study that surveyed the general United States population observed that, although overall prevalence of nasal colonization with S. aureus (ie, all strains of S. aureus, both methicillin susceptible and methicillin resistant) decreased from 32.4% to 28.6% from the period 2001–2002 to the period 2003–2004, MRSA colonization increased from 0.8% to 1.5%. Certain characteristics increase the risk for nasal colonization and infection with MRSA, including hemodialysis, human immunodeficiency virus (HIV) infection, residence in long-term care facilities, previous antibiotic exposure, and prior hospitalization or surgery. Although colonization of the anterior nares seems to be predominant, MRSA colonization can occur at other sites, that is, axillae, inguinal regions, oropharynx, pressure sores, and gastrointestinal tract. Therefore, surveillance strategies assessing nasal colonization may overlook a proportion of MRSA carriers. In intensive care units, a large proportion of S. aureus carriers are colonized in the nares at admission. In surgical intensive care units, nosocomial acquisition of MRSA colonization has been associated with having tracheostomy tubes, decubitus ulcers, or enteral nutrition. Burn patients may be more likely to acquire MRSA, compared with other ill surgical patients. Recently, infections due to USA300 strains—the predominant clone of community-associated MRSA in the United States by pulsed-field gel electrophoresis—have been observed in burn units, both at admission and as nosocomial infections. With the epidemic spread of this new strain of MRSA, understanding colonization—nasal and extranasal—is essential. The study by Reighard et al in this issue evaluates nasal and extranasal colonization or infection among patients in a burn trauma unit. The 5-year study includes a case-control analysis between patients with S. aureus nasal colonization who had concurrent or subsequent recovery of S. aureus isolates from another body site (case patients) versus patients with S. aureus nasal colonization only (control subjects). Prolonged hospitalization, diabetes, and residence in long-term care facilities were significant risk factors for S. aureus isolation at extranasal sites. Nasal colonization with MRSA was more common among case patients than among control subjects, with 2 case patients having USA300 strains isolated from both nasal and extranasal sources. Furthermore, on the basis of pulsed-field gel electrophoresis patterns, 89.5% of case patients had nasal isolates that were similar to isolates from extranasal sites. This study raises several interesting questions with regard to S. aureus colonization. A large proportion of enrolled patients were found to be colonized with S. aureus within 48 hours after unit admission (89.5% of case patients and 55.6% of control subjects). Nasal colonization with MRSA occurred in most patients who had positive culture results for extranasal samples (ie, case patients)—89.5% of case patients versus 53.1% of control subjects. Furthermore, 57.9% of case patients had extranasal colonization or infection within 48 hours after unit admission. The colonization burden at admission may reflect patients with particular comorbidities, prior healthcare exposures (case patients were more likely to not be admitted from home and to have had long-term care facility residence), or characteristics specific to burn trauma patients. The findings of Reighard et al also highlight the importance of adhering to standard infection control practices, es-