Abstract: Utility of a combination of molecular typing methods for characterizing methicillin resistant Staphylococcus aureus (MRSA) isolates and detecting USA 300 (21st Annual Scientific Meeting of the Society for Healthcare Epidemiology of America (April 1-4, 2011))

Sunday, April 3, 2011

Trinity Ballroom (Hilton Anatole)

Jo-anne M. Salangsang, MD, MS , University of Pittsburgh Medical Center, Pittsburgh, PA

Emily E. Ricotta , University of Pittsburgh, Pittsburgh, PA

Jessica L. Gee , University of Pittsburgh, Pittsburgh, PA

Mary M. O'Leary , University of Pittsburgh Medical Center, Pittsburgh, PA

A. William Pasculle, ScD , University of Pittsburgh Medical Center, Pittsburgh, PA

Jane W. Marsh, PhD , University of Pittsburgh, Pittsburgh, PA

Carlene A. Muto, MD, MS , University of Pittsburgh Medical Center, Pittsburgh, PA

Lee H. Harrison, MD , University of Pittsburgh, Pittsburgh, PA

Background : Pulsed field gel electrophoresis (PFGE) is the gold standard for MRSA molecular epidemiology but more objective typing methods are available.

Objective : To determine the utility of combinations of molecular typing methods for categorizing MRSA isolates into traditional healthcare-associated (HCA)-MRSA (SCCmec type II) and community-associated (CA)-MRSA (USA 300), characterizing HCA and SCCmec type IV MRSA isolates, and determining clonal lineage.

Methods : MRSA isolates including a random selection of nasal and clinical isolates, consecutive blood isolates and consecutive colonizing nasal isolates collected from 2/2008 to 8/2009 were investigated. All were SCCmec typed and SCCmec IV isolates underwent SCCmec junkyard region (J) PCR subtyping. All SCCmec IV and a subset of consecutive nasal SCCmec II MRSA isolates underwent PFGE, spa typing, and pvl/arcA detection. A convenience sample of SCCmec IV and the consecutive nasal SCCmec II subset underwent MLST, spa-BURP, and MLST BURST analysis. Sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) of combinations of SCCmec subtype IVa, spa type t008, pvl, and/or arcA positivity in identifying USA 300 vs. non-USA 300 MRSA among SCCmec IV isolates were calculated.

Results: A total of 1124 were typed. SCCmec II HCA-MRSA was predominant, n=757 (67%). SCCmec IV-USA 300 was the second most common strain type, n= 211 (19%); the majority was SCCmec subtype IVa and pvl/arcA (+). The majority in the consecutive SCCmec II subset (n=51) was USA 100 and pvl/arcA (-). A variety of spa types was observed for both USA 300 and USA 100 MRSA, although most were spa t008 and t002, respectively. For SCCmec IV MRSA isolates (n=344), (+) arcA detection alone and in combination with SCCmec-J IVa gave the highest sensitivity in detecting USA 300, followed by combinations with (+) pvl (Table). Combinations with arcA and SCCmec J subtyping followed by pvl had the highest specificity. Most SCCmec IV isolates belonged to spa-clonal complex (CC) 008 and MLST-CC8. The SCCmec II subset was predominantly spa-CC002 and MLST-CC5.

Conclusions : In our hospital, SCCmec typing efficiently identified HCA (SCCmec II) and possible CA-MRSA (SCCmec IV). The addition of SCCmec J subtyping, arcA, and pvl detection had excellent performance characteristics while spa typing had excellent specificity/ PPV but lower sensitivity/NPV for detecting USA 300 isolates among SCCmec IV MRSA. There was good correlation between CCs identified by spa typing and MLST.

429 Utility of a combination of molecular typing methods for characterizing methicillin resistant Staphylococcus aureus (MRSA) isolates and detecting USA 300