Comparison of BD Max StaphSR and BD Max MRSA XT for Screening of Staphylococcus aureus Clinical Isolates Collected from Hospitals in the United States

Abstract

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 ϭ 93) and MSSA (n ϭ 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.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 sin...

Prospective Evaluation of the BD MAX StaphSR Assay for the Screening of Methicillin-Susceptible and -Resistant Staphylococcus aureus from Nasal Swabs Taken in Intensive Care Unit Patients

“…Indeed, third-generation PCR assays targeting the SCCmec-orfX right-extremity junction and the mecA gene were found to reduce the occurrence of false-positive results due to mecA dropout, but they do not eliminate it in the case of patients colonized by both S. aureus - mecA dropout and the CoNS-carrying mecA gene. In a collection of strains isolated in the US, 7.1% of MSSA isolates were compatible with the genotype of the S. aureus empty-cassette variant [ 13 ]. Lee et al reported a prevalence of mecA dropout- S. aureus of 4.8% in atopic dermatitis samples from a Korean cohort [ 24 ].…”

“…These assays can distinguish methicillin-susceptible S. aureus (MSSA) from MRSA given the presence of both the methicillin-resistant gene ( mecA/C ) and the Staphylococcal Cassette Chromosome mec (SCC mec ) embedded into the attB site of the S. aureus chromosome located at the 3′ end of orfX . Several studies have highlighted the excellent performances of the fully automated BD Max StaphSR assay using positive blood cultures, wound swabs, and strains collections [ 9 , 10 , 11 , 12 , 13 , 14 ]. Two other studies have evaluated the performances of this assay on nasal swabs for the detection of S. aureus and MRSA, but these studies were conducted in the United States and never in Europe, where the rate of nasal colonization by MRSA is lower than in the United States [ 15 , 16 , 17 ].…”