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395 Chlorhexidine (CHG) and Mupirocin (MUP) Susceptibility of Methicillin-Resistant Staphylococcus aureus (MRSA) from Long-Term Care Facility (LTCF) Patients

Sunday, April 3, 2011
Trinity Ballroom (Hilton Anatole)
Jennifer Kroeger, MS , The University of Iowa College of Medicine and College of Public Health, Iowa City, IA
Daniel Diekema, MD , University of Iowa College of Medicine and Hospitals and Clinics; Iowa City Veterans' Affairs Medical Center, Iowa City, IA
Courtney Reynolds, MS , School of Social Ecology and Division of Infectious Diseases, University of California Irvine School of Medicine, Irvine, CA
Victor Quan, BA , Division of Infectious Diseases and Health Policy Research Institute, University of California, Irvine School of Medicine, Irvine, CA
Diane Kim, BS , Division of Infectious Diseases and Health Policy Research Institute, University of California, Irvine School of Medicine, Irvine, CA
Ellena Peterson, PhD , Department of Pathology and Laboratory Medicine, University of California School of Medicine, Irvine, CA
Kay Evans, BS , Department of Pathology and Laboratory Medicine, University of California School of Medicine, Irvine, CA
Grace Tan, BA , Department of Pathology and Laboratory Medicine, University of California School of Medicine, Irvine, CA
Mary Hayden, MD , Rush University Medical Center, Chicago, IL
Susan S. Huang, MD, MPH , Division of Infectious Diseases and Health Policy Research Institute, University of California, Irvine School of Medicine, Irvine, CA

Background: CHG is widely used as a surface antimicrobial, for bathing, oropharyngeal care, catheters and catheter-site dressings.  With increasing use comes greater concern for resistance (R) emergence, but standardized CHG susceptibility test methods are lacking.  Recent data suggest the potential for CHG-R associated with presence of qacA/B genes (Batra, et al. Clin Infect Dis 2010;15:210), and failure of decolonization when qacA/B is present in conjunction with low-level MUP resistance (Lee, et al.  50th ICAAC, abstract K-1981).  LTCF patients have high rates of MRSA carriage and infection, and serve as one source to introduce MRSA into acute care facilities. 

Objective: To examine a large collection of MRSA obtained from nares carriers in multiple LTCFs, to determine the rate of CHG-R and low-level MUP-R in the LTCF population.

Methods:   From October 2008 to September 2010, we performed a multicenter observational study of MRSA carriage among residents of 17 LTCFs in Orange County, CA.  A total of 723 MRSA isolates collected during this study were examined. These isolates were obtained from bilateral nares swabs taken from LTCF patients during admission prevalence (N=223) or point prevalence surveys (N=500), up to a maximum of 100 samples per survey per LTCF.  CHG susceptibility testing was performed by broth microdilution method, and qacA/B genes were detected by PCR.  Low-level MUP-R was detected by disk diffusion method using 5 mcg mupirocin disks. 

Results: Only 3 isolates were found to harbor the qacA/B gene loci, each of which carried qacA (0.4%), and all isolates tested had CHG minimum inhibitory concentrations (MICs) <= 4mcg/ml (range, 0.5-4 mcg/ml, MIC50/90, 2/4 mcg/ml). All 3 isolates with the qacA gene had a CHG MIC of 4 mcg/ml.  Low-level MUP-R was more common, found among 84 isolates (12%).  There were no isolates that were both MUP-R and harbored the qacA/B genes.

Conclusions: The qacA/B gene loci were rare among MRSA isolates from these LTCF patients, and no isolates were found to harbor both qacA/B and MUP-R.  The broth dilution CHG testing method clusters MICs in a narrow range between 0.5-4 mcg/ml.  More study is needed to better understand mechanisms of CHG-R, and to develop standardized methods of in vitro testing that correlate with in vivo activity.