365. ACME, a USA300 Virulence Element, Is Present in MSSA Isolates from Uncomplicated Skin and Soft Tissue Infections (uSSTIs) in a Multicenter Trial (DMID Prot. 07-0051)
Session: Poster Abstract Session: MRSA, MSSA, Enterococci
Thursday, October 3, 2013
Room: The Moscone Center: Poster Hall C
Background:

The Arginine Catabolic Mobile Element (ACME) may contribute to the success of epidemic USA300 MRSA, the predominant cause of uSSTIs in the U.S. The arcA gene, a marker for ACME, can enhance the ability of USA300 to survive on human skin. Virtually all USA300 MRSA strains carry both ACME and the Panton Valentine leukocidin genes (pvl). USA300 MRSA is almost always ST8 by multi-locus sequence typing (MLST). However, little is known about the genetic backgrounds of MSSA isolates causing uSSTIs and whether ST8 MSSA carry arcA (ACME) and pvl

Methods:

At 4 U.S. medical centers in 2009-2011, adult and pediatric subjects with large uSSTIs, multiple uSSTIs, or nonpurulent cellulitis were enrolled in a double-blinded, randomized trial comparing clindamycin or trimethoprim-sulfamethoxazole therapy.  Purulent lesions were cultured at enrollment. All S. aureus isolates from uSSTIs underwent PCR assays for pvl, arcA, and mecA. Isolates positive for all 3 were considered to be USA300 MRSA by criteria that we have previously demonstrated; the rest underwent MLST. mecA+ isolates were considered to be MRSA and underwent SCCmec typing.

Results:

Of 524 subjects, 211 (40%) had purulent lesions at enrollment that grew S. aureus; 44 (21%) grew MSSA and 159 (75%) grew MRSA. Eight other subjects (4%) had both MSSA and MRSA isolated and were excluded from analysis. Among the MSSA isolates, 84% (37/44) were ST8 and 68% (30/44) were pvl+. 13% (4/30) of pvl+ MSSA isolates were also arcA+; all were ST8 and from a single center in Chicago where 85% (17/20) of MSSA were pvl+ and 20% (4/20) of MSSA were both pvl+ and arcA+. 94% of MRSA isolates (149/159) were pvl+ and arcA+ and were assigned to pulsotype USA300. Of non-USA300 MRSA isolates, 90% (9/10) were pvl+, ST8, SCCmec type IV and lacked the arcA gene. A single MRSA isolate was pvl+, ST30, SCCmec type IV and lacked the arcA gene.

Conclusion:

1) Nearly two-thirds of MSSA isolates from uSSTIs were pvl+, 84% were ST8, and 13% were ST8, pvl+ and carried arcA, a proxy for ACME, a putative virulence element in USA300 MRSA. The ACME element may thus also confer a fitness advantage to these MSSA isolates. 2) As expected, USA300 MRSA was the most common genetic background of S. aureus isolated from uSSTIs in this trial at all 4 centers.

Michael Z David, MD PhD1, Henry F. Chambers, MD2, Loren Miller, MD, MPH3, C. Buddy Creech, MD, MPH4, Susan Boyle-Vavra, PhD5, Neha Kumar, M.D.6, Julia Sieth, B.A.7 and Robert Daum, MD6, (1)The University of Chicago, Chicago, IL, (2)University of California, San Francisco General Hospital, San Francisco, CA, (3)Harbor-University of California, Los Angeles Medical Center, Torrance, CA, (4)Pediatric Infectious Diseases and Vanderbilt Vaccine Research Program, Vanderbilt University School of Medicine, Nashville, TN, (5)Pediatrics, Infectious Disease, The University of Chicago, Chicago, IL, (6)Pediatrics, University of Chicago, Chicago, IL, (7)University of Chicago, Chicago, IL

Disclosures:

M. Z. David, None

H. F. Chambers, None

L. Miller, None

C. B. Creech, None

S. Boyle-Vavra, None

N. Kumar, None

J. Sieth, None

R. Daum, None

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