Antimicrobial susceptibility among gram-negative isolates collected in the USA between 2005 and 2011 as part of the Tigecycline Evaluation and Surveillance Trial (TEST)

Background: The Tigecycline Evaluation and Surveillance Trial (T.E.S.T.) was designed to monitor in vitro antimicrobial susceptibility to tigecycline and comparator agents. We present susceptibility data on Gram-negative organisms collected between 2005 and 2011 from nine United States census regions. Methods: T.E.S.T. was conducted using standardized CLSI methodologies or FDA-approved breakpoints. Results: Tigecycline was highly active (MIC90 <= 2 mg/L) against Enterobacteriaceae irrespective of species or region of collection (N = 25011). The isolates were also highly susceptible to the carbapenems when all regional data are combined, except for ESBL-producing Klebsiella pneumoniae (MIC90 16 mg/L) and Acinetobacter baumannii (MIC90 >= 32 mg/L). In addition, 883 (30%) of 2900 A. baumannii isolates were classified as multidrug-resistant (MDR): these MDR organisms were most susceptible to tigecycline (MIC90 2 mg/L) and minocycline (MIC90 8 mg/L) when all regional data are considered together. Susceptibility patterns also varied widely among the regions. Conclusions: The findings highlight the importance of monitoring antimicrobial susceptibility patterns and implementing effective methods to curb increased resistance and also confirm that additional studies to determine the efficacy of tigecycline in vivo, especially for treating infections with MDR organisms, are warranted.