Comparative in vitro antimicrobial activity of tigecycline, a new glycylcycline compound, in freshly prepared medium and quality control

Abstract

The in vitro spectra of activity of tigecycline and tetracycline were determined for 2,490 bacterial isolates representing 50 different species or phenotypic groups. All isolates were tested simultaneously by broth microdilution using freshly prepared Mueller-Hinton broth and by disk diffusion. Portions of these data were submitted to the Food and Drug Administration (FDA) in support of the sponsor's application for new drug approval. In a separate study, MIC and disk diffusion quality control ranges were determined. The tigecycline MICs at which 50%/90% of bacteria were inhibited were (in microg/ml) as follows: for Streptococcus spp., 0.06/0.12; for Moraxella catarrhalis, 0.06/0.12; for Staphylococcus spp., 0.12/0.25; for Enterococcus spp., 0.12/0.25; for Listeria monocytogenes, 0.12/0.12; for Neisseria meningitidis, 0.12/0.25; for Haemophilus spp., 0.25/0.5; for Enterobacteriaceae, 0.05/2.0; for non-Enterobacteriaceae, 0.5/8.0. Tigecycline was consistently more potent than tetracycline against all species studied. The data from this study confirm the FDA-approved MIC and disk diffusion breakpoints for tigecycline for Streptococcus spp. other than Streptococcus pneumoniae, enterococci, and Enterobacteriaceae. Provisional breakpoints for Haemophilus spp. and S. pneumoniae are proposed based on the data from this study. The following MIC and/or disk diffusion quality control ranges are proposed: Staphylococcus aureus ATCC 29213, 0.03 to 0.25 microg/ml; S. aureus ATCC 25923, 20 to 25 mm; Escherichia coli ATCC 25922, 0.03 to 0.25 microg/ml and 20 to 27 mm; Pseudomonas aeruginosa ATCC 27853, 9 to 13 mm, Enterococcus faecalis ATCC 29212, 0.03 to 0.12 microg/ml; S. pneumoniae ATCC 49619, 0.015 to 0.12 microg/ml and 23 to 29 mm; Haemophilus influenzae ATCC 49247, 0.06 to 0.5 microg/ml and 23 to 31 mm; and Neisseria gonorrhoeae ATCC 49226, 30 to 40 mm.

FIG. 1.

Scattergrams of tigecycline MICs versus zone diameters (15-μg disks). (A) All staphylococci combined (including methicillin-resistant isolates; n = 130), (B) Streptococcus pneumoniae (n = 209) and nonpneumococcal streptococci (n = 60) combined, (C) all Enterococcus spp. combined (n = 216), (D) all Enterobacteriaceae combined (n = 1,352) (122/125 [97.6%] of the isolates with a tigecycline MIC of ≥4 μg/ml were Morganella, Proteus spp. other than P. vulgaris, or Providencia spp.), (E) Enterobacteriaceae without Morganella, Proteus spp. other than P. vulgaris, or Providencia spp. (n = 1,054), (F) Haemophilus spp. (n = 113). Horizontal lines represent proposed susceptible (lower line) and resistant (upper line) MIC breakpoints; vertical lines represent proposed susceptible (right line) and resistant (left line) zone diameter breakpoints. Abbreviations: n, number of strains tested; VM, very major errors; M, major errors; m, minor errors; I+2, intermediate MIC breakpoint plus 2 log₂ dilutions; I−1 to I+1, intermediate MIC breakpoint plus or minus 1 log₂ dilution; I−2, intermediate MIC breakpoint minus 2 log₂ dilutions.