From the Editor

Friday, December 23, 2016 Published in On-Demand Summer 2011 Written by David Persing M.D., Ph.D., Chief Medical and Technology Officer, Cepheid

This edition of the Newsletter focuses on the challenges of detecting methicillin-susceptible Staphylococcus aureus (SA) and methicillin-resistant S. aureus (MRSA) by using molecular assays. Several companies today offer a range of products for SA and MRSA, so users understandably may be confused as to which assay to use. Dr. Ellen Jo Baron explains some of the obstacles that test manufacturers face when developing assays and keeping them current as this elusive microbe changes under our noses (pun unintended) in ways that could affect test performance. Her article highlights the differences between the two Cepheid nasal surveillance assays and presents our opinion on the optimal use of each assay. In fact, there is even a quiz for you to assess your understanding.

Decisions, decisions: Which Staphylococcus Surveillance Assay Should You Use?

Friday, December 23, 2016 Published in On-Demand Summer 2011 Written by Ellen Jo Baron, Ph.D., D(ABMM), Prof. Emerita, Stanford University Director of Medical Affairs, Cepheid

QUIZ: How should this result be reported to your clinicians?

No two healthcare systems are alike, nor are the patient-care needs of your clinicians, so Cepheid offers choices for detecting both staphylococcal infections and staphylococcal colonization. In fact, there are more FDA-cleared Staphylococcus aureus (SA) and methicillin-resistant Staphylococcus aureus (MRSA) assays available for GeneXpert® System than for any other diagnostic platform. Although it is more obvious in which situations the Xpert® MRSA/SA Blood Culture Assay or the skin and soft tissue infection (SSTI) assay (Xpert® MRSA/SA SSTI) should be used, the choice between the two nasal surveillance assays is less clear. The performance characteristics of each assay are slightly different, so each assay has a unique place in an overall surveillance program. The original surveillance tool, the Xpert® MRSA assay, is the most utilized molecular MRSA surveillance method in the U.S. This assay detects only a single staphylococcal target, which is the junction where the staphylococcal cassette chromosome mec (SCCmec) [i.e., the genetic element that contains the mecA gene, which confers resistance to the extended-spectrum penicillins and cephalosporins], integrates into the S. aureus chromosome. Published data suggest that decades ago, a wild-type S. aureus strain [i.e., one susceptible to oxacillin, methicillin, and other extended-spectrum beta-lactam agents] acquired a large DNA segment containing mecA and the genes that enable it to integrate into another DNA molecule, from another organism (probably Staphylococcus sciuri). Once the DNA cassette was integrated into the chromosome, the mecA gene proved to be fully functional and the newly resistant organism began to spread.1 The integration site of the cassette into the staphylococcal genome is located in the middle of open reading frame X (orfX), a genetic region of unknown function, but one that is unique to S. aureus strains. SCCmec, with its internal mecA gene, inserts into the orfX and in so doing, splits orfX into two parts. The Xpert MRSA surveillance assay targets one set of primers that bind to orfX in the wild type staphylococcal chromosome and additional primers that bind to the outer edge of the newly incorporated SCCmec element, bridging the integrated DNA and chromosomal DNA, insuring that the SCCmec element is present in an S. aureus strain and not some other staphylococcal species. This target is called the SCCmec/orfX junction region (Figure 1).

Prize-Winning Case Report

Friday, December 23, 2016 Published in On-Demand Summer 2011 Written by Ellen Jo Baron, Ph.D., D(ABMM), Prof. Emerita, Stanford University Director of Medical Affairs, Cepheid

From Dr. Anne Dubouix, Director of Laboratoire de Microbiologie, Clinique de L'Union, Toulouse, France. (Figure 1)

A 44-year-old male presented to the Emergency Department of our institution on November 29, 2009 with inability to ambulate and pain in the right knee. On examination, the patient had a large knee effusion with significant erythema.

The patient reported that his problems with his knee had started nearly two weeks before he finally presented to the hospital. He also stated that because he had been previously diagnosed with sinusitis, he had been taking pristinamycin (similar to quinupristin/dalfopristin, trade name Synercid®) for the previous 8 days as prescribed by his general practitioner.

Laboratory tests revealed a slightly above-normal leukocyte count (11,300/mm3) mainly due to an increase in polymorphonuclear leukocytes (9,800/mm3) and an elevated C-reactive protein (230 mg/l).

Summaries of selected abstracts presented in Europe recently on the use of GeneXpert® MRSA/SA SSTI

Friday, December 23, 2016 Published in On-Demand Summer 2011 Written by Ellen Jo Baron, Ph.D., D(ABMM), Prof. Emerita, Stanford University Director of Medical Affairs, Cepheid