Susceptibility testing.

Susceptibility to ceftobiprole and comparator agents was tested using current CLSI methods (36, 37). JMI prepared frozen-form broth microdilution panels for susceptibility testing. Ceftobiprole was supplied by Basilea Pharmaceutica International Ltd., and comparator agents were acquired from Sigma-Aldrich Chemical Co. (St. Louis, Missouri, USA) or similar suppliers. Interpretive criteria were those of CLSI and EUCAST (36, 38), where applicable. Only EUCAST interpretive criteria were applied to ceftobiprole, because corresponding interpretive criteria have not been published by CLSI. FDA criteria were used as an alternative breakpoint source for tigecycline (39). JMI Laboratories followed current CLSI quality assurance practices when performing the susceptibility tests (36). MIC values were validated by concurrently testing CLSI-recommended American Type Culture Collection (ATCC) quality control reference strains (36) that included S. aureus ATCC 29213, E. faecalis ATCC 29212, E. coli ATCC 25922 and ATCC 35218, P. aeruginosa ATCC 27853, and S. pneumoniae ATCC 49619 (36). The susceptibilities of pathogen groups without specific published interpretive criteria for ceftobiprole were evaluated using the EUCAST non-species-specific breakpoint of 4 mg/liter (38).