JCM Accepts, published online ahead of print on 28 May 2008

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J. Clin. Microbiol. doi:10.1128/JCM.00154-08
Copyright (c) 2008, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Molecular Epidemiology of Methicillin-Resistant Staphylococcus aureus (MRSA) Bloodstream Isolates in Urban Detroit

Thea Chua, Carol L. Moore, Mary B. Perri, Susan M. Donabedian, William Masch, Dora Vager, Susan L. Davis, Kaitlin Lulek, Benjamin Zimnicki, and Marcus J. Zervos^*

Henry Ford Health System, Detroit, MI; Boston University School of Medicine, Boston, MA; Eugene Applebaum College of Pharmacy and Health Sciences, Wayne State University School of Medicine, Detroit, MI

^* To whom correspondence should be addressed. Email: mzervos1{at}hfhs.org.

To gain a better understanding of epidemiology of resistance in Staphylococcus aureus, we describe the molecular epidemiology of methicillin-resistant Staphylococcus aureus bloodstream isolates in Urban Detroit. Bloodstream isolates from July 2005 to February 2007 were characterized. Two hundred and ten bloodstream isolates from 201 patients were evaluated. Patient characteristics were: median age 54 years, 56% male, 71% African-American. Seventy-six percent of infections were health care-associated, with 55% community onset and 21% hospital acquired, and 24% were community-associated. The most common sources were skin/wound (25%), central venous catheters (24%), unknown source (20%) and endocarditis (9%). Ninety percent and 5 % of isolates had MIC to vancomycin of 1.0 mg/L, using automated dilution testing and E-test, respectively. Six percent of isolates showed heteroresistance to vancomycin, all occurring with isolates having vancomycin E-test MIC 1.5 mg/L. Results of PFGE showed seventeen strain types. The predominant strains were USA100 (104 isolates) and USA300 (74 isolates). Forty-nine percent of the isolates were SCCmec II and 56% were agr II. All USA300 isolates were positive for the PVL toxin genes and agr I. Forty-seven percent of USA300 bloodstream infections were health care-associated (35% community onset and 12% hospital onset). USA300 strains were more common in injection drug users with skin/wound as the predominant source of infection. Thirty percent of the USA100 strains were closely related to vancomycin-resistant Staphylococcus aureus isolates. The results of this study show that vancomycin MICs using automated dilution testing with Vitek-2 and E-test is highly discordant. Most MRSA causing bacteremia are health care-associated, commonly have MICs to vancomycin that are high within the susceptible range, and are not detected by routine automated dilution testing and have significant diversity of molecular characteristics. USA100 strains that are closely related to VRSA isolates and USA300 strains are common as causes of both hospital and community onset infection. Infection control measures should focus not only on prevention of spread of community strains in the hospital but also prevention of spread of hospital strains associated with VRSA into the community.

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