JAC Advance Access originally published online on May 17, 2009
Journal of Antimicrobial Chemotherapy 2009 64(1):69-72; doi:10.1093/jac/dkp169
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Original research |
Influence of testing methodology on the tigecycline activity profile against presumably tigecycline-non-susceptible Acinetobacter spp.
1 Microbiology Department, Hospital Universitario Reina Sofía, Avenida Menéndez Pidal s/n, 14004 Cordoba, Spain 2 Laboratories International for Microbiology Studies, International Health Management Associates Inc., Schaumburg, IL, USA 3 Microbiology Department, Hospital Universitario Infanta Cristina, Avenida de Elvas s/n, 06006 Badajoz, Spain 4 Microbiology Department, Hospital Universitari de Bellvitge, IDIBELL, University of Barcelona, Feixa Llarga s/n, 08907 Hospitalet de Llobregat, Barcelona, Spain 5 Microbiology Department, Hospital Clínico Universitario de Valladolid, Avenida Ramón y Cajal 3, 47005 Valladolid, Spain 6 Microbiology Department, School of Medicine, Universidad Complutense, Avenida Complutense s/n, 28040 Madrid, Spain 7 Microbiology Department, Hospital Vall d'Hebron, P. Vall d'Hebron 119-129, 08035 Barcelona, Spain 8 Medical Department, Wyeth Farma S.A., Ctra. N-I, km. 23 Desvío Algete Km.1, San Sebastián de los Reyes, Madrid, Spain
Received 3 March 2009; returned 14 April 2009; revised 16 April 2009; accepted 17 April 2009
* Corresponding author. Tel: +34-91-3941505; Fax: +34-91-3941511; E-mail: laguilar{at}med.ucm.es
Objectives: To compare the tigecycline activity profile against Acinetobacter spp. by Etest versus broth microdilution in isolates with high Etest MIC.
Methods: Acinetobacter spp. isolates with tigecycline MICs of 
0.5 mg/L determined by commercially developed Etests strips (January 2006 to July 2007) in five Spanish hospitals were considered. Values were rounded to the nearest upper double-dilution. Susceptibility by broth microdilution following CLSI (formerly NCCLS) recommendations, as the reference method, was determined in a central laboratory. BSAC breakpoints were used: susceptible 
1 mg/L; intermediate = 2 mg/L; and resistant >2 mg/L.
Results: One hundred and forty-eight isolates were collected: 12 isolates with a tigecycline Etest MIC of 0.5 mg/L, 14 with 1 mg/L, 86 with 2 mg/L, 31 with 4 mg/L and 5 with 8 mg/L. Isolates with Etest MICs of 0.5–1 mg/L showed the same values by broth microdilution. Among isolates with Etest MICs of 2 mg/L, only 5.8% of strains showed the same value by both methods (88.4% showed values that were one or two dilutions lower by microdilution). None of the 36 isolates with Etest MICs of 4–8 mg/L showed the same value by both methods, with values at least two dilutions lower by microdilution. Weak correlation (R = 0.238; P 0.001) was found between both methods. All 26 Etest susceptible isolates, 80/86 (93.0%) Etest intermediate and 32/36 (88.9%) Etest resistant strains were susceptible by microdilution.
Conclusions: Caution should be taken in interpreting Etest MICs of 2 mg/L for Acinetobacter spp. since strains with Etest MICs of 2–4 mg/L are susceptible when tested by microdilution. False non-susceptibility by Etest may exclude tigecycline as a therapeutic option in a field where multiresistance is the rule.
Keywords: Etest , broth microdilution , BSAC breakpoints