Identification of Methicillin-Resistant or Methicillin-Susceptible Staphylococcus aureus in Blood Cultures and Wound Swabs by GeneXpert

Abstract

Letters to the Editor Identification of Methicillin-Resistant or Methicillin-Susceptible Staphylococcus aureus in Blood Cultures and Wound Swabs by GeneXpert ᰔ Until a decade ago, clinicians could use epidemiological clues to select empirical therapy for methicillin-susceptible Staphylococcus aureus (MSSA) or methicillin-resistant S. aureus (MRSA) (5). The emergence of MRSA as a community pathogen and the documentation of the inferiority of nonbeta-lactam antibiotics in treating MSSA bacteremia greatly complicate initial antibiotic choice (2-4, 7, 8, 9). Early identification and determination of antibiotic susceptibility might help focus initial antibiotic therapy. We compared a multiplex PCR that identifies MSSA and MRSA to standard microbiologic techniques for evaluating the results of blood cultures (BCs) and wound swab (WS) cultures. Blood was cultured in BacT/Alert, and drug susceptibility was determined with a Vitek 2 system (both from BioMerieux, Durham, NC). For BCs judged to contain gram-positive cocci in clusters (GPCCl), 1-ml aliquots were centrifuged (2 min at 3,000 rpm) to remove charcoal, and the supernatant was studied in a GeneXpert system (Cepheid, Sunnyvale, CA). WS samples were streaked to standard media (blood, chocolate, McConkey, and colistin-nalidixic acid) and then studied in the GeneXpert system within 48 h of collection. GeneXpert realtime PCR detects proprietary sequences of the S. aureus protein A gene, the staphylococcal cassette chromosome, and the methicillin resistance element (1). Of 223 blood samples, 68 yielded S. aureus by culture, 47 with MRSA and 21 with MSSA. PCR correctly identified 67/68 (98.5%) S. aureus isolates (Tables 1 and 2), including 46/47 (97.9%) MRSA and 21/21 (100%) MSSA isolates. No BC (155/155; 100%) that contained GPCCl without S. aureus contained S. aureus by PCR. Of 321 WS samples, 106 yielded MRSA and 51 MSSA by culture. PCR identified 104/106 (98.1%) MRSA isolates correctly but misidentified 2 MRSA isolates as MSSA (Table 1). Of 51 MSSA isolates, 47 (92.2%) were identified correctly, 3 incorrectly as MRSA, and 1 incorrectly as no S. aureus by PCR.

Rapid identification of Gram-positive pathogens and their resistance genes from positive blood culture broth using a multiplex tandem RT-PCR assay

“…Our data, with 100 % sensitivity for the detection of Staphylococcus aureus and mecA, are similar to contemporary work with the various RT-PCR assays employed in previously published studies detecting meticillin-sensitive Staphylococcus aureus (MSSA) and MRSA, with sensitivities of 100 % and between 97.9 and 100 %, respectively (Paule et al, 2005;Parta et al, 2009;Wolk et al, 2009;Jukes et al, 2010). The MT-PCR assay described here has the added advantage of also being a rapid and accurate diagnostic tool for the detection of Streptococcus pneumoniae and Enterococcus species, including VRE.…”

Impact of Results of a Rapid Staphylococcus aureus Diagnostic Test on Prescribing of Antibiotics for Patients with Clustered Gram-Positive Cocci in Blood Cultures

“…Four CNSs gave an invalid result on the GeneXpert, which was due to the operator error of adding too much BC fluid. Compared with the phenotypic result, the sensitivity and specificity of the second-generation GeneXpert for differentiating S. aureus from non-S. aureus isolates were 100% and 96.7%, respectively (when invalid results were considered to be false positives), similar to previous reports for the first-generation product (5,7,12).…”

“…When a positive blood culture (BC) reveals Gram-positive cocci in clusters (GPCC), the choice of initial antibiotics is a balance between breadth of coverage for the likely pathogen and avoidance of unnecessary antibiotic use. From the time a blood culture signals positive, oxacillin susceptibility testing for S. aureus can take as long as 48 h using standard laboratory methods or as little as 90 min for targeted molecular methods (4,5,7,8,10,12). There are few prospective studies investigating whether the earlier knowledge of a MRSA, methicillin-susceptible S. aureus (MSSA), or coagulase-negative staphylococcus (CNS) isolate influences antibiotic prescribing (9).…”

Real-Time Identification of Bacteria and Candida Species in Positive Blood Culture Broths by Matrix-Assisted Laser Desorption Ionization-Time of Flight Mass Spectrometry

“…In addition, automatic rapid systems require 3.5 h for bacterial identification, versus only 20 min for MALDI-TOF-MS. PCR-based techniques have been used for bacterial identification directly from blood culture broth. Some methods require the use of specific targets (8,19,20,24). Despite the fact that these techniques are sensitive, they remain expensive and are specific for one or a few pathogens.…”