In vitro Susceptibility of Tigecycline against Multidrug-Resistant Gram-Negative Strains: Etest versus Agar Dilution

Abstract

Background and Aim: Tigecycline is a semi-synthetic tetracycline with activity against most multidrug-resistant (MDR) bacteria. Methods: We studied in vitro activity of tigecycline by agar dilution (AD) and Etest methods to evaluate their correlation. The study included 206 isolates of extended-spectrum β-lactamase (ESBL)-producing Escherichia coli, Klebsiella pneumoniae and MDR Acinetobacter baumannii recovered from blood cultures of patients of Baskent University between 2008 and 2010. Results: ESBL-producing E. coli had MIC₅₀/MIC₉₀ values of 0.5/0.5 µg/ml by AD and 0.25/0.5 µg/ml by Etest. ESBL-producing K. pneumoniae had MIC₅₀/MIC₉₀ values of 1/2 µg/ml by AD and 0.75/2 µg/ml by Etest, whereas MDR A. baumannii had MIC₅₀/MIC₉₀ values of 4/4 µg/ml by AD and 2/4 µg/ml by Etest. The correlation between AD and Etest was weak for ESBL-producing E. coli and strong for ESBL-producing K. pneumoniae and MDR A. baumannii. Tigecycline MIC values for ESBL-producing E. coli were lower than the tigecycline concentration, while they were higher than the concentrations attainable by treatment doses for A. baumannii. Conclusion: Tigecycline is an appropriate agent in the treatment of E. coli bacteremia, but it is not for treating A. baumannii bacteremia. Tigecycline could be used for K. pneumoniae bacteremia treatment after determining its MIC value. Determining the MIC value by gold-standard methods is more appropriate due to the correlation between Etest and AD at high MIC values.