engleski

Comparison of two different methods for tigecycline susceptibility testing in Acinetobacter baumannii

SUMMARY Tigecycline susceptibility testing (TST) is a huge problem for clinical microbiologists. Previous studies have shown that Epsilometer test (E-test) and Vitek 2 system significantly overestimate the minimum inhibitory concentrations (MICs) of tigecycline compared to broth dilution method, leading to major errors (i.e., resistant instead of sensitive). The aim of this study was to compare the aforementioned two different methods for TST in carbapenem-resistant and carbapenem-susceptible A. baumannii and to analyze the changes in tigecycline susceptibility between two time periods (2009- 2012 and 2013-2014). Broth microdilution method was considered the gold standard for TST. Using the EUCAST criteria the rate of resistance was found to be 54.5% (6/11) for OXA-23 MBL positive strains, 29.4% (5/17) for OXA-23 MBL negative and 2.7% (1/37) with carbapenemase negative strains, with broth microdilution method. The OXA-24/40 and OXA-58 producing organisms did not exhibit any resistance. With E-test, all OXA-23 MBL positive organisms (11/11), 23.5% (4/17) of OXA-23 MBL negative, and 4.1% of OXA-24/40 (3/74) displayed resistance to tigecycline. There were no resistant strains among the OXA- 58 and carbapenemase negative isolates with E- test. The resistance to tigecycline emerged during 2013 to 2014. Although tigecycline does not display cross-resistance, the highest rates of resistant isolates were observed among isolates producing VIM- MBL regardless of the testing method. According to our results, the commercial E-test method does not provide reliable results. Further confirmation with the dilution method should be recommended, particularly in the case of serious infections.

Acinetobacter baumannii ; tigecycline, testing

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