Program Schedule

Prevalence of qac A/B among Methicillin-Resistant Staphylococcus aureus (MRSA) Isolates Recovered from Active Surveillance Cultures of the Anterior Nares in the Setting of Chlorhexidine Bathing

Session: Poster Abstract Session: MRSA and VRE
Thursday, October 9, 2014
Room: The Pennsylvania Convention Center: IDExpo Hall BC

Chlorhexidine (CHG) body washes prevent methicillin-resistant Staphylococcus aureus (MRSA) transmission and nosocomial blood stream infections among intensive care unit (ICU) patients. CHG tolerance can be conferred by qac A/Bresistance genes. The long-term use of CHG bathing may result in the emergence of chlorhexidine-tolerant MRSA strains.

To determine the frequency of qac A/B and high-level mupirocin (MUP) resistance among MRSA isolates recovered from active surveillance cultures from the anterior nares. We compared frequencies before and after the introduction of a CHG daily body wash intervention in a surgical intensive care unit.


Analysis included a random sample of banked MRSA isolates recovered before (2005) and after (2008-2010, 2012) institution of a CHG bathing protocol. PCR was used to detect the presence of qacA/B.  Disk diffusion with a 200 ug mupirocin disk (Oxoid) was used to detect high level MUP resistance; PCR for detection of mupA was performed on all resistant isolates.  SCCmec typing was performed using a multiplex PCR assay for SCCmec types I-V. Power analysis assumed a baseline qacA/B prevalence of 1%; to detect an increase in prevalence of 1% per year with a statistical power of 80% 63 samples per year were required.  


Sixty-three MRSA isolates per year (out of approximately 250 banked isolates per year), were randomly selected. Of the 315 selected isolates, 30 (9.5%) were qac A/B positive and 26 (8.2%) were MUP resistant. There was no significant difference in the prevalence of qac A/B positive MRSA isolates before (2005: 4, 6.3%) and after (2008: 1, 1.5%; 2009: 9, 14%;  2010: 11, 17%; 2012: 5, 8%) use of CHG bathing (p=0.23).The frequency of MUP resistant isolates per year was: 2005: 6 (9%); 2008: 7 (11%); 2009: 4 (6.3%); 2010: 5 (8%); 2012: 4 (6.3%).  For the high-level MUP resistant and/or qac A/B positive isolates for which SCCmec typing was available (n=36), SCCmectype II (24/36, 66%) was predominant.   


Implementation of a daily CHG bathing protocol among surgical ICU patients did not significantly increase the frequency of qac A/B genes in MRSA isolates recovered from the anterior nares over a 6 year period.  The frequency of high level MUP resistance in these strains also remained stable.

Martin Prager, MD, Infectious Diseases, Washington University in St. Louis, St Louis, MO, Meghan Wallace, B.A., Pathology & Immunology, Washington University School of Medicine, St. Louis, MO, Kerry M. Bommarito, PhD, MPH, Division of Infectious Diseases, Washington University School of Medicine, St. Louis, MO, Carey-Ann Burnham, PhD, Pediatrics, Pathology and Immunology, Washington University School of Medicine, St Louis, MO and David K. Warren, MD, MPH, FIDSA, FSHEA, Division of Infectious Diseases, Department of Medicine, Washington University School of Medicine, St. Louis, MO


M. Prager, None

M. Wallace, None

K. M. Bommarito, None

C. A. Burnham, Thermofisher Scientific: Consultant, Consulting fee

D. K. Warren, None

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