Bacterial isolates

Archived isolates of S. aureus from 869 unique pediatric and adult patient bloodstream infections were included in this study (Supplementary Table 1). Isolates were obtained from clinical blood culture specimens submitted to the Department of Pathology and Laboratory Medicine at Dartmouth-Hitchcock Medical Center (DHMC), New Hampshire, USA, from December 2010–February 2022. The first significant blood culture isolates from each patient is routinely archived (freezer space permitting) in case of future need for patient care, epidemiologic, public health, or laboratory quality studies. Upon subculture, isolates were assigned a study number, and all patient identifiers were removed with only the date of collection and results of phenotypic antimicrobial susceptibility testing linked to the study number. Ethical approval was granted by the Committee for the Protection of Human Subjects of Dartmouth-Hitchcock Medical Center and Dartmouth College. This study protocol was deemed not to be human subjects research. Samples used in the study were subcultured bacterial isolates that had been archived in the routine course of clinical laboratory operations. No patient specimens were used, and patient-protected health information was not collected. Therefore, informed consent was not required.

All isolates were tested in vitro on a commercial automated broth microdilution testing platform (MicroScan Walkaway, Beckman Coulter, Inc., La Brea, CA) against a panel of antimicrobial agents, including cefoxitin (screening well), daptomycin, oxacillin, penicillin, and vancomycin. Clinical breakpoints were interpreted according to Clinical Laboratory Standards Institute (CLSI) guidelines⁶⁹. Methicillin resistance was determined per the manufacturer’s guidelines and CLSI guidelines by growth in the cefoxitin screening well (>4 μg/mL) and/or by an oxacillin minimum inhibitory concentration (MIC) of >2 μg/mL. All isolates were stored in a DMSO solution at −80 °C.