Susceptibility to Tigecycline and Linezolid Among Gram-Positive Isolates Collected in the United States as Part of the Tigecycline Evaluation and Surveillance Trial (TEST) Between 2004 and 2009

Abstract

Background The Tigecycline Evaluation and Surveillance Trial (TEST) was initiated in 2004 to chart the activity of tigecycline and comparator antimicrobial agents against gram-positive and gram-negative organisms globally. Objectives This study aimed to provide an analysis of the antimicrobial susceptibility of gram-positive organisms collected from the 9 census regions of the United States between 2004 and 2009. Methods The MICs and antimicrobial susceptibility were determined using Clinical and Laboratory Standards Institute methodology. For tigecycline, US Food and Drug Administration susceptibility criteria were used. Results A total of 8782 Staphylococcus aureus isolates (54.5% methicillin-resistant S aureus) were collected, with the highest percentage of MRSA isolates collected from the South Central region (67.9%). All S aureus isolates were susceptible to tigecycline, linezolid, and vancomycin. Overall, 4.6% of Enterococcus faecalis (n = 3753) and 69.1% of Enterococcus faecium (n = 1417) isolates were vancomycin resistant, with the highest rates in the East North Central region for E faecalis (7.1%) and the South Atlantic region for E faecium (79.5%). Small numbers of linezolid nonsusceptible E faecalis (n = 13) were identified. MIC 90 values for tigecycline were ≤0.25 mg/L against E faecalis and 0.12 mg/L against E faecium. Of the Streptococcus pneumoniae isolates collected (n = 4541), 1.1% were penicillin resistant. All S pneumoniae isolates were susceptible to linezolid and vancomycin; susceptibility to tigecycline varied between 80.9% (Pacific region) and 95.2% (West North Central region). Conclusions The rates of MRSA and vancomycin-resistant Enterococcus spp varied among the 9 census regions; however, susceptibility to linezolid, vancomycin, and tigecycline remained consistent, with low MIC 90 values and high rates of antimicrobial susceptibility.