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
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.
K. M. Bommarito, None
C. A. Burnham, Thermofisher Scientific: Consultant, Consulting fee
D. K. Warren, None