engleski

COMPARISON OF THREE DIFFERENT METHODS FOR TIGECYCILINE SUSCEPTIBILITY TESTING IN IN ACINETOBACTER BAUMANII

Background and aim: Acquired carbapenem resistance is an emerging problem in A. baumannii due to the production of acquired carbapenemases of class A (KPC), class B metallo-β-lactamases (MBLs) of IMP, VIM, SIM and NDM family and class D carbapenem- hydrolyzing oxacillinases (CHDL) (OXA-23-like, OXA-40-like, OXA-58-like, OXA-143-like and OXA- 238-like. Tigecyline and colistin are often the last resort antibiotics for the treatment of infections associated with carbapenemase producing organisms. There are no guidelines established by CLSI or EUCAST for tigecycline susceptibility testing for Acinetobacter baumannii so far. In most studies breakpoins for Enterobacteriaecae are applied with susceptibility breakpoint of ≤ 1 mg/L and resistance breakpoint of ≥ 4 mg/L. The aim of the study was to compare the three different methods for the tigecycline susceptibility testing in A. baumannii: broth microdilution, E test and Vitek 2 automated method. Material and methods: In total 74 carbapenem- resistant Acinetobacter baumanii strains collected in General hospital Pula, University Hospital Zagreb, General Hospital Varaždin, General Hospital Sisak, University Hospital Merkur, Clinic for Infectious diseases, General Hospital Bjelovar and Godan nursing home in Zagreb during 2009-2014 from various clinical specimens, were analyzed.The susceptibility to tigecycline was determined by broth microdilution metod, E test and Vitek2 automated method. Pseudomonas aeruginosa ATCC 27853 and A. baumannii ATCC 19606 were used as quality control strains. Results: According to EUCAST criteria for defining resistance with breakpoint of 4 mg/L, the resistance rate with Vitek 2 was 15% (11/74), with E test 23% (17/74) and with broth microdilution 46% (34/74). If broth microdilution method was considered as gold standard for antibiotic susceptibility testing, there are 23 and 17 resistant isolates classified as susceptible or intermediate susceptible by Vitek 2 and E test, respectively, demonstrating a major error of these two methods. Minor errors with susceptible isolates being identified as resistant or intermediate susceptible were not reported. MIC90 obtained by Vitek 2 and E test was 4 mg/L while broth microdilution method yielded MIC90 of 8 mg/L. MIC50 varied between 1 and 4 mg/L depending on the method. Conclusions: Tigecycline testing by Vitek2 automated method yielded significantly lower MICs compared to broth microdilution or E test, yielding to a major error. Huge discrepancies were found between three methods, particularly between dilution method and Vitek2. According to our results Vitek2 testing does not provide reliable results and thus the results should be confirmed by E test or dilution method, particularly in the case of serious infections. Broth microdilution method is gold standard method but is laborious, time consuming and requires educated staff.

Acinetobacter baumannii, tigecyline, susceptibility methods, major error

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