Tigecycline susceptibility of multidrug-resistant Acinetobacter baumannii from intensive care units in the western Balkans

Abstract

Tigecycline can be effective to treat infections of carbapenem-resistant Acinetobacter baumannii (CRAB) however, no interpretive criteria have been approved so far. The objectives of this study were to evaluate the proportion of CRAB isolates and to compare gradient test with a broth microdilution (BMD) method for tigecycline susceptibility testing of A. baumannii.This study included 349 multidrug-resistant (MDR) Acinetobacter spp. collected from Serbia, Montenegro, Bosnia and Herzegovina in 2016 and 2017. Antibiotic susceptibility testing was performed by disk diffusion, VITEK2, gradient, ComASP Colistin. Tigecycline susceptibilities were interpreted according to breakpoints of European Committee on Antimicrobial Susceptibility Testing (EUCAST) and Food and Drug Administration (FDA).Majority of the tested isolates were CRAB (92.8%). Tigecycline MIC50/MIC90 values were 4/8 μg/mL by BMD and 0.5/4 μg/mL by gradient test. Essential agreement for BMD and gradient test amounted to 65.1%. With EUCAST breakpoints, categorical agreement (CA) was achieved in 38% isolates. Major discordance (MD-false susceptibility/resistance) and minor discordance (mD-false categorization involving intermediate results) were observed in 10% and 57% A. baumannii, respectively. With FDA breakpoints, CA, MD and mD were observed in 44%, 16% and 47% isolates, respectively. Colistin resistance was 2.1%.The study highlights a high proportion of CRAB and several discordances between BMD and gradient test which may lead to inappropriate therapy.

Keywords: Acinetobacter baumannii; broth microdilution; gradient strip test; tigecycline.