Evaluation of Surrogate Tests for the Presence of mecA -Mediated Methicillin Resistance in Staphylococcus capitis, Staphylococcus haemolyticus, Staphylococcus hominis, and Staphylococcus warneri

Abstract

Testing of staphylococci other than Staphylococcus aureus (SOSA) for mecA-mediated resistance is challenging. Isolates of Staphylococcus capitis, Staphylococcus haemolyticus, Staphylococcus hominis and Staphylococcus warneri were evaluated by cefoxitin and oxacillin broth microdilution (BMD) and disk diffusion (DD) and PBP2a immunoassay and compared to mecA PCR results. No phenotypic susceptibility test correlated well with PCR results across all species, although PBP2a immunoassay yielded 100% correlation. Oxacillin BMD testing by current Clinical and Laboratory Standards Institute (CLSI) SOSA breakpoints led to 2.1% very major errors (VME) and 7.1% major errors (ME). Adjusting this breakpoint up by a dilution (susceptible, ≤0.5μg/mL; resistant, ≥1.0 μg/mL) led to 2.8% VME and 0.3% ME. Among species evaluated, S. haemolyticus had unacceptable VMEs with this new breakpoint (6.4%), as did S. hominis (4.0%). MEs were acceptable by this new breakpoint, ranging from 0-1.2%. Oxacillin DD yielded high ME rates (20.7-21.7%) using CLSI or European Committee on Antimicrobial Susceptibility Testing breakpoints. VMEs ranged from 0% - 5.3%. Cefoxitin BMD led to 4.9% VMEs and 1.6% MEs. Cefoxitin DD performed best when interpreted with CLSI SOSA breakpoint with 1.0% VMEs and 2.9% MEs. This study led CLSI to adjust the oxacillin MIC breakpoints for SOSA. Laboratories should be aware that no individual phenotypic test correlates well across all species of SOSA with mecA PCR results. Molecular testing for mecA or evaluation for PBP2a is the preferred approach.

Multidrug-resistant Staphylococcus cohnii and Staphylococcus urealyticus isolates from German dairy farms exhibit resistance to beta-lactam antibiotics and divergent penicillin-binding proteins

“…Our study illustrates that the prediction of AMR phenotypes by genomic approaches in the case of rarely investigated staphylococcal species such as S. cohnii and S. urealyticus needs further research and improvement of established gene databases. Moreover, as already shown for other NAS species [20][21][22] , the specificity of cefoxitin testing for the detection of mecA or mecC genes and respective methicillin resistance in S. cohnii and S. urealyticus seems to be quite low. As reported for other Staphylococcus spp., oxacillin testing may be more reliable, but this needs to be confirmed on a larger strain collection.…”

“…Determination of beta-lactam antibiotic resistance in S. aureus, a well investigated staphylococcal species, and the identification of methicillin-resistant S. aureus (MRSA) is routinely performed by cefoxitin resistance testing and detection of the mecA or mecC gene. However, several studies have reported that the determination of beta-lactam antibiotic resistance in NAS using cefoxitin susceptibility testing might be misleading and the use of oxacillin should be favored [20][21][22] .…”

Comparing BioFire FilmArray BCID2 and BCID Panels for Direct Detection of Bacterial Pathogens and Antimicrobial Resistance Genes from Positive Blood Cultures

“…This was the case for BSI pathogens, such as Escherichia coli, K. pneumoniae (with the KPC gene), S. aureus (with the mecA gene), or Enterococcus faecalis (with the vanA gene), which are the most common bacterial pathogens (5). With the BCID2, the opportunity to identify MRSA (5) or S. epidermidis and S. lugdunensis (6) among SOSA may be clinically useful (7), whereas the incapacity to identify SOSA such as S. haemolyticus or other coagulase-negative Staphylococcus species may be detrimental, particularly in the presence of mecA-positive isolates (8,9). Because the reference method helped us to interpret BCID2 (or BCID) results, we believe that the FilmArray BCID2 panel may be a valuable adjunct to culture-based diagnostics.…”

What happened to microbiological diagnosis in 2020 beyond COVID-19?

“…However, testing of SOSA for mecA -mediated resistance is challenging. In the study of Humphries et al [ 11 ] isolates of Staphylococcus capitis (n=50), Staphylococcus haemolyticus (n=50), Staphylococcus hominis (n=50), and Staphylococcus warneri (n=48), were evaluated by cefoxitin and oxacillin broth microdilution (BMD), disk diffusion (DD), and PBP2a immunoassay, and the results were compared to mecA PCR results. No phenotypic susceptibility test correlated well with PCR results across all species, although the PBP2a immunoassay yielded 100% correlation.…”