LB 13 Incidence of qacA/B genes in Methicillin-resistant Staphylococcus aureus Isolated during 2010 in a Regional U.S. Surveillance Network

Sunday, April 3, 2011: 3:15 PM
Coronado BCD (Hilton Anatole)
Patrick McGann, PhD , Walter Reed Army Institute of Research, Silver Spring, MD
Paige Waterman, MD , Walter Reed Army Institute of Research, Silver Spring, MD
James Cummings, MD , Walter Reed Army Institute of Research, Silver Spring, MD
David Craft, PhD , Walter Reed Army Institute of Research, Silver Spring, MD
Emil Lesho, DO , Walter Reed Army Institute of Research, Silver Spring, MD
Background: qacA/B genes are plasmid-borne and encode multidrug efflux pumps and resistance to surface antiseptics such as chlorhexidine.  These antiseptics are increasingly being used to prevent methicillin-resistant Staphylococcus aureus (MRSA) infection and transmission.  Data on the prevalence and incidence of qacA/B genes in MRSA isolates from U.S. healthcare facilities are limited.

Objective: Determine the incidence of qacA/B in MRSA isolated in 2010 from a regional surveillance network in the eastern U.S.

Methods: All unique (one isolate per patient) MRSA isolated from clinical infections or surveillance activities from January to December 2010 were screened for the presence of qacA/B genes using real-time (RT) PCR and species-specific primers optimized to address recent concerns  (Clin Infect Dis 2010;50:218-20). Two sets of primers for each gene were designed using Primer Express 2.0 (Life Technologies, CA) based on gene sequences deposited at Genbank and used at a final concentration of 1 µM.  Amplification was performed in 20 μl volumes (1X SYBR Green Master Mix (Roche Applied Sciences, Indianapolis, IN), 1 µM forward and reverse primers, 2 μl template DNA) on a Light Cycler 480 (Roche Applied Sciences, IN) using a 96 well plate format.  Amplification conditions consisted of an activation step at 95oC for 5 minutes, followed by 40 cycles of 95oC for 10 seconds, 56oC for 10 seconds, and 72oC for 10 seconds. Melting temperature calling and crossing point values were calculated using the melting curve analysis software and 2nd derivative maximum method respectively, as outlined by Roche Applied Sciences, IN. 

Results: Four community hospitals and one major referral hospital submitted a total of 262 MRSA isolates to the surveillance network. This represented all MRSA that were identified by culture (non-molecular) methodology.  MRSA were confirmed by the presence of femA and mecA by RT-PCR in all 262 isolates.  All were screened for the presence of qacA/B, and none contained these genotypes by RT-PCR.

Conclusions: Despite increasing selection pressure from the widespread use of surface antiseptics, and despite their ease of plasmid-borne transmission, qacA/B genes do not yet appear to have entered the population from which the most resent isolates in our repository were recovered.  To our knowledge, our screening represents the largest effort in the U.S. to date.  The absence of qacA/B is consistent the only other report in the U.S. we could find (Antimicrob Agents Chemo 2007;51:3235-9).  We are broadening our screening to include our earliest archived isolates (2003) and newly submitted isolates from more hospitals, as our surveillance network grows.  Prospective qacA/B screening will become part of our future surveillance and characterization efforts.