In vitro activity of tigecycline: MICs, MBCs, time-kill curves and post-antibiotic effect

The minimum inhibitory concentrations (MICs), minimum bactericidal concentrations (MBCs), time-kill curves, and postantibiotic effect (PAE) of tigecycline, the first in the glycylcycline class of antibiotics, were evaluated. MICs were determined against 749 clinical isolates. Time-kill curves were performed against two isolates each of Enterococcus faecalis, MSSA, and MRSA. The presence of PAE, against the same isolates, was investigated. MIC(90)s (microg/mL) were the following: Escherichia coli 0.25; Klebsiella spp 0.5; Enterobacter spp 1; Acinetobacter spp. 2; Staphylococcus aureus (MSSA+MRSA) 0.25; CNS (MS+MR) 0.25; vancomycin-susceptible Enterococcus faecalis 0.12. Tigecycline exerted bacteriostatic activity against all the tested isolates, MBC(90)values being 32xMIC. Time-kill experiments showed a marked reduction in bacterial growth. A PAE at 1- to 20-fold the MIC was observed against the two enterococcal isolates (1.5-3.2h, range) and the four staphylococci (1.6-3h, range). Our findings confirm the excellent antimicrobial activity of tigecycline, adding informations on its bacteriostatic activity vs enterococci and staphylococci, whether methicillin-resistant or -susceptible, and its PAE.