Two types of colistin heteroresistance in <i>Acinetobacter baumannii</i> isolates

In this study, we investigated the colistin heteroresistance patterns in <i>Acinetobacter baumannii</i> isolates. To identify colistin heteroresistance, population analysis profiling was performed for six <i>in vitro</i> colistin-susceptible <i>A. baumannii</i> isolates. Survival rates with and without prior exposure to colistin (at concentrations between 0 and 32 mg/L) were measured in media with and without colistin. Amino acid substitutions were also detected in colonies that survived in media with 4 mg/L colistin without further antibiotic treatment in six <i>A. baumannii</i> isolates. A stability test was also performed to investigate whether colistin resistance is maintained without antibiotic treatment. Although only three isolates showed typical colistin heteroresistance pattern, colistin-resistant populations were identified even without prior exposure to colistin in all <i>A. baumannii</i> isolates. Nearly all colonies of typical colistin-heteroresistant isolates (Type I heteroresistance) that survived after exposure to high colistin concentrations were found to be colistin-resistant, whereas no resistant colonies were identified in the other isolates (Type II heteroresistance). Stability tests showed that most of the surviving populations in media with 4 mg/L colistin without further antibiotic exposure failed to preserve resistance to colistin. Colistin-resistant populations also showed either no change in amino acid sequences, or diverse amino acid substitutions. We identified two types of colistin heteroresistance in <i>A. baumannii</i> isolates. Because Type I colistin-heteroresistant <i>A. baumannii</i> isolates could not be eradicated <i>in vitro</i> by high concentrations of colistin, differentiating two colistin heteroresistance types would be important for the treatment of <i>A. baumannii</i> infections using colistin.