Tigecycline activity tested against multidrug-resistant Enterobacteriaceae and Acinetobacter spp. isolated in US medical centers (2005–2009)

Abstract

We evaluated the activity of tigecycline against Enterobacteriaceae (9563 isolates) and Acinetobacter spp. (835) with various resistance phenotypes collected from 31 US medical centers in 2005–2009. The isolates were tested for susceptibility by the reference broth microdilution method against tigecycline and various comparators. Among Escherichia coli and Klebsiella spp., 6.8% and 15.4% exhibited an extended-spectrum β-lactamase (ESBL) phenotype, respectively; and 22.2% of Enterobacter spp. strains were ceftazidime-resistant. Tigecycline was active against E. coli [minimum inhibitory concentration (MIC 50/90), 0.12/0.25 μg/mL; 100.0% susceptible] independent of ESBL phenotype or resistance to other antimicrobials. Among Klebsiella spp., 97.9% of ESBL-producing Klebsiella spp. and 98.2% of imipenem–non-susceptible strains were susceptible to tigecycline (MIC 50/90, 0.5/1 μg/mL for both subsets). Tigecycline was active against Enterobacter spp. (MIC 50/90, 0.25/1 μg/mL; 98.4% susceptible), including ceftazidime-resistant strains (MIC 50/90, 0.5/2 μg/mL; 97.1% susceptible). Tigecycline inhibited 94.4% of Acinetobacter spp. overall (MIC 50/90, 0.5/2 μg/mL) and 86.2% of imipenem–non-susceptible (MIC 50/90, 1/4 μg/mL) strains at ≤2 μg/mL. No trend toward decreased tigecycline activity overtime was observed for any of the organisms or resistant subsets during the study period. These results indicate that tigecycline has sustained potent in vitro activity and a broad spectrum against these clinically important Gram-negative pathogens causing infections in US medical centers, including multidrug-resistant organism subsets.