Changes in the molecular epidemiological characteristics of methicillin-resistant Staphylococcus aureus in a neonatal intensive care unit.

Objective: To determine whether the molecular epidemiological characteristics of methicillin-resistant Staphylococcus aureus (MRSA) had changed in a level III neonatal intensive care unit (NICU).

Design: Retrospective review of medical records.

Setting: Level III NICU of a university-affiliated children's hospital in New York, New York.

Patients: Case patients were neonates hospitalized in the NICU who were colonized or infected with MRSA.

Methods: Rates of colonization and infection with MRSA during the period from 2000 through 2008 were assessed. Staphylococcal chromosomal cassette (SCC) mecA analysis and genotyping for S. aureus encoding protein A (spa) were performed on representative MRSA isolates from each clonal pulsed-field gel electrophoresis pattern.

Results: Endemic MRSA infection and colonization occurred throughout the study period, which was punctuated by 4 epidemiologic investigations during outbreak periods. During the study period, 93 neonates were infected and 167 were colonized with MRSA. Surveillance cultures were performed for 1,336 neonates during outbreak investigations, and 115 (8.6%) neonates had MRSA-positive culture results. During 2001-2004, healthcare-associated MRSA clones, carrying SCC mec type II, predominated. From 2005 on, most MRSA clones were community-associated MRSA with SCC mec type IV, and in 2007, USA300 emerged as the principal clone.

Conclusions: Molecular analysis demonstrated a shift from healthcare-associated MRSA (2001-2004) to community-associated MRSA (2005-2008).