Antimicrobial susceptibility of extended-spectrum β-lactamase producers and multidrug-resistant Acinetobacter baumannii throughout the United States and comparative in vitro activity of tigecycline, a new glycylcycline antimicrobial

As part of the Tigecycline Evaluation and Surveillance Trial, isolates of Escherichia coli, Klebsiella pneumoniae, and Acinetobacter baumannii were collected in the United States between January 2004 and January 2006. Determinations of antimicrobial susceptibility and extended-spectrum β-lactamase (ESBL) production were carried out according to the Clinical and Laboratory Standards Institute guidelines. A high percentage of ESBL-producing K. pneumoniae (≥19.0%) was detected in New Jersey, Massachusetts, New York, and Missouri, and for E. coli, in the District of Columbia (9.5%). Against ESBL-producing isolates, the lowest MIC90s were for tigecycline (0.5–2 μg/mL) and imipenem (0.5–8 μg/mL). Overall, 282 (27.5%) A. baumannii isolates were resistant to ≥3 antimicrobial classes. The most common phenotype (33.0%) was resistance to cefepime, ceftazidime, ceftriaxone, levofloxacin, and piperacillin–tazobactam. Against multidrug-resistant A. baumannii, tigecycline and minocycline were the most active agents (MIC90, 2 and 8 μg/mL, respectively).