LETTER
We recently reported the performance of the BD Max StaphSR assay for screening of methicillin-resistant Staphylococcus aureus (MRSA), as well as the prevalence in the U.S. of S. aureus carrying a staphylococcal cassette chromosome (SCCmec) element lacking mecA (so-called dropout mutant) (1). This assay targets the nuc and mecA/C genes and 11 sequences of the SCCmec-orfX right-extremity junction (MREJ) region. Dropout mutants were defined as those isolates showing positive results for nuc (i.e., S. aureus) and the MREJ region but negative results for mecA/C. This report documented an overall prevalence of dropout mutants of 7.1% among a diverse and contemporary U.S. collection of methicillin-susceptible S. aureus (MSSA) isolates that contained fragments of the SCCmec components (1).
These results are of paramount importance, since dropout mutants may generate false-positive MRSA reports with assays screening the MREJ region only (2). Such results can cause inappropriate patient management care, such as unnecessary decolonization treatment, additional precautionary measures, possibly the unjustified use of glycopeptides, and unnecessary expenses of infection control practices (3). Here, we report a comparison analysis of results obtained by both the BD Max StaphSR and BD Max MRSAXT methods and the performance of MRSAXT for the detection of MRSA and dropout mutants.
Subsets of MRSA (n = 93) and MSSA (n = 102) isolates utilized in the previous study (1) were subjected to the MRSAXT kit. All isolates where false-positive and -negative MRSA results were previously obtained with the BD Max StaphSR kit in comparison with the “gold standard,” as well as all 64 dropout mutant isolates detected by the BD Max StaphSR kit, were included. These isolates were tested for susceptibility and defined as MRSA or MSSA on the basis of oxacillin and/or cefoxitin susceptibility results obtained by the reference broth microdilution and/or disk diffusion methods (4–6). Isolates were subjected to MRSAXT according to the manufacturer's instructions, with the same slight modifications applied for the BD Max StaphSR samples (1).
All MRSAXT results generated with MRSA and MSSA were in agreement with those obtained previously by BD Max StaphSR, including the presence of three false-negative and eight false-positive MRSA results (1). Sensitivity and specificity values for the MRSAXT kit were 96.8% (90/93) and 92.2% (94/102), respectively. Among the eight false-positive MRSA results, six were, in fact, mecA-positive methicillin-susceptible isolates (a corrected specificity of 97.9% [94/96]; Table 1) (1). In addition, all 64 MSSA isolates initially categorized as dropout mutants by BD Max StaphSR were correctly excluded by the MRSAXT kit.
TABLE 1.
BD Max MRSAXT assay performance compared with phenotypic methicillin (oxacillin and cefoxitin) susceptibility results
| Category (no. of isolates tested)a | No. of BD Max MRSAXT resultsb |
|
|---|---|---|
| MRSA positive | MRSA negative | |
| MRSA (93) | 90 | 3 |
| MSSA (102) | 8c | 94 |
| Total (195) | 98 | 97 |
MSSA and MRSA clinical isolates were defined by the oxacillin and/or cefoxitin susceptibility results obtained by reference broth microdilution and/or disk diffusion methods according to the CLSI (M02-A12, M07-A10, and M100-S25).
Sensitivity and specificity of 96.8% (90/93) and 92.2% (94/102), respectively.
Six MSSA isolates were mecA/C positive by an in-house PCR screening assay, and the mecA gene was confirmed by sequencing analysis. This would provide a corrected specificity of 97.9% (94/96).
These results suggest that the MRSAXT and StaphSR assays provide equivalent results for screening of MRSA while also correctly identifying dropout mutants as MSSA (or MRSA negative). Both systems use the same sets of primers and probes targeting the MREJ region, nuc, and mecA/C. However, while MRSAXT generates a single reportable result (MRSA positive or negative on the basis of MREJ and mecA/C status), StaphSR reports results as positive or negative for S. aureus and for MRSA according to the results from all of the targets. Another study also reported equivalent MRSA status obtained by MRSAXT and StaphSR from nasal swab samples, with a sensitivity of 94.3% and a specificity of 97.7% for both assays (7). However, it is important to emphasize that the selection of the isolates included here was biased, since all of the isolates that previously generated false-positive and -negative MRSA results by the StaphSR kit were intentionally selected.
ACKNOWLEDGMENTS
We thank the following staff members at JMI Laboratories (North Liberty, IA) for technical support: M. Janechek, J. Oberholser, P. Rhomberg, J. Ross, J. Schuchert, J. Streit, and L. Woosley.
This study was supported by an educational/research grant from BD Diagnostic Systems (Sparks, MD). JMI Laboratories also received compensation fees for services with regard to manuscript preparation, which was funded by BD Diagnostic Systems. JMI Laboratories, Inc., also received research and educational grants in 2014 and 2015 from Achaogen, Actavis, Actelion, Allergan, American Proficiency Institute (API), AmpliPhi, Anacor, Astellas, AstraZeneca, Basilea, Bayer, BD, Cardeas, Cellceutix, CEM-102 Pharmaceuticals, Cempra, Cerexa, Cidara, Cormedix, Cubist, Debiopharm, Dipexium, Dong Wha, Durata, Enteris, Exela, Forest Research Institute, Furiex, Genentech, GSK, Helperby, ICPD, Janssen, Lannett, Longitude, Medpace, Meiji Seika Kasha, Melinta, Merck, Motif, Nabriva, Novartis, Paratek, Pfizer, Pocared, PTC Therapeutics, Rempex, Roche, Salvat, Scynexis, Seachaid, Shionogi, Tetraphase, The Medicines Co., Theravance, Thermo Fisher, VenatoRX, Vertex, Wockhardt, Zavante, and some other corporations. Some JMI employees are advisors/consultants for Allergan, Astellas, Cubist, Pfizer Cempra, and Theravance. In regard to speakers bureaus and stock options, we have none to declare.
Funding Statement
This study was supported by an educational/research grant from BD Diagnostic Systems (Sparks, MD). JMI Laboratories also received compensation fees for services with regards to manuscript preparation, which was funded by BD Diagnostic Systems.
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