Resistance and heteroresistance to colistin among clinical isolates of Acinetobacter baumannii

Colistin is one of the most effective alternatives for treating Acinetobacter baumannii infections. The aim of this study was to determine colistin resistance and heteroresistance rates in A. baumannii from clinical samples in Hacettepe University clinical microbiology laboratory between June 2016 and January 2017. A total of 200 isolates were included in the study. In vitro susceptibility to amikacin, gentamicin, ceftazidime, piperacillin/tazobactam, meropenem, ciprofloxacin, and tigecycline were determined by disk diffusion test. Most isolates were multiresistant as they exhibited resistance to aminoglycosides, β-lactams, and fluoroquinolones. Colistin susceptibility was determined by broth microdilution (BMD) test (EUCAST standards) and was compared with E-test (bioMérieux, France) in 120 isolates. In 14 blood isolates that were susceptible to colistin (MIC ≤ 2 mg/L), heteroresistance was investigated with the population analysis profile (PAP) method. Overall resistance (n = 200) to colistin was 28% by BMD. Among the 120 isolates where the two tests were compared, resistance to colistin was 25.8% versus 4.2% with BMD and E-test, respectively. Three blood isolates (21.4%) were heteroresistant to colistin. With E-test, a majority of the resistant isolates are overlooked and in vitro susceptibility to colistin should be determined with broth dilution method. This is the first study in Turkey reporting heteroresistance in A. baumannii isolates by the PAP method and emphasizes the need to test for heteroresistance in relation to clinical outcome in serious infections due to A. baumannii.