Evaluation of two commercial broth microdilution systems for dalbavancin susceptibility testing of MRSA and other resistant Gram-positive cocci

Objectives This study aims to evaluate two commercial broth microdilution (BMD) systems, E1-185-100 (Merlin) and FDANDPF (ThermoFisher) for dalbavancin susceptibility testing in comparison with reference broth microdilution assay. Methods Study collection was composed of 200 nonreplicate multidrug-resistant Gram-positive cocci of clinical origin, including 180 methicillin-resistant Staphylococcus aureus, 10 vancomycin resistant enterococci, 7 linezolid resistant Staphylococcus epidermidis, and 3 methicillin-resistant coagulase negative staphylococci. S. aureus ATCC 29213 and Enterococcus faecalis ATCC 29212 reference strains were also included as controls. Testing was performed according to the ISO 20776-1 standard, starting from the same bacterial inoculum, and results were compared according to the ISO 20776-2 standard. Results Reference BMD showed that 92.6% (187/202) of the strains were susceptible to dalbavancin, while few staphylococci and all VanA-producing enterococci showed a resistant phenotype. In comparison with the reference method, Category Agreement and Essential Agreement of 98% (198/202; 95% CI, 95.4%-99.3%) and 98% (198/202; 95% CI, 95.4-99.3%), for both Merlin and ThermoFisher panels. A few false susceptibilities were observed, for both commercial systems, with dalbavancin-resistant staphylococci. BIAS values of 11% and 3% were calculated for the Merlin and ThermoFisher systems, respectively. Conclusions This study, reporting the first evaluation of the two commercially available BMD assays for dalbavancin susceptibility testing, revealed an overall good correlation with reference BMD, although with some underestimation tendency of MIC values by both commercial systems. Further studies involving a higher number of resistant isolates will be necessary to better evaluate this issue.