Performance of BD Max StaphSR for Screening of Methicillin-Resistant Staphylococcus aureus Isolates among a Contemporary and Diverse Collection from 146 Institutions Located in Nine U.S. Census Regions: Prevalence of mecA Dropout Mutants

Abstract

This study determined the performance of BD Max StaphSR and the rate of methicillin-resistant Staphylococcus aureus (MRSA) with an unrecognized staphylococcal cassette chromosome mec (SCCmec) right-extremity junction (MREJ) region among 907 methicillin-resistant S. aureus (MRSA) and 900 methicillin-susceptible S. aureus (MSSA) isolates. The rate of mecA/mecC dropout mutants was also evaluated. Only three MRSA isolates (99.7% sensitivity; 904/907) were classified as MSSA by the BD Max StaphSR assay, due to negative results for MREJ. Eight MSSA isolates (99.1% sensitivity; 892/900) were assigned as MRSA. However, six of these MSSA isolates had the mecA gene confirmed by PCR and sequencing (99.8% sensitivity; 898/900). Overall, 7.1% (64/900) of MSSA isolates showed results compatible with a mecA dropout genotype. Several studies have reported a decline in the incidence of hospital-acquired methicillin-resistant Staphylococcus aureus (HA-MRSA) and invasive infections in US and European hospitals (1-6). However, the incidence of community-onset (CO) MRSA infection has varied according to geographic region (7-10). Despite variability in the occurrence of CO-MRSA and HA-MRSA invasive diseases, S. aureus persists as the most common organism responsible for human infections, and methicillin resistance remains the most commonly identified resistance in medical institutions (11). Therefore, proper infection control practices and antimicrobial stewardship strategies play important roles in controlling MRSA infections (12, 13).Screening for MRSA carriers has become an important tool for early detection and to help prevent MRSA spread (14). Early generations of molecular assays targeting the mecA gene may provide false-positive results due to the copresence of methicillin-resistant staphylococci other than S. aureus (i.e., coagulase-negative staphylococci [CoNS]) (15). Performance evaluations of second-generation assays targeting the staphylococcal cassette chromosome mec (SCCmec)-orfX right-extremity junction (MREJ) region reported the presence of S. aureus carrying a genetic element that lacked the mecA (so-called dropout) mutant, again resulting in false-positive reports (16). Newer approaches targeting both mec and MREJ region sequences have been developed to minimize the likelihood of false-positive results, thus minimizing unnecessary isolation precautions (17). However, a false-positive reaction can still occur in the presence of mixed populations of methicillinresistant CoNS and a dropout S. aureus mutant.This study aimed to (i) determine the relative percentage rate of mecA/mecC dropout mutants among methicillin-susceptible S. aureus (MSSA) isolates collected from U.S. hospitals and (ii) determine the relative percentage rate of MRSA with unrecognized MREJ region sequences. A total of 907 MRSA and 900 MSSA isolates were included (at least 100 MRSA and 100 MSSA from each U.S. Census region). Isolates were collected from 146 U.S. hospitals during the 2013 SENTRY Antimicrobial Surveillance Program (see Table S1 in the s...

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

“…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 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.…”

“…e 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.…”

“…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).…”

Multicenter Evaluation of the Xpert MRSA NxG Assay for Detection of Methicillin-Resistant Staphylococcus aureus in Nasal Swabs

“…The MRSA NxG assay evaluated in this study showed similar performance characteristics, with an overall specificity of 97.2%. Previous generations of MRSA molecular assays were prone to false-positive results when tested on strains containing an empty SCCmec cassette (i.e., mecA dropout strains), which are estimated to occur in about 7% of methicillin-susceptible S. aureus (MSSA) isolates in the United States (33). However, this problem was remedied in subsequent assays through the inclusion of primers and probes targeting both the junction of the SCCmec cassette with the orfx gene and the mecA gene.…”