Acinetobacterspp. susceptibility to tigecycline determined by Etest and microdilution

Abstract number: P801

Objective: To compare susceptibility to tigecycline through Etest and broth microdilution in Acinetobacter baumannii strains resistant to imipenem.

Methods: Isolates from patients admitted to our hospital between January 2007 and October 2009 were studied, considering only one isolate per patient. Identification and study of susceptibility were conducted with Vitek-2 system (Biomerieux®) and, in parallel, the susceptibility to tigecycline was studied with Etest® (AB BIODISK) in Mueller-Hinton agar (BioMedics). BSAC (British Society for Antimicrobial Chemoterapy) breakpoints were used – susceptible leqslant R: less-than-or-eq, slant1, intermediate = 2, and resistant geqslant R: gt-or-equal, slanted2 mg/l. The values read were rounded up to the upper dilution in base-2 system. We considered different MICs where variation existed in at least 1 dilution.

Results: 52 imipenem-resistant A. baumannii were isolated – 15 in 2007 (28.85%), 15 in 2008 (28.85%), and 22 in 2009 (42.3%). The allocation among services was 44.2% ICU, 23.1% Reanimation, 9.6% Internal Medicine, 7.7% Surgery, and 15.4% in other services. 36 (69.2%) isolates were susceptible to tigecycline by Etest (MIC: 0.5–1 mg/l) – amongst them, 13 (36.1%) shown a MIC value similar to the one obtained by microdilution, 14 (38.9%) a greater MIC value, and 9 (25%) a lesser value – 2 mg/l by microdilution. The remaining 16 (30.8%) strains shown an intermediate susceptibility to tigecycline (MIC=2 mg/l) by Etest. In 7 (43.75%) isolates, this value was similar to the one obtained by microdilution, whereas in 2 cases (12.5%) we obtained a greater MIC value by Etest – with the interpretation changing from intermediate to susceptible. We could not found any strain with MIC > 2 by Etest. A correlation between both methods was found (r = 0.73 pleqslant R: less-than-or-eq, slant0.05). The percentages of intermediate or tigecycline-resistant strains were 30.8% by Etest and 27% by microdilution. As to the evolution in time, in 2007 a 33.3% of intermediate or resistant strains is found, 6.6% in 2008 and 45% in 2009.


1The greatest percentage of isolates was found in the critical care units.

2Even though microdilution is the reference method, we realized that 17.3% strains found susceptible by Etest were intermediate by microdilution, which suggests that using Etest as the only study method for susceptibility might lead to mistaken interpretations and result in therapeutic failure.

3We observed a significant increase of resistance to tigecycline in our hospital during the last year.

Session Details

Date: 10/04/2010
Time: 00:00-00:00
Session name: Abstracts 20th European Congress of Clinical Microbiology and Infectious Diseases
Location: Vienna, Austria, 10 - 13 April 2010
Presentation type:
Back to top