Antimicrobial susceptibility testing.

Broth microdilution susceptibility testing of all isolates was performed in the reference monitoring laboratory and followed the Clinical and Laboratory Standards Institute (CLSI) M07-A10 document (35). Bacterial inoculum density was monitored by colony counts to ensure an adequate number of cells for each testing event. MIC values were validated by concurrently testing CLSI-recommended quality control reference strains (Staphylococcus aureus ATCC 29213, Enterococcus faecalis ATCC 29212, and Streptococcus pneumoniae ATCC 49619) (18). MIC interpretations were based on the CLSI M100-S27 (2017) breakpoint criteria, as available (18); however, tigecycline MIC results were interpreted using U.S. Food and Drug Administration criteria (36). Isolates resistant to erythromycin but susceptible to clindamycin were subjected to the CLSI broth microdilution inducible clindamycin resistance screening test (18, 35).